Perinatal winter conditions affect later reproductive performance in Romanian women: intra and intergenerational effects.

OBJECTIVES: To compare epidemiological studies on Austrian and New Zealand women that report below average reproductive performance after birth in summer, with a similar study in Romanian women.
METHODS: We examined the association between birth month and offspring count analyzing census data from Romania obtained from IPUMS International (Population and Housing Census of Romania, year 2002), totaling 411,270 women aged older than 45 years. We investigated whether socio-economic status affected this association, and whether intergenerational effects exist for mother's birth month in relation to their daughter's offspring number.
RESULTS: The time series of mean offspring count per birth month has a highly significant period of 12 months. Contrary to our expectations, average offspring count is highest in women born in June and lowest in those born in December, with a strong coincidence between negative peak values of residuals of mean offspring count per birth month and corresponding monthly means of ambient temperature in Romania. The association between winter birth and lower offspring number is significant in poorly educated women only. For mothers born in winter there is also an association with a lower daughters' offspring count.
CONCLUSIONS: Conflicting results exist between Romania and Austria/New Zealand, and may be most easily explained by women's different exposure to winter conditions in these countries.