The season‐of‐birth paradox

Abstract

Vital statistics data show a remarkably consistent seasonality in U.S. birth patterns, with peaks in late summer and winter months, and a valley in the spring. An attitude survey of college students suggests that peaks in the actual birth distribution occur in unpopular months in which to give birth; the valley in the actual birth distribution occurs in popular months. This paradoxical finding is named the Season‐of‐Birth Paradox. Explanations to resolve the paradox include biological and psychological components. A psychological mechanism—named the Misinformed Reproducer Hypothesis—is tested using NSFG data from the 1973–75 and 1979–81 cycles. Results suggest that women stop contracepting with the expectation that they will get pregnant almost immediately. When it takes several months on the average for a successful conception to occur, the actual birth distribution is shifted away from the preferred birth distribution. These results suggest that psychological as well as biological mechanisms underlie the consistent seasonality patterns in U.S. births.     

The seasonality of births in Canada

Earlier studies on the seasonality of births indicate that a major peak occurs in August and September and a minor peak in January and February. This study uses the 1984 Canadian Fertility Survey data on reproductive history to examine birth and pregnancy seasonalities, and shows that the 'worst' months for births are January and February and the 'best' months are March, April and May. There is no systematic pattern in the seasonality of pregnancies, possibly because effective birth control allows couples to plan the timing of births.     

Seasonality of births is associated with seasonality of marriages in Malta

This study was carried out to quantify secular trends in seasonal variation in births in Malta, a small Mediterranean country where the vast proportion of births occur in wedlock due to a predominantly Roman Catholic population. It also related such variations to seasonal variation in marriages. Annual seasonal peaks of marriages and births were analysed over the period 1950-1996 by X11 ARIMA. A significant peak in marriages (n = 111,932) in the third quarter of the year was found for almost the entire period under study. This was paralleled by a peak in births (n = 299,558) for the period 1970-1996, which lagged after the peak in marriages by 13-14 months. For the period 1994-1996, when monthly data for monthly pregnancies were available by pregnancy order, the peak in births was caused by first pregnancies only. Seasonal patterns in births occur almost universally due to cultural and/or biometeorological factors. The best known patterns include those of the southern United States, where births decline in April and May, and in northern Europe, where births peak in March and April. In Malta, the late summer peak in births appears to be due to a practical and planned approach by Maltese couples to contraceptive planning, probably influenced by the Roman Catholic ethos and social pressures, with unprotected intercourse occurring only after marriage. In Malta, birth control, albeit by so-called natural methods, was introduced in the 1960s. Prior to this period, births peaked towards the beginning/end of the year, and this may be the more natural seasonality of births in Malta.     

Decreased fecundability in Hutterite couples sharing HLA-DR.

To study the effects of parental HLA sharing on pregnancy outcome, we initiated population-based studies in the Hutterites. We previously reported longer intervals from marriage to each birth among couples sharing HLA, particularly HLA-DR. In the present report, we present the results of a prospective, 5-year study of fecundability and fetal loss rates in this population. Between April 1986 and April 1991, 154 pregnancies were observed in 104 couples. The median number of months of unprotected intercourse to a positive pregnancy test was significantly longer among couples sharing HLA-DR who stopped nursing prior to the first menses as compared with couples not sharing HLA-DR who stopped nursing prior to the first menses (5.1 vs. 2.0 mo, respectively; P = .016). Fetal loss rates were increased among couples sharing HLA-B as compared with couples not sharing HLA-B (.23 vs. .12, respectively; P = .041, adjusted for age, gravidity, and kinship). These data suggest that our earlier observations of increased birth interval lengths among Hutterite couples sharing HLA were predominantly due to longer intervals until a clinical pregnancy among couples sharing HLA-DR and, to a lesser degree, were due to increased fetal loss rates among couples sharing HLA-B.     

Effects of maternal birth season on birth seasonality in the Canadian population during the seventeenth and eighteenth centuries

Birth records of the French-Canadian population for the period 1621-1765 were analyzed retrospectively to examine the effect of maternal birth season on the seasonal distribution of births. Preliminary examination indicated that there was a bimodal pattern in birth seasonality: a major peak in early spring, a trough in early summer, a minor peak in autumn, and a trough around December. Because this seasonality was strongly biased at the level of the first birth by the month of marriage, which was concentrated in November, the seasonality of nonfirst births (n = 32,926) was examined in relation to the four seasons of maternal birth. Mothers born in May-July showed a flatter monthly distribution of nonfirst births at a maternal age of 28 years or more. Analysis of marriage-first birth intervals indicated that mothers who married in August-October showed a lower percentage of immediate conception (intervals of 8-10 months), whereas those mothers born in May-July had a higher percentage of immediate conception. This difference in birth seasonality shown by mothers born in May-July is similar to results from early twentieth-century Japan. Some seasonal infertility factors could have affected the embryos at the earliest stage of pregnancy, modifying a part of the seasonal variation in birth rate.     

