Seasonal variation of livebirths, stillbirths, extramarital births and twin maternities in Switzerland.

A study was made of the seasonal variation in all births, and births according to marital status, multiplicity and birth status (live and still) in Switzerland recorded between 1876 and 1990. To obtain seasonal variation in as pure as possible form, our analyses are based on rates. When comparing the seasonality in data sets showing markedly different levels, standardised indices were used. Assuming the length of pregnancies with twins to be about one month shorter than for pregnancies with singletons, lagged twinning rates were calculated but, in comparison with actual twinning rates, the general seasonal variation remained. Therefore, this study was based on actual twinning rates. A monotonic increase in the amplitude of the seasonal variation in general births was noted for the period 1876-1930, with strong seasonal variation holding for 1921-1980. After that, a marked decline in the amplitude can be observed. Seasonality of both all births and twin maternities showed very similar pattern for the periods 1876-1930 and 1969-1990, with maxima in the spring (March-May) and troughs in late autumn (October-December). Twin maternities showed a strong seasonality for the period 1876-1930, being about 20% higher in March than in October. The twinning rate in the period 1876-1930 was about 2.6 per thousand units higher than in the period 1969-90. For twin maternities there was also a stronger seasonal variation during the earlier period than during the later one. The pattern of the seasonal variation for extramarital births, showing a maximum in February (conceptions in May-June) and a minimum in August (conceptions in November-December) with a difference of no less than 24% was more marked than for the marital births. It seems likely that this seasonality of extra-marital maternities was due mainly to seasonal variation of coital rates and multiple ovulation in the early summer months coinciding with optima of light, temperature and food supply. A strong reduction in the rate of stillbirths (gestational age more than 29 weeks) was observed during the twentieth century. The stillbirth rate declined from about 40 per 1000 in the 1870s to fewer than 5 per 1000 in the 1980s. Irrespective of this strong decline in the stillbirth rate, the same seasonal rhythm was noticed throughout the period with high stillbirth rates among births around March and low rates during the summer and autumn.     

Twin seasonality in a rural Catalonian population

The seasonality of twinning in the Spanish populations has not been studied until now. Differences between seasonal distribution of the twin conceptions and those of the single births have been observed in other populations. The aim of this work is to explore the frequency of twinning in a rural population from Catalonia during the nineteenth century, as well as the seasonality patterns characterizing each of the twinning types. Data corresponding to all births recorded at Tortosa (South Catalonia) from 1801 to 1900 have been analyzed in order to study the twinning distribution. The distribution of the moving averages of the monthly rates of twins shows a peak in autumn. Twinning distribution differs from the total births' distribution in Tortosa. This fact is very clear in the case of unlike-sexed twins that have their greater incidence in the last quarter of the year, while the total maternities have their peak in the first one.     

Statistical analysis of the seasonal variation in demographic data

There has been little agreement as to whether reproduction or similar demographic events occur seasonally and, especially, whether there is any universal seasonal pattern. One reason is that the seasonal pattern may vary in different populations and at different times. Another reason is that different statistical methods have been used. Every statistical model is based on certain assumed conditions and hence is designed to identify specific components of the seasonal pattern. Therefore, the statistical method applied should be chosen with due consideration. In this study we present, develop, and compare different statistical methods for the study of seasonal variation. Furthermore, we stress that the methods are applicable for the analysis of many kinds of demographic data. The first approaches in the literature were based on monthly frequencies, on the simple sine curve, and on the approximation that the months are of equal length. Later, "the population at risk" and the fact that the months have different lengths were considered. Under these later assumptions the targets of the statistical analyses are the rates. In this study we present and generalize the earlier models. Furthermore, we use trigonometric regression methods. The trigonometric regression model in its simplest form corresponds to the sine curve. We compare the regression methods with the earlier models and reanalyze some data. Our results show that models for rates eliminate the disturbing effects of the varying length of the months, including the effect of leap years, and of the seasonal pattern of the population at risk. Therefore, they give the purest analysis of the seasonal pattern of the demographic data in question, e.g., rates of general births, twin maternities, neural tube defects, and mortality. Our main finding is that the trigonometric regression methods are more flexible and easier to handle than the earlier methods, particularly when the data differ from the simple sine curve.     

Association between the rates of multiple maternities.

