Dr. Christos Coutifaris of the University of Pennsylvania's Perelman School of Medicine in Philadelphia said that after a previous study by Chinese researchers stating that frozen embryos were better for women with PCOS, "a lot of people jumped to the conclusion that we should always do frozen".
In the second, of 782 women without PCOS, live birthrates were 33.8 percent for the frozen-embryo group and 31.5 percent for the fresh, again an insignificant difference.
Frozen embryos result in just as many live births as fresh embryos for in vitro fertilization.
The study, published in The New England Journal of Medicine, investigated nearly 800 women who had infertility not related to polycystic ovarian syndrome (PCOS).
For the study, women were given one cycle of IVF where either frozen or fresh embryos were implanted.
There were no significant differences in the live-birth rate between the frozen-embryo group and the fresh-embryo group (48.7% and 50.2%, respectively; relative risk, 0.97; 95% confidence interval, 0.89 to 1.06; p = 0.50).
Mol warns that although many clinics are moving away from using fresh embryo transfers for IVF, the freezing process does make IVF more expensive, despite not resulting in higher rates of live births: "Couples concerned about such unnecessary costs of freezing all embryos do not need to go down that path, and will still have the same live birth success rate".
But whether frozen embryos contributed to the same pregnancy and live birth rates as fresh embryos wasn't clear. In those cases, fertility experts believe that freezing the embryos to give the woman time between when the eggs are extracted and fertilized, after she has received hormones to produce more eggs than her normal cycle would, may provide a more receptive uterine environment for an embryo to grow.
Chen and colleagues noted, however, that "caution is warranted in the interpretation of this finding because the analysis was post-hoc and the overall rates of pregnancy loss did not differ significantly between the two groups".
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.
"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". Vitrification does not harm the embryos and they can be stored indefinitely.
The researchers also found that using frozen embryos resulted in a lower risk of the woman developing ovarian hyperstimulation syndrome, a condition that sometimes affects women undergoing fertility treatment.
But the study suggests it may not increase the chances of a live birth compared to fresh embryos in the study population.
The study by Vuong et al was supported by My Duc Hospital in Ho Chi Minh City, Vietnam.