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.     

Secular trends of twinning rate in Sweden.

The change in parity and maternal age in Sweden accounts for the decline in the twinning rate from the 19th century to the middle of the 20th century. The later decline in twinning rate cannot be explained by changes in maternal age and parity. Fecundity appears to be higher in mothers of twins than in other mothers.     

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.     

Twinning in humans: maternal heterogeneity in reproduction and survival

While humans usually give birth to singletons, dizygotic twinning occurs at low rates in all populations worldwide. We evaluate two hypotheses that have differing expectations about the effects of bearing twins on maternal lifetime reproduction and survival. The maternal depletion hypothesis argues that mothers of twins will suffer negative outcomes owing to the higher physiological costs associated with bearing multiples. Alternatively, twinning, while costly, may indicate mothers with a greater capacity to bear that cost. Drawing from the vast natural fertility data in the Utah Population Database, we compared the reproductive and survival events of 4603 mothers who bore twins and 54 183 who had not. These mothers were born between 1807 and 1899, lived at least to the age of 50 years and married once to men who were alive when their wives were 50. Results from proportional hazards and regression analyses are consistent with the second hypothesis. Mothers of twins exhibit lower postmenopausal mortality, shorter average inter-birth intervals, later ages at last birth and higher lifetime fertility than their singleton-only bearing counterparts. From the largest historical sample of twinning mothers yet published, we conclude that bearing twins is more likely for those with a robust phenotype and is a useful index of maternal heterogeneity.     

Population based study of rates of multiple pregnancies in Denmark, 1980-94.

OBJECTIVE: To study trends in multiple pregnancies not explained by changes in maternal age and parity patterns. DESIGN: Trends in population based figures for multiple pregnancies in Denmark studied from complete national records on parity history and vital status. POPULATION: 497,979 Danish women and 803,019 pregnancies, 1980-94. MAIN OUTCOME MEASURES: National rates of multiple pregnancies, infant mortality, and stillbirths controlled for maternal age and parity. Special emphasis on primiparous women > or = 30 years of age, who are most likely to undergo fertility treatment. RESULTS: The national incidence of multiple pregnancies increased 1.7-fold during 1980-94, the increase primarily in 1989-94 and almost exclusively in primiparous women aged > or = 30 years, for whom the adjusted population based twinning rate increased 2.7-fold and the triplet rate 9.1-fold. During 1989-94, the adjusted yearly increase in multiple pregnancies for these women was 19% (95% confidence interval 16% to 21%) and in dizygotic twin pregnancies 25% (21% to 28%). The proportion of multiple births among infant deaths in primiparous women > or = 30 years increased from 11.5% to 26.9% during the study period. The total infant mortality, however, did not increase for these women because of a simultaneous significant decrease in infant mortality among singletons. CONCLUSIONS: A relatively small group of women has drastically changed the overall national rates of multiple pregnancies. The introduction of new treatments to enhance fertility has probably caused these changes and has also affected the otherwise decreasing trend in infant mortality. Consequently, the resources, both economical and otherwise, associated with these treatments go well beyond those invested in specific fertility enhancing treatments.     

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 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.     

On the standardisation of the twinning rate.

In many studies the twinning rate, being strongly dependent on maternal age (and parity), has been standardised according to the maternal age distribution. The direct method requires very informative twinning data for the target population. The indirect method is used when the data for the target population is not sufficiently informative or when the target population is small. We have earlier introduced an alternative indirect technique for standardising the twinning rate. Our technique requires even less of the twinning data. Besides maternal age, parity is an influential factor, and should, if possible, be taken into account. In this study we present the traditional standardisation methods based on both maternal age and parity, we propose a new direct standardisation method and we develop our standardisation methods so that they take into account both maternal age and parity. We apply these standardisation methods to data from Finland, 1953-1964, from St. Petersburg, Russia, 1882-92, from Canada 1952-1967, and from Denmark, 1896-1967. These methods all give results very similar to those for the Finnish data, but the effect of parity is strongest with the direct methods. This may be due to the fact that, among extramarital maternities, parity has a strongly increasing effect on the twinning rate. This may be attributed to a higher reproduction capacity among unmarried mothers. Standardisations of the Canadian and the Danish data also give reliable results. With the St. Petersburg data, however, the different standardisations show notable discrepancies. These discrepancies are compared with Allen's findings.     