We analyze the association between the rates of multiple maternities. Correlation analysis is suitable if there are at least monotonic relationships between the variables. A decreasing tendency can be observed in the rates of multiple maternities in Sweden and in its 25 counties for the period 1751-1960 and the effect of external variables can be assumed to be monotonic. After 1960, the rates of multiple maternities show marked increases, mainly caused by ovulation stimulants and the introduction of other artificial reproduction techniques. We transformed the triplet and quadruplet rates according to Hellin's law in order to obtain rates on the same scale as the twinning rate. The time trends for the multiple maternities in Sweden as a whole were quite similar. The regional differences in the twinning rates could not be satisfactorily explained by maternal age. Alternative attempts to eliminate the effect of other time-dependent factors were to study partial correlation coefficients when the time was kept fixed and the correlation coefficients based on cross-sectional regional data. Both the ordinary and the partial correlation coefficients showed strong regional variation. Cross-sectional analyses gave correlation coefficients similar to the partial correlation coefficients for Sweden as a whole. The variations in the correlation coefficients between the twinning and the triplet rates seem to be caused by other time-dependent factors and the effects of these factors show strong regional variation. After elimination of such factors, the correlation between the twinning and the triplet rates is moderate.     

A multiple pregnancy register in the north of England.

A regional population-based Multiple Pregnancy Register was established in 1998, with the aim of collecting detailed information on multiple pregnancies to enable research into mortality and morbidity in multiples. Multiple pregnancies are notified to the Register as soon as they are detected, irrespective of whether they resulted in a spontaneous abortion, termination of pregnancy or registered birth. Nine hundred and twenty-six twin pregnancies were recorded during 1998-99, giving a twinning rate of 14.8 per 1000 maternities (rate at birth 13.0 per 1000 maternities). Sixty one per cent of twin pregnancies were detected before 13 weeks of gestation. Chorionicity was determined in 82.6% of 849 twin maternities with at least one stillbirth or livebirth. The fetal loss rate before 24 weeks of gestation was 10.5% (194/1852). The perinatal and infant mortality rates were 40.6 per 1000 births and 32.6 per 1000 livebirths respectively. A prospective Multiple Pregnancy Register not only allows monitoring of trends in multiple birth rates and mortality, but also etiological research and long-term follow-up studies.     

Weinberg's differential rule reconsidered

A central problem in research on twins is the estimation of the rates of monozygotic and dizygotic twin maternities. The estimation usually follows Weinberg's differential rule. According to this rule, the rate of dizygotic twinning is twice the rate of twin maternities in which the twins are of opposite sex. The monozygotic twinning rate is the difference between the rates of same-sex and opposite-sex twin sets. Weinberg's differential rule is implicitly based on the assumptions that the probability of a male equals the probability of a female and that the sexes in a dizygotic twin set are independent. Although Weinberg's differential rule has been the target of continuing discussions, the reliability of these assumptions has never been conclusively verified or rejected. In this study we present new variance formulas for the monozygotic and dizygotic twinning rates obtained using Weinberg's differential rule and stress that these new formulas have to be used. We analyze the accuracy of Weinberg's differential rule by considering alternative attempts. Especially, we build a general likelihood function and show that the maximum-likelihood estimates differ only slightly from the rates obtained using Weinberg's differential rule. In addition, our methods are applied to twinning data from Finland and Sweden. We compare our results with findings in the literature. In conclusion, our findings indicate that Weinberg's differential rule is rather robust and that despite its simplicity, it gives reliable results when official birth registers are analyzed.     

Demographic analysis of the variation in the rates of multiple maternities in Sweden since 1751