Changes in contraceptive use and fertility: El Salvador, 1978-88

In El Salvador from 1978 to 1988, contraceptive use among married women 15-44 years of age increased from 34% to 47%, and the total fertility rate declined from 6.3 to 4.6 children per woman. Most of this change took place from 1978 to 1985. Sterilization is the most prevalent method used, but nearly one-half of the women who are sterilized did not use any contraception before their operation. Few young couples use reversible methods of contraception to space births or delay the start of childbearing. On average, women wait 8 years after marriage and have nearly three children before they use contraception.     

Maternal exposure to exogenous sex hormones in relation to birth weight of offspring

Birth weight was analyzed among singleton live births (N = 665) in Upstate New York in 1974 to women who used oral contraceptives (OC) in comparison to live births to women who used no contraceptives (N = 716), within 11 months prior to last menstrual period (LMP). In addition, birth weight was examined among live births to women who received hormone support therapy (N = 97) and hormone pregnancy tests (N = 75) during pregnancy. There was no evidence for a reduction in mean birth weight, or an increase in frequency of lower weights, among births to OC users, including those who stopped using OC within 2 months of LMP. Generally similar findings held within three maternal age groups (less than 25, 25-29, and 30-39 years). There was no evidence for a reduction in birth weight among offspring of women who received hormone pregnancy tests. Mean birth weight was relatively low among male and female births to women who received hormone therapy for "threatened abortion," but this may reflect the selection of women for such treatment rather than an effect of exogenous hormones on fetal growth.     

Social-demographic influence on first birth interval in China, 1980-1992

This study examines the delay between first marriage and first live birth in China among a sample of women who married between 1980 and 1992. Most couples in China only use contraception after the first child is born. Most sample women had their first child within 2 years of marriage. However, there are significant rural-urban differences in the first birth interval, indicating that there was most probably deliberate fertility regulation after marriage among many urban couples. Survival analysis shows that place of residence, level of education, age at first marriage and marriage cohort affect the first birth interval.     

Perceived contribution of children to marriage and its effects on family planning behavior.

The impact of the perceived consequences of having children on a couple's contraceptive practices and the tendency to plan births are examined. 2 separate measures of family planning were utilized, contraceptive pattern and proportion of unplanned pregnancies. It was found that the more a woman viewed children as an essential part of the marital experience the less likely she was to use reliable contraception early in marriage. The influence of the evaluation of children on contraceptive practices also was contingent upon the number of children a couple had had and the number of years they had been married. Differences in incidence of unplanned pregnancies among women was more a result of socioeconomic circumstance and the tendency to have a large family than attitude towards children. These 2 aspects of family planning have different implications for policy decisions. Changing the incidence of unplanned pregnancies would entail attacking those conditions which handicap couples in changing their life circumstances. It would involve educating them to the concept of family planning and the techniques of birth control and also increasing economic opportunity. Changing contraceptive patterns, on the other hand, might involve encouraging alternative adult roles, and creating differential incentives for childbearing.     

Gender relations: husband-wife fertility and family planning decisions in Kenya

Although Kenya's fertility rate has declined from 6.7 births per woman in the mid-1980s to 5.4 births per woman in 1993 (NCPD, 1994), population growth is still high, yielding a doubling time of 35 years. This study uses the 1993 Kenya Demographic Health Survey data collected from 1257 couples to examine the socioeconomic and sociodemographic characteristics of married men and women and their communication with their spouses over fertility and family planning decision-making practices. The logistic regression analysis shows that education for both men and women, discussion of fertility and family planning between spouses, male approval of use of contraception and male family size desires are important factors that influence ever-use of family planning.     