Congenital malformations and variations in reproductive performance in the ferret: effects of maternal age, color and parity

Demographic data of ferrets from a commercial breeding colony were analyzed for the effects of maternal age, parity and strain on reproductive performance and the frequency of gross congenital abnormalities observed at parturition. Litter size (mean +/- SEM) was found to be greatest for young, primiparous females (10.3 +/- 0.2) and decreased with advancing maternal age and parity to a cohort mean of 8.1 +/- 0.1 for third parity females 16 months of age. Age, parity or strain had no effect on 24-hour neonatal mortality (7%) or mortality from birth to weaning (20%) and an examination of the causes for death suggested that these rates can be reduced. The malformation rate from two cohorts of females whelping at different times of the year was low (less than 1.0%) and not significantly different. A higher frequency of malformed offspring was detected in females of low previous parity (0-2) than in those with three or more. Based on data obtained in this survey, the ferret would seem a valuable alternative, nonrodent species for teratologic investigations using currently recommended protocols.     

An aetiological study of 290 XXY males,with special reference to the role of paternal age

Summary Data on 290 non-mosaic 47,XXY males have been analysed for possible associations with parental ages at birth, season of birth, sex ratio among sibs, and twinning. Comparison with matched population controls revealed a highly significant association with parental age, which was fully explained by dependence on maternal age and maternal age alone. The maternal age effect was determined with greater precision than in an earlier study of the same material, in which siblings were used as controls, and was estimated to result in an increased risk of between 5% and 10% per annum (p.a.). The estimated independent effect of paternal age, after fitting maternal age, was marginally (but not significantly) negative, and excluded an increased risk in excess of 3% p.a. Paternal age therefore appears to have little if any independent significance in the aetiology of 47,XXY. After correcting for seasonal variations in the population birth rate and smoothing, there was a peak of XXY births in March and a trough in November. Though not statistically significant, the pattern resembled that reported in previous studies, and was similar for both younger and older mothers. The twinning rate for both the XXYs and their sibs, and the sex ratio among the latter, were close to the corresponding population values.     

Reproductive mortality and its relation to different methods of birth control

This report evaluates the decrease in maternal mortality and its relation to family planning methods in Sweden during the years 1911-80. In the 1930s fertility was low but illegal abortions were at a high level and the associated maternal death rate was 18.5 per 1000 women. With the legalization of abortion and the introduction of modern contraceptive methods, the crude reproductive mortality rate in 1965-70 was 1.7 per 100,000 women and this was reduced still further, especially for younger women, by the late 1970s. Standardized reproductive mortality was then 80% higher than the crude rate, indicating the importance of modern family planning methods. Mortality associated with oral contraceptive or IUD use in Sweden during the 1960s and 1970s was lower than in England and the US. Mortality associated with sterilization was 6.2 per 100,000 procedures.     

Maternal smoking and twinning.

In order to investigate a possible association between maternal smoking during pregnancy and twinning, information on 1,096,330 single births and 12,342 twin births in 1983-95 was obtained from the Swedish Medical Birth Registry (MBR). All odds ratios (OR) were estimated after stratification for year of birth and maternal age, parity, and educational level. Smoking women, compared with non-smoking women, were at increased risk of having dizygotic (DZ) twins, but the risk increase was only evident among multiparas. A strong association between previous involuntary childlessness and dizygotic (DZ) twinning (especially in primiparas) was found. The strongest association between maternal smoking and DZ twinning was found among multiparas without any history of involuntary childlessness (OR: 1.35, 95%CI:1.22-1.49), whereas among women who had experienced involuntary childlessness, the opposite was seen (OR: 0.82, 95%CI:0.66-1.00, no difference between parity strata). Weinberg's differential method was used to estimate the number of monozygotic (MZ) twins, and a method of estimating stratified ORs among mothers of MZ twins was presented. No association was found between MZ twinning and maternal smoking (OR: 0.96, 95%CI:0.86-1.07), and no confounding by parity or previous involuntary childlessness was indicated. Several non-causal explanations to the positive association between DZ twinning and maternal smoking among multiparas were discussed, but homogeneity over strata indicated that maternal smoking may be a true risk factor for double ovulation.     

