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.

Seasonal variation in reproductive failures is expected, as many of the putative causes change over the seasons. Many studies have documented such seasonal variation in reproductive failures, but none has addressed the potential source of bias related to seasonal planning of pregnancies. 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 25-44 years in different parts of Europe. Data on pregnancy planning stem from personal interviews. Altogether, we analyzed 4,731 pregnancies. Results show that pregnancy planning is not evenly distributed over the seasons, with summer the preferred time for starting pregnancy. The most fecund will conceive within the preferred time, but those who are subfecund may not succeed until later. Since subfecund women have a higher risk of some reproductive failures (spontaneous abortions, for example), 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 have only a small impact under the present conditions in Europe.