To elucidate the causes and mechanisms of twinning and higher multifetal maternities, we have taken advantage of the statistical sources of Sweden, where continuous statistics for the whole population are the oldest available. We found strong secular and regional fluctuations. The rates of multiple maternities were the highest during the last three decades of the 18th century, when the twinning rate was more than 17 per 1,000, the triplet rate was more than 3 per 10,000, and the quadruplet rate was almost 7 per 1 million maternities. During 1849-1873 the twinning rate in Sweden was 14.2 per 1,000, but this rate showed great regional differences, being 18.0 per 1,000 on the island of Gotland and 12.6 per 1,000 in the county of Alvsborg. During this period the twinning rate in the countryside in the county of Stockholm was 20.4, but in the city of Stockholm it was only 14.1 per 1,000. In Sweden after the 1930s there was a marked decrease in the twinning rate, which by the 1960s had fallen to only about half of what it had been two centuries earlier. The corresponding reductions for triplet and quadruplet rates were about 75%. The aim of this paper was to study the temporal and regional variations in multiple maternities in Sweden from 1751 to 1960 based on demographic and some socioeconomic data for the counties. We confirmed our earlier studies that maternal age and parity cannot satisfactorily explain the secular and regional differences in the twinning rates. In contrast to studies in France (1901-1968), we found no unequivocal association between the twinning rates and the crude birth rates. The correlation coefficients between the twinning rate and the crude birth rate showed statistically significant regional and temporal variations. After eliminating the temporal trends, regional differences in the correlation coefficients remained. The twinning rates for the counties seem to converge toward a common low level, 10-12 per 1,000. The observed convergence toward relatively similar levels may be caused by the increased matrimonial migration distances and decreased endogamy of the citizens as a consequence of better communications. The increased urbanization and industrialization that started in the last decades of the 19th century broke up the old static agrarian isolates and caused Sweden, within 2-3 generations, to develop from a poor nation to one of the most prosperous in the world. A more urban and affluent lifestyle, a better diet, and increased stress and sedentary occupations may have reduced the physical capacity of mothers to carry gestations with multiple embryos or fetuses to completion.     

Low twinning rate and seasonal effects on twinning in a fertile population,the Hutterites

This paper analyzes from the mid 18th century to 1987 the birth records of the Dariusleut, one of the three subgroups of the Hutterite population. The aim of this study is to describe several aspects of the twinning rate in a fertile population. The overall rate of twinning was 0.90%:103 twins among all 11492 maternities. The rate peaked at the 7th birth order and at the maternal age of 40 years and over. Until the mid 19th century when the Hutterites lived in Russia, the twinning rate was higher (1.5%), and it decreased during the migration period in the second half of the 19th century (0.7%). After the group had settled in the USA and Canada, the population maintained a twinning rate of 1.0% until 1965. After 1965 the rate decreased to 0.7%, partly due to a decline in fertility among women aged 30 years and over. There was a significant seasonal variation: the twinning rate decreased to 0.5% in May–July compared to 1.2% for the other three seasons during the years up to 1965 (P<0.01), while more recent mothers did not show such a seasonal variation. The incidence of twin births in this population seems to have been influenced by environmental factors, which would change their effect seasonally and secularly.     

Twinning and heteropaternity in chimpanzees (Pan troglodytes)

Unlike monozygotic (MZ) twins, dizygotic (DZ) twins develop from separate ova. The resulting twins can have different sires if the fertilizing sperm comes from different males. Routine paternity testing of a pair of same-sexed chimpanzee twins born to a female housed with two males indicated that the twins were sired by two different males. DNA typing of 22 short-tandem repeat (STR) loci demonstrated that these twins were not MZ twins but heteropaternal DZ twins. Reproductive data from 1926-2002 at five domestic chimpanzee colonies, including 52 twins and two triplets in 1,865 maternities, were used to estimate total twinning rates and the MZ and DZ components. The average chimpanzee MZ twinning rate (0.43%) equaled the average human MZ rate (0.48%). However, the chimpanzee DZ twinning rate (2.36%) was over twice the human average, and higher than all but the fertility-enhanced human populations of Nigeria. Similarly high twinning rates among African chimpanzees indicated that these estimates were not artifacts of captivity. Log-linear analyses of maternal and paternal effects on recurrent twinning indicated that females who twinned previously had recurrence risks five times greater than average, while evidence for a paternal twinning effect was weak. Chimpanzee twinning rates appear to be elevated relative to corresponding estimated human rates, making twinning and possibly heteropaternity more important features of chimpanzee reproductive biology than previously recognized.     

Monthly trend and seasonal variation in twinning rate in Japan, 1975–1994

We examined birth records from Japanese statistics, 1975–1994, to investigate the seasonality of twin births. We could identify 198 924 pairs of twins (97.9% of all the registered twin records) and estimated the numbers of mono- and dizygotic twin pairs. The seasonal index of the twinning rate for each month was calculated by dividing the crude rate by the estimated trend value for the month. There were significant variations in the seasonal index for overall, dizygotic and monozygotic twinning rates. Peak months with values more than 3% higher than expected were July and October–December for dizygotic twins, and April and June for monozygotic twins; these seasonalities were statistically significant by analysis of variance and the patterns were similar in recent years, with a sharp increase in the total twinning rate. When observed year-by-year, however, there were years that did not show these typical seasonalities. It is suggested that the mechanisms for probable seasonal variations in twinning rates are different for dizygotic and monozygotic twin pregnancies, and that factors involved in these variations are not effective every year. Received: 2 September 1998 / Revised: 6 May 1999 / Accepted: 26 May 1999     

Time trends in twin perinatal mortality in northern England, 1982-94. Northern Region Perinatal Mortality Survey Steering Group.