Differences in maternal age-specific rates of Down syndrome between Jews of European origin and of North African or Asian origin.

Rates of Down syndrome in livebirths in West Jerusalem in 1964-1975 were studied in relation to the mother's continent of birth or, if she was born in Israel, to the maternal grandfather's continent of birth. In women of European origin the crude livebirth rate of Down syndrome was 1.3 per 1,000 livebirths. This crude rate and the maternal age-specific rates in this group were very close to those observed in a Swedish study and two studies of white livebirths in the United States. For West Jerusalem women of North African or Asian origin the crude rate was about 2.4 per 1,000 livebirths, and at all maternal ages except the youngest their rates were higher than for women of European origin. The summary adjusted relative risk for a Down syndrome livebirth for all those of North African or Asian origin, compared to those for women of European origin, was about 1.56. If attention is restricted to mothers born outside of Israel, the adjusted relative risk for mothers born in Europe, the Americas or English speaking countries of the British commonwealth compared to those born in North Africa or Asia was 1.97, consistent with a two-fold difference in the likelihood of a Down syndrome livebirth between thes two groups. To our knowledge this is the first report of ethnic differences in maternal age specific rates of Down syndrome that cannot be plausibly explained by differences in ascertainment.     

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.     

Age-related patterns of reproductive success among female mountain gorillas

A key goal of life history theory is to explain the effects of age and parity on the reproductive success of iteroparous organisms. Age-related patterns may be influenced by changes in maternal experience or physical condition, and they may reflect maternal investment trade-offs between current versus future reproduction. This article examines the influences of age and parity upon the interbirth intervals (IBI), offspring survival, and birth rates of 66 female mountain gorillas in the Virunga Volcano region from 1967-2004. Fertility was relatively low for females below age 12; improved as they matured; and then declined as they aged further. Primiparous mothers had 50% higher offspring mortality and 20% longer IBI than second-time mothers, though only the difference with IBI was statistically significant. The length of subsequent IBI was positively correlated with birth order but not with the mother's age. Mountain gorillas showed no evidence of an extended postreproductive lifespan. Age-related patterns seem most likely to reflect changes in the physical condition of the mother, but more detailed studies are needed to quantify those physical differences, and to obtain behavioral evidence that would provide more direct measures of maternal investment and experience.     

The epidemiological characteristics of twinning in Northwest India

The data on twin births from four different places in Northwest India were analyzed to study epidemiological characteristics of twinning. Gujjars manifested the highest incidence of twinning (30.2 per 1000 births). The twinning rate at Amritsar for the year 1987 was 19.20. At Bathinda and Jammu, the average twinning rate over the period 1984–93 were 10.70 and 11.40 respectively. Stillbirth rate among twins was much higher in Gujjars as compared to other three samples. Regional trends of twinning rate showed the highest incidence in the state of Uttar Pradesh, while the lowest in Tamilnadu. There were no significant differences between states for MZ twinning rate, while differences in the DZ twinning rate were significant in some instances. Maternal age and parity specific twinning rate showed the highest incidence at parity 4 and in the maternal age group 30–34 years. There was evidence of both seasonality and secular trends in twin births. The highest incidence of twin births over the period 1984–93 was noted in the summer season followed by rainy season, while the lowest in autumn. The average incidence of twinning in the Jammu region decreased from 13.76 during the years 1984–89 to 9.07 during the year 1990–93.     

