The shift to frozen embryos was triggered by a landmark study in the field published in 2012, which found that frozen embryos actually were more likely to successfully implant in the uterus when transferred than fresh embryos - and theoretically more likely to result in a full-term pregnancy and live birth.
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.
Previous research conducted on women undergoing IVF with infertility problems linked to polycystic ovary syndrome (PCOS), found frozen-embryo transfer led to more live births.
Vuong added that the results of the current study are specific to a common freezing method called Cryotech vitrification, so they may not apply to all current embryo freezing techniques: "Further research will be needed to compare pregnancy outcomes and live birth rates from other embryo freezing techniques".
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".
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. There were no differences in pregnancy rate or live birth rate, nor pregnancy complications such as high blood pressure.
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. 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.
"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. Each woman received up to two cleavage-stage embryos.
"Given the differences in the uterine condition and embryo characteristics between cleavage-stage embryo transfer and blastocyst transfer, our results may not be applicable to cycles with blastocyst transfer", they wrote.
Live births were recorded in 33.8 per cent of the frozen-embryo group and 31.5 per cent of women in the fresh-embryo group. They also implanted, on average, two at a time.
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 study by Chen et al was supported by grants from the National Key Research and Development Program of China, the Major Program of the National Natural Science Foundation of China, and the State Key Program of the National Natural Science Foundation of China.