The dynamics of perinatal mortality rates (PNMR) and causes of death in twin pregnancies over 13 years in the Northern Region of the National Health Service in England is described. All twin perinatal deaths occurring between 1982-1994 were identified from the Northern Region Perinatal Mortality Survey. The twinning rate increased from 9.9 per 1000 maternities in 1982 to 12.0 in 1994. There was a total of 10,734 twin pregnancies and of these 421 resulted in 530 perinatal deaths. The perinatal mortality rate in twins significantly decreased over time (1982-87, 55.4 per 1000; 1988-94, 44.4 per 1000; P = 0.01). The PNMR was significantly higher for twins from like-sexed than from unlike-sexed pairs (53.5 and 34.4 per 1000 respectively, P < 0.001). Despite no improvement in birthweight distribution in the twin population, birthweight-specific perinatal mortality rates for both like and unlike-sexed twins decreased for each birthweight category in 1988-94 compared with 1982-87. Twins with very low birthweight (< 1500 g) comprised 69%, and preterm twins (< 37 completed weeks of gestation) 74.9% of all twin perinatal deaths. The major immediate cause of early neonatal death was pulmonary immaturity (63%); antepartum anoxia caused 76.9% of antenatal deaths. Unexplained preterm labour and intrauterine death were the leading obstetric factors underlying death in twins. Despite a decrease over the 13 years, the perinatal mortality rate in twins in the Northern Region remains high. Continued monitoring of trends in twinning and mortality rates is needed to inform health care planning.     

The effects of assisted reproduction on the trends and zygosity of multiple births in England and Wales 1974-99.

Assisted reproductive techniques have led to an increase in the proportion of maternities that are multiple. Though predominantly dizygotic, they are at greater risk of monozygotic division than those spontaneously conceived. England and Wales data 1974-99 on stillbirths and livebirths were analysed for 4 periods: 1974-80 (pre-assisted reproduction; 1982-8; 1989-91 (pre-redefinition of stillbirth); 1993-9 (post-redefinition of stillbirth). For twin data, Weinberg's rule was applied to estimate the proportions that were mono- (MZ) and dizygotic (DZ). Compared with the period before assisted reproduction, the most recent period shows an increase in twin maternities of 3.81 per 1,000 comprised of 3.22 (95% CI 3.10 to 3.33; p < 0.0001) DZ and 0.60 (95% CI 0.51 to 0.68; p < 0.0001) MZ twins. It is estimated that 15.7% of assisted reproduction twins are MZ. Higher order multiple births showed an increase of 3.06 (95% CI 2.85 to 3.29; p < 0.0001) per 10,000 maternities. Stillbirth rates in MZ twins are of the same order of magnitude as those in higher order multiple births but higher than those in DZ twins. The improvement in stillbirth rates over the 26 year study period is of the same order magnitude in singletons, DZ and MZ twins and higher order multiples. Assisted reproduction has led to a significant increase in the proportion of MZ twins. These are at high risk of fetal death and this needs to be considered when local stillbirth and perinatal mortality rates are used in auditing obstetric services.     

Frequency of twinning in southwest Nigeria

BACKGROUND:

In the human species, twin is a type of multiple birth in which the mother gives birth to two offspring from the same pregnancy. The occurrence and frequency of twinning, however, varies across human populations. The maternal age, socio-environmental factors, increase in the use of contraceptives, the race of human population, increase in the spontaneous abortion rate, and seasonal variations are among the factors that could influence twinning rate. Information on twinning rates in southwest Nigeria is limited.

AIMS:

This study presents information on the frequency of twinning, as well as its analysis by maternal age, in four urban settings in southwest Nigeria. This is with the aim of extending current knowledge on the frequency of twinning in southwest Nigeria and contributing to the demographic studies in the country.

MATERIALS AND METHODS:

Data on single births and twin births from January 1995 to December 2004 were collected from the Oyo State General Hospital (OSGH), Wesley Guild Hospital (WGH), Obafemi Awolowo University Teaching Hospital (OAUTH), and Ekiti State Specialist Hospital (ESSH) in Ogbomoso, Ilesa, Ile-Ife, and Ado-Ekiti respectively. These were analyzed by year and maternal age groups of 15-19, 20-24, 25-29, 30-34, 35-39, 40-44, and 45-49 years according to the standard method.

