ARE SEASONAL PREFERENCES IN PREGNANCY PLANNING A SOURCE OF BIAS IN STUDIES OF SEASONAL-VARIATION IN REPRODUCTIVE OUTCOMES

Seasonal variation in reproductive failures is expected, as many of th e putative causes change over the seasons. Many studies have documente d such seasonal variation in reproductive failures, but none has addre ssed the potential source of bias related to seasonal planning of preg nancies. Our aim was to quantify this bias under realistic assumptions . We used data from the European Study of Infertility and Subfecundity , which is a study based upon representative samples of women age 25-4 4 years in different parts of Europe. Data on pregnancy planning stem from personal interviews. Altogether, we analyzed 4,731 pregnancies. R esults show that pregnancy planning is not evenly distributed over the seasons, with summer the preferred time for starting pregnancy. The m ost fecund will conceive within the preferred time, but those who are subfecund may not succeed until later. Since subfecund women have a hi gher risk of some reproductive failures (spontaneous abortions, for ex ample), the seasonal planning differences could in themselves lead to seasonal variations in reproductive failures. A simulation model shows that bias related to differential pregnancy planning is likely to hav e only a small impact under the present conditions in Europe.