Couples undergoing in vitro fertilization (IVF) have many difficult decisions to make on the road to starting a family. Among them is the decision to use fresh or frozen embryos.

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. Most IVF clinics today take advantage of embryo freezing.

But whether frozen embryos contributed to the same pregnancy and live birth rates as fresh embryos wasn’t clear. In two papers published in the New England Journal of Medicine, an international group of researchers led by scientists in Vietnam and China report that pregnancy and live birth rates are similar among women who used fresh or frozen embryos.

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The two studies involved nearly 3,000 women undergoing IVF who were randomly assigned to receive either frozen or fresh embryos. In one study, led by researchers at a single clinic in Vietnam, 36% of the women implanted with frozen embryos became pregnant, while 34% of those implanted with fresh embryos did. Among the women in this study, 34% of those with frozen embryos had a live birth, compared to 31% of those with fresh embryos.

In the other study, led by researchers in China, women at multiple clinics were randomly assigned to receive fresh or frozen embryos. The live birth rate among those with frozen embryos was 49%, compared to 50% among those with fresh embryos.

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“The importance of these papers is that it documents what we know,” says Dr. Jamie Grifo, program director of the New York University Fertility Center, who was not involved with the studies. “The bottom line is that there probably isn’t a big difference between fresh and frozen embryos.”

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.

In addition, studies showed that some women who experience a condition called polycystic ovarian syndrome (PCOS), in which the ovaries develop fluid-filled follicles and fail to release eggs, increased their chances of having a live baby when they used frozen embryos. 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.

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.

Together, these factors prompted many clinics to advocate freezing of all embryos.

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The current studies show that the practice may not harm women’s chances of having a baby. But some experts point out that the findings could also justify use of fresh embryos as well. If the fresh and frozen embryos result in similar outcomes, then some families may opt for using fresh embryos, since storing frozen ones results in added costs.

“We shouldn’t be freezing embryos across the board [for all IVF cycles] if it makes no difference,” says Dr. Christos Coutifaris, president of the American Society for Reproductive Medicine and chief of reproductive endocrinology at University of Pennsylvania, who was also not involved in the study.

Coutifaris says that it’s clear that frozen embryos are more effective for some women undergoing IVF, specifically women who have PCOS or who may be prone to hyperstimulation when they are treated with hormones. But for other women, fresh embryos, according to the two studies, are just as likely to result in ongoing pregnancies and live births.

“Based on these papers, there is no harm in freezing embryos,” says Grifo. “And that’s reassuring for people who do a lot of frozen embryo transfer, which most of us are doing.”

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