A lower risk of ovarian hyperstimulation syndrome among women in the frozen-embryo group was detected (0.6% frozen-embryo vs 2.0% fresh-embryo; RR, 0.32, 95% CI; 0.14 to 0.74; p = 0.0005). The improvement came primarily from a lower rate of pregnancy loss.
However, many women going in for in vitro fertilization (IVF) are rather wary about the success of frozen embryos in the process as compared to the transfer of fresh embryos.
"Now these two papers, equally large and done in non-PCOS patients, show that in terms of live birth, which is what we care about, there is no difference", he told Reuters Health by phone. The researchers observed no significant difference between the groups in the live-birth rate (48.7 versus 50.2 percent for the frozen-embryo and fresh-embryo groups, respectively; relative risk, 0.97 [95 percent confidence interval, 0.89 to 1.06; P = 0.5]).
Rates of ongoing pregnancy occurred in 36 per cent of the frozen-embryo group and 34.5 per cent in the fresh-embryo group, according to the study.
Freezing embryos also allows couples to take advantage of genetic testing, which can pick up potentially lethal genetic diseases and guide doctors in deciding which embryos to implant. Coutifaris was not involved with the study.
"Even with 800 odd patients in the study".
In a 12-month post-hoc analysis, the median time to pregnancy was slightly higher in the frozen-embryo group compared with the fresh-embryo group (3.6 months versus 2.2 months), but the cumulative ongoing pregnancy rate 1 year after randomization was similar (54.2% versus 53.5%).
The researchers noted that their results are specific to a common freezing method known as Cryotech vitrification, so it may not apply to all embryo freezing techniques now being used.
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.
"There are many examples in medical history where people jump to innovation and new things, and then after a while it turns out that the new thing is not necessarily so much better", Mol said.
FET is a cycle where a frozen embryo from a previous fresh IVF cycle is thawed and transferred back into a woman's uterus.
But until now, it was not known whether this was also the case for women confronted with fertility problems due to other reasons.
"The cost for freezing embryos is about 30 percent more than that for fresh transfer", said Dr. Vuong.
"It's not a lot, but it's a couple of hundred of dollars that you're talking about, so obviously that could be part of the choice [for patients]", he said. "Some programs around the country won't do fresh transfers anymore", said Dr. Christos Coutifaris of the University of Pennsylvania's Perelman School of Medicine in Philadelphia, who was not connected with the new research.
"They only looked at day-three embryos, whereas the general practice in Australia today is day-five transfers", he said. Prior studies of women with PCOS have suggested that frozen embryo transfer is more successful.