Impact of maternal age on birth outcomes: a population-based study of primiparous Brazilian women in the city of São Paulo

The aim of this study is to analyse the impact of maternal age at first birth on low birth weight, preterm birth and low Apgar scores at one minute and at five minutes among live births delivered to primiparous Brazilian women in the city of S?o Paulo. Analyses were based on 73,820 birth records from the 1998 birth cohort. Logistic regression was used to assess the association between maternal age and each outcome variable, controlling for the following risk factors: delivery mode, plurality, sex, maternal education, number of prior losses, prenatal care, race, parity and community development. Maternal ages below 20 and above 30 years were significantly associated with the risks of low birth weight and preterm birth, but no association was found between maternal ages and Apgar score, with the exception that ages 15-19 reduced the odds of a low one-minute score. Even though this result seems to be inconsistent, low birth weight, preterm birth and low Apgar scores measure different dimensions of newborn well-being, and the association of each measure with maternal age is expected to diverge.     

Frequency of twin births among Holstein-Friesian cows in a warm dry climate

The objectives of this study were to examine 1) the effect of parity of dam and season of birth on the incidence of twins in Holstein-Friesian cattle in a warm, dry climate and 2) the sex ratio, gestation period and birth weight of calves born. Natural calvings (n = 24,843) from nine dairy farms in Saudi Arabia were evaluated over an 8-year period. The calendar year was divided into six bimonthly intervals to identify a possible seasonal effect on the number of natural twins born (twinning rate). The twinning rate increased (P < 0.05) from 1.0% (103 10,094 ) at first calving to 8.0% (1,178 14,749 ) for all the subsequent calvings. The twinning rate was similar with the second calving (P(2)) at 7.0% (417 5,929 ), the third calving (P(3)) at 7.5% (218 2,896 ), and the fourth calving (P(4)) at 7.8% (147 1,864 ). Correspondingly, cows having >/= five calves had a higher (P < 0.01) twinning rate at 9.0% (164 1,805 ) than cows with too, three or four calvings collectively at 7.3% (782 10,689 ). The seasonal effect on twinning rate was significant within all parity calvings except in primiparous females. The peak twinning rate occurred in the third season interval (May/June) at 9.3% (66 708 ) with P(2), at 13.0% (34 262 ) with P(3), at 13.8% (27 196 ) with P(4), and at 12.5% (23 184 ) with P(5) females. The male to female ratio was different (P < 0.01) for multiparous females (55:46; n = 12,305). The ratio of males to females (55:45) was also different (P < 0.01) for all twin pairs (n = 1,151), evaluated in the study. The gestation period for cows with single calves was 8.5 days longer (P < 0.05) than that of cows carrying twin calves. The birth weight of single calves was 13.1 kg greater (P < 0.05) than that of twin calves. The reason for the marked increase in the twinning rate during the warmest months may be explained by greater embryo mortality in the hot vs cool months.     

Extrinsic Mortality Effects on Reproductive Strategies in a Caribbean Community

Extrinsic mortality is a key influence on organisms’ life history strategies, especially on age at maturity. This historical longitudinal study of 125 women in rural Domenica examines effects of extrinsic mortality on human age at maturity and pace of reproduction. Extrinsic mortality is indicated by local population infant mortality rates during infancy and at maturity between the years 1925 and 2000. Extrinsic mortality shows effects on age at first birth and pace of reproduction among these women. Parish death records show huge historical variation in age-specific mortality rates. The infant mortality rate (IMR) in the early 1920s was low, increased dramatically beginning in 1929, and reached a maximum in the 1950s, at which point IMR declined steadily to its present low rate. The mortality rate early in life showed a quadratic association with age at first birth. Women who experienced conditions of low IMR early in life reproduced relatively late in life. Those born into moderately high levels of infant mortality tended to reproduce earlier than those born at low levels. At very high infant mortality levels early in life, women went on to delay reproduction until relatively late, possibly as a result of somatic depletion and energetic stress associated with the conditions that lead to high IMR. Population mortality rates at age of maturity also showed a quadratic association with age at first birth. The pace of reproduction, estimated as number of surviving offspring controlled for maternal age, showed a similar quadratic effect. There were complex interactions between population mortality rates in infancy and at maturity. When extrinsic mortality was high during infancy, extrinsic mortality later in life had little effect on timing of first birth. When extrinsic mortality was low to moderate in infancy, extrinsic mortality later in life had significant effects on adult reproduction. I speculate that these effects are mediated through development of personality facets associated with reproduction.     

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.