Seasons in the sun: the impact on IVF results one month later

Background: Several retrospective studies have evaluated seasonal variations in the outcome of IVF treatment. Some also included weather conditions, mostly temperature and hours of daylight. The results were conflicting. Methods: In a retrospective study we analysed all fresh cycles (N = 9865) that were started between January 1, 2007 and December 31, 2012. Because some patients were included more than once, correction for duplicate patients was performed. We focused on individual variables provided as monthly results by our national meteorological institute. We evaluated if weather conditions determined by temperature, rain and sunshine at the start of ovarian stimulation had an effect on the outcome of IVF in terms of number of mature and fertilized oocytes, pregnancy and live birth rates. We shifted the results in IVF outcome to the weather results of one month earlier, as we supposed that the selection of good quality oocytes might start in the weeks before ovarian stimulation is initiated. Results: There was a clear trend towards better results when the "early" weather conditions (one month before the treatment cycle) were good. There was a statistically significant correlation between the number of rainy days (Pearson Correlation -0.326; p < 0.01) and the rain flow (Pearson Correlation -0.262; p < 0.05) on the one hand and the live birth rate per cycle on the other. The live birth rate per cycle was statistically different between cohorts of patients that were stratified into quartiles of sunshine hours (p < 0.01) and of number of rainy days (p < 0.05) during the month before the start of ovarian stimulation. Conclusions: Weather conditions during the month before IVF treatment have an impact on live birth rate.