"Previous research has shown that women who experience infertility because of PCOS benefit from significantly higher live birth rates from frozen embryos in IVF procedures, but evidence was lacking for this approach in non-PCOS patients", said Professor Ben Mol, from the University of Adelaide. The findings may encourage doctors to implant only one embryo at a time, reducing the risks associated with implanting more.
But it wasn't clear whether fresh and frozen embryos offered better chances of success in women who ovulate normally, the study authors explained.
"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 study, however, did find that frozen-embryo transfers resulted in a significantly lower risk of ovarian hyperstimulation syndrome (OHSS) than women using fresh-embryo transfers.
For most women undergoing in vitro fertilization, fresh embryos work just as well as frozen ones.
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.
In the other study, led by researchers in China, women at multiple clinics were randomly assigned to receive fresh or frozen embryos.
In a Vietnamese study, rates of live births after the first transfer were comparable between groups: 33.8% with frozen embryos and 31.5% with fresh embryos, according to Lan N. Vuong, MD, PhD, of the University of Medicine and Pharmacy at Ho Chi Minh City, and colleagues. 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. Doctors typically implanted two embryos per attempt.
They also acknowledged several limitations, such as the limited generalizability due to the study's single center nature, and using only the Cryotech vitrification method.
The rates of the syndrome in the Chinese study were 0.6 percent with frozen embryos and 2.0 percent with fresh.
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. Secondary outcomes included rates of pregnancy complications (ovarian hyperstimulation syndrome, ectopic pregnancy, congenital anomaly), birth weight, rates of pregnancy loss, and comparisons of biochemical pregnancy and clinical pregnancy characteristics.
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%). "And that's reassuring for people who do a lot of frozen embryo transfer, which most of us are doing".