Oct. 31, 2000 -- In nature, some animals tend to give birth in the spring. Do humans have similar seasonal variations in fertility and birthing? An Israeli study shows that fertilization rates do vary by the seasons of the year in some women undergoing in vitro fertilization.
"This observation suggests the existence of hereditary factors in seasonally breeding primates," the authors write in a recent issue of the journal Fertility and Sterility.
In vitro fertilization is a process by which a woman's eggs are removed from her body and then fertilized with sperm in a laboratory. The sperm has also been harvested previously. In vitro fertilization, also called IVF, is a commonly used method of helping couples with fertility problems to become pregnant.
Spring is the most opportune time for fertilization and also the time when embryos that are conceived through this method are at their highest quality, according to study author Nathan Rojansky, MD, and colleagues. The average fertilization rate overall was just over 70%, and the spring fertilization rate was 75%.
Do the findings on seasonal variability mean that a woman undergoing IVF should pin her hopes on a spring harvesting of her eggs? Should she expect that this strategy would enhance her chances of becoming pregnant?
Not necessarily -- the conception and pregnancy rates did not show the same seasonal variability. Among the women in the study, about one in four became pregnant. "The seasonal variability in conception rates, which showed a trough in the spring and rose gradually to an autumn acme, did not reach statistical significance," the authors write. "No correlation between pregnancy rate and any of the seasonal parameters was found."
This discrepancy may mean that these seasonal fluctuations in fertilization and quality of embryos are interesting but have limited ability to achieve the ultimate goal of producing a child, say several experts.
"The study presents a viewpoint that has long been looked at in reproductive medicine. It doesn't really offer any new information," David Diaz, MD, tells WebMD. "What is applicable in one part of the country and one part of the world cannot be generalized to other parts of the world. It's very difficult to isolate season with regards to fertility." Diaz, who was not involved in the current research, is the medical director of reproductive medicine and fertility at Fountain Valley Regional Hospital and Medical Center in Fountain Valley, Calif.
In addition, this study was done before the use of medications that control pituitary gland output, and therefore ovulation, says Carolyn Kaplan, MD. "We have much better control now over egg development and hormones reaching the ovaries," she tells WebMD. "I think there probably is some residual impact of the seasons on our reproduction, but it's more important for women who are not using fertility drugs. When they do take fertility drugs, we can control those variables much more effectively." Kaplan is an assistant clinical professor of obstetrics-gynecology at Emory University in Atlanta, where she is the director of in vitro fertilization at Georgia Reproductive Specialists.
"This study seems to have manipulated the statistics to get significance," Sam Najmabadi, MD, tells WebMD, "Embryo quality, which was emphasized by the authors, is a very qualitative assessment. Our endpoint is pregnancy, and there was no difference in pregnancy rates between the seasons." Najmabadi, who was not involved in the current research, is the director of the Center for Reproductive Health and Gynecology in Los Angeles.