Michael Chapman, Professor of Obstetrics and Gynaecology at UNSW and President of the Fertility Society of Australia - who was not involved in the study - said the slight percentage difference in groups is not significant.
In-Depth [randomized controlled trial]: This multicenter, randomized controlled trial enrolled 2157 ovulatory women with infertility to undergo either frozen-embryo or fresh-embryo transfer in a 1:1 block randomization pattern.
In the first study, of 2,157 women without PCOS, 50.2 percent of fresh embryo transfers resulted in a live birth, and so did 48.7 percent of the frozen transfers.
"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", Christos Coutifaris, a doctor at the University of Pennsylvania's Perelman School of Medicine in Philadelphia, told Reuters.
Before freezing methods were refined, fresh embryos were the only option, but as cryopreservation techniques improved, freezing embryos so that not all of the healthiest ones had to be transferred to the woman immediately became more popular.
In women without polycystic ovaries, the pregnancy rates and live births were comparable when implanted with either fresh or frozen embryos, according to the study published in The New England Journal of Medicine on Thursday. 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.
For their study, Chen and colleagues enrolled women at 20 clinical sites across China between March 2015 and November 2015.
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Vuong and her colleagues also discovered that women with high levels of progesterone might be better off getting implanted with a thawed frozen embryo. Multiple embryos result in multiple births, but also come with many complications.
The rates of the syndrome in the Chinese study were 0.6 percent with frozen embryos and 2.0 percent with fresh.
"It's not an easy process to go through... the probability of falling pregnant was quite low back then through IVF, for me certainly", Monroe, 51, said.
Zhang said that the findings will immediately impact patients and clinicians in their evaluation and consideration with regard to the risk and benefit of different embryo transfer strategies.
After the first completed cycle of IVF, ongoing pregnancy occurred in 36% of women in the frozen embryo group, and in 35% of the fresh embryo group. This could challenge the long-held belief of fertility specialists that freezing and thawing embryos before implanting them offers a better chance of successful pregnancy and birth.
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.