RESULTS:

A frequency of twin births of 46.5 per 1000 deliveries and 46.2 per 1000 deliveries was recorded for Ilesa and Ile-Ife respectively. The frequency recorded for Ogbomoso and Ado-Ekiti was 38.5 and 22.1 per 1000 deliveries respectively. The overall average frequency of 40.2 per 1000 deliveries for the four hospitals ranks among the highest recorded rates of twin births in the world. The maternal age group of 25-29 years had the highest occurrence of twin births, while the lowest was recorded in the 45-49 years age group.

CONCLUSION:

This analysis reveals high incidence of twinning in the studied areas and supports previous assertion that the southwestern part of Nigeria has the highest twinning rate in the country and in the whole world. It is our opinion that diet, maternal history of twinning, and some socio-environmental factors may have influenced the results.     

Twinning rates in a rural area of Bangladesh

In this study we investigate the incidence of twin births over a period of 16 years in a rural area of Bangladesh using data from the Demographic Surveillance System of the International Centre for Diarrhoeal Disease Research. Over the study period twinning rates fluctuated between 7.8 and 11.2 per 1000 live births. The twinning rate was strongly correlated with maternal age; the rate for mothers over 35 years of age was about 3 times higher than for mothers younger than 20 years. The variation in twinning rate with maternal age is due to the variation in dizygotic twinning; the rate of monozygotic twinning is almost constant for all ages. Twinning rates were higher in the treatment area than in the comparison area after controlling for maternal age and parity. The rates were lower for monozygotic twinning and higher for dizygotic twinning in the treatment area than in the comparison area. Seasonality was observed for both twins and singletons, but the peak for twinning precedes that for singleton births by more than a month.     

Selection for increased brood size in historical human populations

Human twinning rates are considered to either reflect the direct fitness effects of twinning in variable environments, or to be a maladaptive by-product of selection for other maternal reproductive traits (e.g., polyovulation). We used historical data (1710-1890) of Sami populations from Northern Scandinavia to contrast these alternative hypotheses. We found that women who produced twins started their reproduction younger, ceased it later, had higher lifetime fecundity, raised more offspring to adulthood, and had higher fitness (individual lambda) than mothers of singletons in all populations studied. For example, an average of 1.2 offspring survived to adulthood from a twin delivery, irrespective of its sex ratio, whereas only 0.8 offspring survived to adulthood from a singleton delivery. Only if mothers started reproduction at very late age (> 37 yr), or had a very long reproductive life span (> 20 yr), was it more beneficial to produce only singletons. These findings suggest that twin deliveries among Sami could not be explained as a maladaptive by-product of selection for other maternal reproductive traits. In contrast, our results suggest that twinning was under natural selection, although the strength of selection was likely to have been context dependent.     

Twinning across the Developing World

Background

Until now, little was known about the variation in incidence of twin births across developing countries, because national representative data was lacking. This study provides the first comprehensive overview of national twinning rates across the developing world on the basis of reliable survey data.

Methods

Data on incidence of twinning was extracted from birth histories of women aged 15–49 interviewed in 150 Demographic and Health Surveys, held between 1987 and 2010 in 75 low and middle income countries. During the interview, information on all live births experienced by the women was recorded, including whether it was a singleton or multiple birth. Information was available for 2.47 million births experienced by 1.38 million women in a period of ten years before the interview. Twinning incidence was measured as the number of twin births per thousand births. Data for China were computed on the basis of published figures from the 1990 census. Both natural and age-standardized twinning rates are presented.

Results/Conclusions

The very low natural twinning rates of 6–9 per thousand births previously observed in some East Asian countries turn out to be the dominant pattern in the whole South and South-East Asian region. Very high twinning rates of above 18 per thousand are not restricted to Nigeria (until now seen as the world''s twinning champion) but found in most Central-African countries. Twinning rates in Latin America turn out to be as low as those in Asia. Changes over time are small and not in a specific direction.

Significance

We provide the most complete and comparable overview of twinning rates across the developing world currently possible.     

