According to a recent Yale-led study, the use of frozen embryos rather than fresh ones for in vitro fertilization may improve the chances of a successful pregnancy in women who suffer from polycystic ovary syndrome — a hormonal disorder that often leads to fertility problems.

The study, led by Yale professor of public health Heping Zhang and professor of public health sciences at Pennsylvania State University Richard Legro, was conducted in collaboration with 13 universities in China. The report was published in the New England Journal of Medicine on Aug. 11. Many women suffering from PCOS turn to in vitro fertilization, particularly when other treatments have failed to bring about a successful pregnancy. According to the U.S. Department of Health and Human Services, between one in 10 and one in 20 women of childbearing age in the U.S. has PCOS. In general, IVF success rates are quite high for cases of infertility caused by polycystic ovary syndrome. Traditionally, the use of fresh embryos is preferred over that of frozen embryos; however, Zhang and Legro’s study provides important evidence in support of the latter, particularly for women diagnosed with PCOS.

“Perhaps elective embryo freezing followed by frozen embryo transfer is a preferential treatment for women with polycystic ovary syndrome,” Legro explained as he described the researchers’ surprised reactions to the outcome.

PCOS causes women to have enlarged ovaries with cysts on the outer edges. In women with PCOS, the ovary does not make all of the hormones needed for an egg to fully mature, and ovulation often fails to occur altogether. Consequently, the infertility rate among women suffering from PCOS is high. According to the U.S. Department of Health and Human Services, PCOS is the most common cause of female infertility.

According to the paper, the researchers randomly assigned 1,508 infertile women with polycystic ovary syndrome in China who were undergoing their first IVF cycle to undergo either fresh-embryo transfer or frozen-embryo transfer. After three days of embryo development, the women underwent the transfer of up to two fresh or frozen embryos. The frozen-embryo transfer resulted in a higher frequency of live births after the first transfer than did the fresh-embryo transfer — 49.3 percent versus 42 percent. Women who underwent frozen-embryo transfer also had fewer incidences of pregnancy loss -— 22 percent versus 32.7 percent.

“This protocol potentially offers immediate benefits to women with PCOS, so practitioners should consider freezing all embryos for these patients,” Legro said.

According to the researchers, the success of frozen embryos can be explained by the fact that frozen embryos take longer than fresh embryos to implant in the uterus. This is important because it allows hormone levels in the uterus to return to normal before the embryo is implanted. During IVF, women are treated with hormones to increase their production of eggs. As such, estrogen levels are “10 times higher than normal,” according to Legro. This initial hormonal imbalance can prevent the successful implantation of fresh embryos, which take less time to implant than their frozen counterparts.

In addition to a greater pregnancy success rate, women given frozen embryos also had fewer instances of hyperstimulation syndrome — a condition that causes the ovaries to swell and become painful and may also lead to other pregnancy complications — than women given fresh embryos.

However, the findings showed that there was a slightly higher risk of potentially dangerous high blood pressure during pregnancy, as well as newborn death, in women who received frozen embryos.

Christos Coutifaris, chief of the division of reproductive endocrinology and infertility at the University of Pennsylvania School of Medicine and writer of an editorial that accompanied the study, questioned whether the difference in pregnancy rates between using frozen or fresh embryos is significant enough to recommend using frozen embryos. Coutifaris said that the delivery rate of women who got pregnant during the fresh IVF cycle was still over 40 percent, which is a “very good rate.” However, Coutifaris conceded that in some cases, especially in women who “over-stimulate,” the approach to “freeze all the embryos is prudent.”

While the study only focused on women suffering from PCOS, it paves the way for future studies into the potential use of frozen embryos to treat unexplained infertility problems, according to the study authors.

According to the Mayo Clinic, the symptoms of PCOS include infrequent or prolonged menstrual periods, excess hair growth, acne and obesity. In adolescents, infrequent or absent menstruation may be indicators for the condition.