Seasonality in the results of in-vitro fertilization.

Seasonal variation has been found in various reproductive outcomes. As known causes for reducing the rate of success of in-vitro fertilization (IVF) cannot explain all the variation in IVF results, we studied whether the season had any additional explanatory power. The study population consisted of 1126 women who were treated for the first time with IVF at the University Hospital in Nijmegen, The Netherlands, between 1987 and 1993. Only first IVF cycles were analysed. After adjusting for confounding by the age of the woman, type of infertility, indication for IVF and year of aspiration, some seasonal variation was observed in the fertilization rate, embryo quality, pregnancy rate and birth rate.