Standardization of the twinning rate

There is a great interest in comparing twinning rates. These comparisons can be performed between different time periods for a specific population, between different regions within the same country, and between different populations. However, there are several factors (maternal age, parity, urbanization, etc.) that influence the twinning rate. The most dominant one is maternal age, and because the age distribution of the mother varies, it is necessary to standardize the data to make these comparisons. If we want to compare the twinning rates in different countries, we have to face the problem that the composition of the data from different countries may differ to a great extent. The applicable method is determined by the data of the lowest quality. Often the available data do not allow the traditional (direct and/or indirect) methods of standardization. Under such circumstances other methods have to be used. Earlier, Fellman and Eriksson (1987) proposed and successfully applied a new method. In this article we discuss the standardization problem in more detail. We suggest different methods and apply them to different data on twinning from Australia, Finland, and Baden-Württemberg (West Germany). The new standardization methods give standardized twinning rates similar to the rates obtained by traditional methods. It is noted that, irrespective of standardization method, changes in maternal age alone cannot explain temporal or regional variations in the twinning rate. Other factors that may raise or lower the twinning rate are decreasing parity, sociodemographic changes with increased communication, which causes the breakup of isolates, and deteriorating physical condition of mothers as a result of increased industrialization and urbanization.     

Twinning propensity and offspring in utero growth covary in rural African women

In humans and other mammals, some females are more likely to experience twin pregnancies than others, but the reasons behind such individual variation are poorly understood. One hypothesis invokes variation in the dynamics of the insulin-like growth factor (IGF) system, which also regulates foetal growth. Using data from a rural African population living in a highly seasonal environment, we test a novel prediction generated by this hypothesis, that maternal twinning status predicts offspring birthweight. We found that among singleton offspring who experience a favourable in utero environment (born January-June), births before and after twins are, respectively, associated with a 134.07 g and 226.41 g increase in birthweight compared with those born to non-twinning mothers. These results were not mediated by maternal anthropometry. This is consistent with a role for the IGF system in individual variation in twinning propensity, a possibility with implications for understanding mechanisms of life-history variation in humans and other vertebrates.     

Insurance ovulation, embryo mortality and twinning.

The 'insurance ova' hypothesis (Anderson, 1990) views dizygotic twinning as a by-product of selection for multiple ovulation which sometimes--in error--results in the birth of twins. From this viewpoint, polyovulation is a mechanism which reduces the risks of fertilization failure or embryo defect/mortality. If DZ twin births are a 'side-effect' of a mechanism which compensates for defective embryos one might predict that embryo defect rates and twinning rates will covary. This prediction is tested using national-level data on twinning rates and rates of trisomy-21 (Down's syndrome), and a strong positive correlation is found, even when controlling for maternal age. One suggestion that follows from this finding is that intra- and interpopulation variation in both twinning rates and Down's syndrome rates may result, in part, from individual variation in pre-implantation rejection of embryos in the very early stages of pregnancy. In this paper it is proposed that the 'insurance ova' explanation for twinning in humans could be expanded to incorporate a model of rejection of anomalous embryos, be they anomalies of number or type. Variation in the efficiency of an embryo rejection mechanism, combined with variation in frequency of polyovulation, would have consequences for individual reproductive success.     

Box-Jenkins modeling of chronological series of twin births and single births

A time-series analysis by Box-Jenkins modeling of the monthly observations of twin births and singleton births was attempted. The study is population based. The single-birth series has been matched to the twin-birth series for maternal birth cohort, parity of the twin birth, and residence of the parents. Three-hundred forty-one twin pairs and 340 singleton births were grouped into chronological series spanning 72 months of observation. Univariate and bivariate time-series analyses were used. Box-Jenkins modeling shows that both series share the same basic demographic and reproductive risk factors, such as ovulation and conception rates, abortions, sexual intercourse, and the less well-known random events that determine the time of delivery. These factors are randomly related to both birth processes but exert a more important influence on twin births. Beyond these crude features, twin- and single-birth series differ by the frequency of their seasonal cycle. Twin births show low-frequency variations because of a rather limited number of factors that impinge markedly on the twinning process. Single births are subjected to higher frequency variations that can be ascribed to a greater number of structuring factors, the action of which is less pervasive. Only the January peak is common to both types of births. Aside from the common features, the bivariate analysis shows that both series of births are independent, neither being related to the other. It is concluded that the subtle structuring of both processes is largely distinct. The large fraction of the unaccounted variation of each birth process (about 70%) suggests that most of the variation is due to as yet unidentified factors. The search for environmental and geocosmic factors at the origin of conception and confinement should be considered in future undertakings.