Best Season for IVF: Spring Temps, daylight hours may affect fertility rates By Liam Carnahan, Newser Staff Posted Jul 17, 2012 12:03 PM CDT 2 comments Comments Researchers find spring is the most successful season for IVF (©Nina Matthews Photography) (Newser) – Infertile couples considering in vitro fertilization might do well to develop a little spring fever, suggests a new study out of Brazil. It found that's the season in which fertilization rates are at their highest, reports the Wall Street Journal. Researchers reviewed the fertilization rates of 1,932 women who underwent a procedure in which a single sperm is injected into an egg cell outside the body; they divided the women into groups depending when the unfertilized egg was retrieved. In spring, fertilization rates (meaning when the sperm and egg successfully formed an embryo) hit 73.5%, far above the rates for fall (69%), summer (68.7%), and winter (67.9%). Researchers speculate that temperature and hours of daylight may be a factor in the varied rates. But one good stat doesn't create another: No matter the season in which the resulting embryo was implanted into the uterus, the average pregnancy rate was the same: 33%.