This blog is part of Fertility FAQ, a series where RMA doctors answer your most pressing questions about treatment. 

Today’s physician expert is Dr. Allison Lange, who practices out of RMA Philadelphia.

Single Embryo Transfer (SET) is, hands down, the fertility industry’s biggest achievement since the advent of In Vitro Fertilization (IVF) over 40 years ago.

But what is it, and how does it work?

How does Single Embryo Transfer work?

SET is exactly what it sounds like – it is the transfer of a single embryo (not two or more) into a woman’s uterus during IVF.

You might think it’s obvious that doctors would only transfer one embryo, but actually, the ability to do this and still achieve high pregnancy rates is revolutionary.

Until recently, it was commonplace for clinics around the world to transfer two or more embryos into a woman’s uterus at one time, hoping at least one would implant and lead to an ongoing pregnancy.

Care teams had imprecise means of determining which embryos were viable and transferred multiple embryos to improve the chances of finding the right one.

Embryo freezing technology was also not as reliable as it is today and embryos were just not as successful after being cryopreserved and thawed compared to if they were transferred immediately.

Issues with transferring multiple embryos

The result of transferring multiple embryos, however, was not always the hoped-for singleton pregnancy. Often, multiple embryos would stick, and the patient would have an ongoing twin or triplet pregnancy.

Why was this a problem?

Because multiple gestation pregnancies are risky for both mom and babies; they are more likely to result in preterm birth, lower birth weight babies, extended NICU stays for babies, as well as multiple health and social complications for moms and families.

To improve these outcomes, fertility researchers, scientists, and doctors (many at RMA) worked to find a way to reliably test embryos for their chromosomal makeup so they could identify whether an embryo was genetically competent prior to transfer.

If this could be done, the thinking went, we could have more confidence in each embryo we transfer.

Genetic testing of embryos

It could be done – this technology is now known as Preimplantation Genetic Testing for Aneuploidy (PGT-A).

In 2013, RMA researchers conducted an important trial to test whether the transfer of one PGT normal embryo did indeed lead to the same pregnancy rates as the transfer of two untested embryos.

They found that ongoing pregnancy rates were equivalent and single embryo transfer dramatically reduced the risk of multiple gestations.

Follow-up studies additionally demonstrated safer pregnancy outcomes in those patients who had a single embryo transferred versus two.

Technological advancements in fertility

Additional technological developments within embryology labs have allowed embryos to develop more reliably, have improved genetic competency, and survive cryopreservation with precision.

This has allowed women who elect not to perform genetic screening to also safely transfer a single embryo with improved confidence and not sacrifice the potential of their additional embryos.

It has now become the standard of care to offer single embryo transfer in most circumstances.

Women and children are both benefitting from the ability to achieve healthier pregnancies and outcomes while preserving a high chance of achieving the dream of a healthy birth.