Determinants of birth intervals in Sudan

Using individual birth history data from the Sudan Fertility Survey, 1979, parity-related differences in fertility are demonstrated, as well as differences between socioeconomic groups. Rural women, women with no education and those married to uneducated husbands show rapid parity progression and its cumulative effects on fertility which are consistent over all birth intervals. Urban women, women with some education and those married to educated husbands, however, go rapidly through their second and third birth intervals and then more slowly at higher parities. A limitation of the study was the inability to control fully for the effects of breast-feeding and contraception. There is evidence for a reduction in high parity births, starting in the 1970s.     

Return of fertility after use of the injectable contraceptive Depo Provera: up-dated data analysis

796 Thai women who stopped using the long-acting injectable contraceptive depot medroxyprogesterone acetate (DMPA, Depo Povera), 437 women who stopped using oral contraceptives and 125 women who had an IUD removed to have a planned pregnancy, were followed up to ascertain the delay to conception after the end of contraception and to determine the proportion of women who did not conceive in the 4 years after discontinuation. The median delay to conception was 5.5 months plus the estimated duration of the effect of the last injection of DMPA, 3 months for oral contraceptives and 4.5 months after discontinuing the IUD. The proportion of women who did not conceive within 9 months after discontinuation of DMPA is similar to that of ex-IUD users, and by 3 years to that of the ex-pill sample. There is no evidence to suggest that prolonged use of DMPA increases the delay to conception. The return of fertility among never pregnant ex-users resembled that of ever pregnant ex-users. There were comparable proportons of live births among ex-DMPA users and ex-pill users and both of these showed higher proportions of live-births than ex-IUD users. There was no evidence to suggest that previous use of DMPA had any significant adverse effect on the outcome of pregnancy of the subsequent births. This study did not show any association between infertility and the previous use of DMPA or other contraceptives.     

Deliveries in teenagers at a Newfoundland general hospital.

A study of deliveries in teenagers was undertaken for the year 1975 in a hospital that had recorded 2797 births, 371 (13%) of which were to women under 20 years of age. Conception had occurred out of wedlock in 314 (85%) of the 371 pregnancies; 124 of the 314 women had married during the pregnancy, most often in the 3rd or 4th month of gestation. The peak months for conception out of wedlock were June and December. This was not the first pregnancy for 65 women (18%), 21 of whom had married during a previous pregnancy. Of the "heads of the households" 36% were labourers and 27% were unemployed. Cesarean section was the method of delivery for 51 (14%) of the women, and 63 (17%) were reported as having had toxemia. There were seven perinatal deaths and seven infants had severe congenital abnormalities. The frequency of low birth weight was 6% overall but 13% for the infants of single women. Five women underwent tubal ligation post partum.     

藏酋猴社群雌体的性行为模式

猕猴属中大部分种类的繁殖类型可划分为季节性繁殖和非季节性繁殖两大类型。但是藏酋猴全年均有交配行为发生, 而产仔仅在1～8月间, 其类型属特殊的非季节性交配-季节性产仔繁殖类型。藏酋猴雌性在妊娠后选择的交配对象主要是高序位的雄性, 但非妊娠雌性则主要选择低序位雄性。妊娠后的雌性交配频率低于非妊娠雌性, 同时它们与成年雄性间理毛行为的发生频率亦低, 反之受到成年雄性攻击的频率却高。     

Correlates of voluntary childlessness in a select population

In order to determine a sociological profile of voluntarily childless couples, 72 couples who had applied to the Association for Voluntary Sterilization for help in obtaining surgical sterilizations were surveyed. No pretense is made that this sample is representative of all childless couples, but this study provides the first step of such research. The average couple in this group was living in a large metropolitan area, about 30 years old, and had been married about 5 years. They tended to be of high socioeconomic status and claimed no religious affiliation. Most had been contracepting for several years, usually with the pill, and were not likely to have had a miscarriage or abortion. Population concerns and health considerations were found to be the 2 most mentioned reasons for these couples' desired childlessness. Other reasons included career commitments, lifestyle, or economic desires. Less common motivations for childlessness included dislike of children and fear of pregnancy. Before significant research on voluntary childlessness can be conducted, some accurate means of selecting a representative sample must be developed.     

Seasonality of birth and conception to teenagers in Texas

We study the births to teenagers during the years 1964-2000 and analyze separately the three main racial/ethnic groups in Texas (White, Hispanic, and African American), as well as married and unmarried teens during the years 1994-2000. By using traditional statistical methods of analysis and a filter based on the multiresolution wavelet analysis, we draw inferences about the times of the year when adolescent females of different racial/ethnic and marital groups have the highest probability for pregnancy ending in live birth. Multiple factors influencing teen pregnancy are identified and associated with temporal features of social, cultural, educational, and familial processes. In particular, we detect links between unmarried teen conception times and school terms, and weekly birth patterns associated with scheduled c-sections that differ according to racial/ethnic groups.     

