Follicle Size Matters: Hormone regimen may reduce pregnancy success

The hormone injections used to induce livestock and women to ovulate may force some eggs to leave ovarian follicles too early to begin and maintain a successful pregnancy, a new study reports. This finding, made in beef cattle, may explain why women who receive this type of fertility treatment have lower pregnancy and birth rates than do women who ovulate on their own.

For convenience, livestock breeders like to artificially inseminate all their cows on the same day. To do this, they frequently provide a standard series of hormone shots starting 9 days before insemination that induce entire herds to grow follicles and release eggs simultaneously.

Although the treatment is identical, each cow’s follicles may grow at a different rate and thus end up being different sizes at ovulation. George A. Perry of the University of Missouri in Columbia and his colleagues wondered whether this variation in follicle size affects whether cows can establish or maintain pregnancies.

Perry’s team started by studying a herd in which all the cows were artificially induced to ovulate. The researchers tracked hormone concentrations in the animals’ blood from the first injection until insemination and did daily ultrasound scans to measure ovarian follicles.

They found that cows with follicles of 11 millimeters or smaller across at ovulation were significantly less likely to become pregnant than cows with larger follicles were. All cows that lost pregnancies, about 13 percent of the herd, had had the smaller follicles at ovulation. Smaller follicles also correlated with lower blood concentrations of progesterone, a hormone necessary for pregnancy.

In a second experiment, Perry and his colleagues followed two sets of cattle: one induced to ovulate and another permitted to ovulate naturally. Among cows induced to ovulate, Perry’s team again found a correlation between small follicle size at ovulation and pregnancy loss. However, the researchers found no correlation between follicle size and capacity to become and stay pregnant among the cows that ovulated naturally.

Results from both experiments will be published in an upcoming Proceedings of the National Academy of Sciences.

Perry speculates that maturity may be the critical difference between eggs released naturally and those spurred on by hormone injection. “It’s possible that some of these eggs [from artificially induced ovulation] are released before they’re mature enough for fertilization,” he says.

Although methods to stimulate ovulation in women are slightly different from those used in cows, Perry points out that many fertility clinics follow a standard protocol that disregards differences in follicle growth.

“This study is very exciting because it shows some of the downside to this technique” of induced ovulation, says Richard Pursley, who studies cattle reproduction at Michigan State University in East Lansing. Personalizing the protocol in fertility clinics might lead to better pregnancy rates and embryo development, he says.