More than 2,000 women who were undergoing their first IVF cycle were randomly assigned to undergo either a frozen-embryo transfer or an un-frozen embryo transfer, by researchers at the Yale School of Public Health (YSPH) and scientists from China.
In the Vietnam study of 782 women undergoing their first or second attempt, the live birth rates after the first transfer were 33.8 percent with frozen and 31.5 percent for fresh.
"Among infertile women without the polycystic ovary syndrome who were undergoing IVF, the transfer of frozen embryos did not result in significantly higher rates of ongoing pregnancy or live birth than the transfer of fresh embryos", one of the studies concluded.
Two studies recently published in the prestigious New England Journal of Medicine attempted to answer that question.
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After an earlier study by the Chinese team showed that frozen embryos were better for women with PCOS, "a lot of people jumped to the conclusion that we should always do frozen".
There is a recent trend in the in vitro fertilization (IVF) community to freeze all of a prospective mother's embryos after an IVF cycle, then do an elective frozen embryo transfer. Overall, there were no significant differences noted in live birth-rates, obstetrical or neonatal complications, or mean birth weight between the two groups. The pregnancy rates, in the past, were much lower with frozen embryo transfer because the freezing technique (slow-freezing) made the embryos susceptible to damage from internal ice crystals.
While women who have polycystic ovary syndrome (PCOS) have a better chance of pregnancy and live birth in using frozen then thawed embryos for implantation, women without PCOS have nearly equally effective results when using either frozen or fresh embryos, Reuters reports.
"It's good that [clinicians] will be able to advise patients that frozen-embryo transfer is as good as fresh, because historically frozen has not been as good".
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But the study suggests it may not increase the chances of a live birth compared to fresh embryos in the study population.
Neither study found a higher risk of neonatal or obstetrical complications in either group, although frozen embryo transfer produced a statistically lower risk of over-stimulated ovaries, which leads to swollen and painful ovaries and is potentially risky.
The current studies show that the practice may not harm women's chances of having a baby. Each woman received up to two cleavage-stage embryos.
"We shouldn't be freezing embryos across the board [for all IVF cycles] if it makes no difference", says Dr. Christos Coutifaris, president of the American Society for Reproductive Medicine and chief of reproductive endocrinology at University of Pennsylvania, who was also not involved in the study. After the first fresh embryo transfer, it will be possible to freeze the remaining embryos and transfer them one by one, which is safe and effective.
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