Reproductive parameters of wild female Rhinopithecus roxellana

On the basis on 6 years of observation, we estimated the reproductive parameters of a Golden snub-nosed monkey (Rhinopithecus roxellana) group in the Qinling Mountains, China. We observed 88 births in 47 females from 2001 to 2006. Two methods were used to calculate the birthrate. The first method is based on the number of births observed in a year, giving 0.49+/-0.07 (mean+/-SD), and the second method is based on the female-years of observation, giving 0.49+/-0.17 births per female per year in this troop. The mean interbirth interval is 21.88+/-6.01 months (mean+/-SD). The mortality of infant born between 2002 and 2005 was 22.4%. The interbirth intervals of females that had lost an infant before the age of 6 months were significantly shorter than that of females whose infants survived for more than 6 months. A female usually gives birth once every 2 years if the previous offspring survives to a weaning age of 5-6 months, or will give birth in the next year if the previous young dies before reaching an age of 6 months. Births were significantly concentrated during March to May of each year. The mean birth date was on April 14, median was April 12; and the standard deviation was 13.98 days. Birth peak occurs 6-7 months after mating peak. From observations on 15 individuals that gave birth for the first time, we concluded that the wild female Golden snub-nosed monkeys in Qinling Mountains start giving birth at an age of 5 or 6 years. We suggest that the seasonal reproductive pattern is an adaptive response to the availability of seasonal food. Our results are consistent with the hypothesis that these reproductive characteristics are a result of adaptation to the seasonality of mountain climate and food resources.     

Reproductive attitudes of couples having a child with cystic fibrosis in Saguenay-Lac-Saint-Jean (Quebec, Canada)

Cystic fibrosis (CF) has a high incidence (1/936 live births) and carrier rate (1/15 inhabitants) in Saguenay-Lac-Saint-Jean. One objective of a major enquiry among several subsets of individuals from this high-risk population for CF was to evaluate the reproductive behaviour of couples with a CF child attending the comprehensive CF clinic in Chicoutimi. The knowledge of the recurrence risk resulted in deciding against further progeny or in reducing the number of children. More reliable contraception methods after the birth of the CF child, but not prenatal diagnosis, were used. Although a minority of parents with a CF child would abort a CF foetus, they apparently started viewing pregnancy interruption for CF after prenatal diagnosis as an acceptable reproductive option.     

Measuring the childbearing motivation of couples referred to the Shiraz Health Center for premarital examinations

Millions of children who were born during the first decade after the Islamic revolution in Iran are now reaching the age of marriage and childbearing. Short spacing between marriage and the birth of the first child has the potential to cause an excessive and costly increase in the growth of population in Iran. Research into the motivations for the birth of first child among newly married couples can create a knowledge base that will enable health centres to help these couples make better decisions about the timing of their first pregnancy. Using a consecutive sampling technique and administering Miller's Childbearing Questionnaire, data were gathered regarding the childbearing motivations and desires of 300 couples who had been referred to the Shiraz Health Center for premarital counselling. The Childbearing Questionnaire, with some minor modifications, was found to be a valid and reliable instrument for measuring the childbearing motivations of newly married couples of Shiraz County, Fars Province, Iran. The utility of these findings for counselling in health centres is discussed. Based on the results, a longitudinal study is being designed that will allow the development of models for predicting the time of first pregnancy after marriage.     

Pregnancy outcome is associated with maternal marital status in Austria--even at the beginning of the 21st century

Human reproduction is a biological phenomenon, however, sociocultural factors such as marital status influence pregnancy outcome and reproductive success. In the present study the impact of maternal marital status on pregnancy outcome was tested for all births, which had taken place in Austria between 1999 and 2004, which met the following criteria: single births, nulliparity, mothers older than 19 years (n = 179 830). The rate of preterm delivery (< 37 beginning weeks of gestation) and rate of low birth weight (< 2500 g) among term births were significantly higher among unmarried mothers in comparison to married mothers. Additionally the newborns of unmarried mothers were significantly lighter and shorter than those of married mothers. This was especially true of immigrant mothers. No significant differences between married and unmarried mothers were found regarding mode of delivery. Even at the beginning of the 21st century unmarried status represents an important stress factor for pregnant women.