Human Reproduction, Vol. 19, No. 5, 1081-1082,
May 2004
© 2004 European Society of Human Reproduction and Embryology
Short Communications |
Brief communication: Birth month influences reproductive performance in contemporary women
1 Research Institute of Wildlife Ecology, University of Veterinary Medicine, Vienna, 2 Department of Zoology, 3 Rector’s Office, 4 Department of Anthropology, and 5 Office of Evaluation and Controlling, University of Vienna and 6 Department of Internal Medicine IV, Medical University of Vienna, AKH, Austria
7 To whom correspondence should be addressed. e-mail: susanne.huber{at}vu-wien.ac.at
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Abstract |
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 Top Abstract IntroductionMaterials and methodsResultsDiscussionReferences |
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BACKGROUND: Season of birth has been reported to affect later reproduction in samples of pre-modern women and contemporary men. METHODS: To examine whether the effect of birth date is also valid in contemporary women, we investigated the association between birth month and measures of reproductive performance (number of live-born children, % childless individuals) in a representative sample of contemporary Austrian women. RESULTS: Among reproducing women, birth month is significantly associated with the number of live-born children (n = 2839, P = 0.032). On average, women born in summer months have fewer children than women born during the remainder of the year. No association between birth month and the percentage of childless individuals was found. CONCLUSIONS: As has been reported in pre-modern women, month of birth also appears to affect later reproduction in contemporary women.
Key words: birth date/female/human/offspring count/reproduction
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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There is evidence that season of birth may impact reproductive performance in pre-modern women and contemporary men. Smits et al. (1997
, 2001) reported a relationship between fecundity and birth month in a historic Dutch population. Lummaa and Tremblay (2003) showed that month of birth predicted reproductive success and fitness in pre-modern Canadian women. Furthermore, Huber et al. (2004) found an effect of birth season on offspring count in a sample of contemporary Austrian men but not women. The latter lack of a detectable association between birth season and reproductive performance in women may reflect the weakness of the female data set which, in contrast to the male data set, has been weighted towards non-reproductive subjects (Huber et al., 2004). However, it may also reflect effects of modern life and contraception use, or of sex differences in the responsiveness to the season of birth. Therefore the present study is aimed at examining whether the effect of birth date is also valid in contemporary women, this time using a representative sample of Austrian women. |
Materials and methods |
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We used the dataset ‘Microcensus Austria 2001 on family survey’ (Statistik Austria, 2001) to analyse differences in reproductive output respective to month of birth. ‘Microcensus Austria’ is a quarterly sample survey carried out in 1% of Austrian households since 1967 by Statistik Austria. The database consists of a representative sample of the current Austrian population. Data are purchasable by the public. In 2001, a special survey of families was performed among adult female members of the households examined during microcensus analysis (n
25 000 households) to obtain data about their natural children and stepchildren. We used the number of live-born children in our analyses. As details about birth date are given on a monthly basis only, data were analysed with regard to birth month. To analyse birth month effects on measures of reproductive performance (number of live-born children, percentage of childless individuals), we used the sample of Austrian women born after 1945 and aged >45 years (n = 3227). We only included the data of women born after 1945 (end of World War II) in our analyses, as birth during World War II might have had long-term effects on subsequent reproductive performance. In addition, we only used the data from women aged >45 years to obtain data about lifetime reproductive success, because >99% of women have finished reproduction by the age of 45 years.
We conducted statistical analyses using SPSS 10.0 for Windows statistical package. We performed comparisons among women born during different months, using Kruskal–Wallis H-test and post-hoc Mann–Whitney U-test (Bonferroni-corrected), as well as 
2-test.
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Results |
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In Austrian women born after 1945 and aged >45 years, no association between birth month and the percentage of childless individuals is found (Table I). Among the reproducing individuals in this sample, however, birth month is significantly associated with offspring count (Figure 1). On average, women born in summer months (June–August) have fewer live-born children than women born during the remainder of the year (Figure 1). The lowest and highest average values differ by 0.3 children, representing 13.4% of the total mean (mean = 2.24 live-born children/reproducing woman, n = 2839).
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Discussion |
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We show that birth month is related to the number of live-born children in a contemporary sample of Austrian women. Average offspring count is lower among reproducing women born in summer (June–August) than those born during any other month. In pre-modern Canadian women, Lummaa and Tremblay (2003
) found that women born in mid to late summer (July, August), early autumn (September, October), or April had below average numbers of live-born children. And in a historic Dutch population, Smits et al. (1997) reported that the portion of subfecunds with childless couples excluded, was highest among those women born in mid and late summer (July, August) or early autumn (September). Thus, our results partially confirm these findings of Smits et al. (1997) and Lummaa and Tremblay (2003), indicating that despite the potential influence of modern life and the use of contraception, birth month effects on later reproductive performance are at least to some degree similar in pre-modern and contemporary women. Whereas in contemporary Austrian men, Huber et al. (2004) found that the average offspring count was highest among males born in spring and lowest among those born in autumn, with intermediate values found in men born in summer and winter, indicating that males and females might respond differently to the season of birth.
An association between birth month and later reproductive performance may have many possible causes. One cause could involve effects of the conditions experienced during fetal and neonatal life on early pre- and postnatal developmental processes. In temperate zones, the external environment varies with the seasons. The conditions experienced early in life are therefore the result of a variety of seasonally and socially varying environmental and maternal factors (e.g. photoperiod, climatic factors, infections, nutrition) that may all affect early development, causing potential downstream effects on later life events. Another, equally likely, cause could be genetic. Basso et al. (1995) reported that in Denmark, women prefer to give birth in spring. Only those with high fecundity succeed, whereas females with lower fecundity and thus a longer waiting time to pregnancy may not succeed until later, giving birth during the summer or autumn. If a genetic cause of lower fecundity is inherited by their daughters, these may end up with lower fecundity, which might explain the observed pattern of lower reproductive performance in summer-born women. Finally, other yet unknown factors might also play a role.
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Acknowledgements |
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We thank the anonymous reviewers for their valuable comments on the manuscript.
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References |
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Basso O, Olsen J, Bisanti L, Juul S and Boldsen J (1995) Are seasonal preferences in pregnancy planning a source of bias in studies of seasonal variation in reproductive outcomes? The European Study Group on Infertility and Subfecundity. Epidemiology 6,520–524.[ISI][Medline]
Huber S, Fieder M, Wallner B, Iber K and Moser G (2004) Season of birth effects on reproduction in contemporary humans. Hum Reprod 19,445–447.
Lummaa V and Tremblay M (2003) Month of birth predicted reproductive success and fitness in pre-modern Canadian women. Proc R Soc Lond B 270,2355–2361.[Medline]
Smits LJ, Van Poppel FWA, Verduin JA, Jongbloet PH, Straatman H and Zielhuis GA (1997) Is fecundability associated with month of birth? An analysis of 19th and early 20th century family reconstitution data from The Netherlands. Hum Reprod 12,2572–2578.
Smits LJ, Jongbloet PH and Zielhuis GA (2001) Season of birth and reproductive performance: an analysis of family reconstitutions of 800 women born in the Netherlands at the end of the 19th century. Chronobiol Int 18,525–539.[Medline]
Statistik Austria (2001) Microcensus, third quarter.
Submitted on December 2, 2003; accepted on March 8, 2004.
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