Single vs Double Embryo Transfer: Why One Is Often Smarter

Single vs Double Embryo Transfer: Why One Embryo Is Often the Smarter Choice

When a couple has more than one embryo ready for transfer, a natural question follows: why not transfer two and improve the chances of pregnancy? It seems like simple logic—two embryos should mean two opportunities to conceive.

While understandable, the reality is more complex. Transferring two embryos only increases the chance of pregnancy slightly in a single IVF cycle, but it dramatically increases the likelihood of twins. Twin pregnancies are not simply "two babies at once." They represent the single biggest avoidable complication associated with IVF, carrying significantly greater risks for both the mother and the babies.

For most couples, transferring one embryo at a time while freezing the remaining embryos provides a similar overall chance of taking home a healthy baby, but with far fewer risks. Here's why.

What Do Single and Double Embryo Transfer Mean?

The terms are straightforward.

• Single Embryo Transfer (SET): One embryo is transferred into the uterus.
• Double Embryo Transfer (DET): Two embryos are transferred during the same IVF cycle.

The most important concept is Elective Single Embryo Transfer (eSET). This means a couple has two or more good-quality embryos available but intentionally chooses to transfer only one while freezing the others for future use.

The decision isn't forced by having only one embryo. It's a planned strategy designed to maximise safety without significantly reducing the chance of success.

Why Two Embryos Don't Double Your Chances of Having a Baby

Many people naturally assume that transferring two embryos should double their chances of pregnancy. Unfortunately, biology doesn't work that way.

Each embryo implants independently. Having two embryos in the uterus does increase the probability that at least one will implant, but the improvement is relatively modest.

Studies have shown that transferring two good-quality blastocysts increases the live birth rate only slightly compared with transferring one. However, what increases dramatically is the likelihood that both embryos implant, resulting in twins.

Another important consideration is that when two embryos are transferred but only one continues developing, the remaining pregnancy may still carry a higher risk of miscarriage and pregnancy complications than if only one embryo had been transferred in the first place.

The Biggest Risk of Double Embryo Transfer: Twins

Twins are often viewed as a welcome bonus by patients, but from a medical perspective, a twin pregnancy is considered high risk.

With double embryo transfer, twin pregnancy rates exceed 30%. In comparison, single embryo transfer results in twins only around 1–2% of the time, similar to natural conception.

Twin pregnancies significantly increase the risk of:

• Premature birth.
• Low birth weight.
• Admission to the Neonatal Intensive Care Unit (NICU).
• Pregnancy loss.
• Newborn complications related to prematurity.

For mothers, twin pregnancies also increase the likelihood of:

• Pre-eclampsia.
• Gestational diabetes.
• Pregnancy-related bleeding.
• Caesarean delivery.

Large national studies consistently show that babies born after single embryo transfer are generally heavier at birth, less likely to be premature, and have lower rates of newborn complications than babies from double embryo transfer.

These safety benefits are one of the main reasons fertility specialists now recommend transferring a single embryo whenever appropriate.

Why One Embryo at a Time Often Gives the Best Overall Outcome

The most meaningful comparison isn't one embryo transfer versus another. It's the overall chance of having a baby from one complete IVF egg collection.

Modern IVF allows embryos to be cultured to the blastocyst stage before selecting the strongest embryo for transfer. Remaining healthy embryos are safely frozen for future Frozen Embryo Transfer (FET) cycles.

If the first transfer is unsuccessful, another frozen embryo can be transferred later, with frozen embryo transfers now achieving success rates similar to fresh transfers.

When all transfers from one egg retrieval cycle are considered together, elective single embryo transfer provides a cumulative live birth rate comparable to, and in some studies even higher than, double embryo transfer—while avoiding most twin pregnancies.

Importantly, choosing one embryo doesn't waste the others. Every suitable embryo still has an opportunity to be transferred—just one at a time.

Why PGT-A Makes Single Embryo Transfer Even More Effective

Preimplantation Genetic Testing for Aneuploidy (PGT-A) further strengthens the case for transferring a single embryo.

PGT-A identifies embryos with the correct number of chromosomes, helping fertility specialists select those with the highest likelihood of implantation and healthy pregnancy.

A single chromosomally normal (euploid) blastocyst already has an excellent chance of resulting in a live birth.

Research has shown that even after a failed transfer of one euploid embryo, transferring two screened embryos during the next cycle does not significantly improve live birth rates. Instead, it simply increases the likelihood of twin pregnancy.

When embryo quality has already been optimised through genetic screening, adding a second embryo usually offers little additional benefit.

When Might Double Embryo Transfer Still Be Considered?

Although single embryo transfer is recommended for most patients, there are situations where transferring two embryos may still be discussed.

• Women of advanced maternal age using unscreened embryos.
• Couples with several unsuccessful embryo transfers.
• Cases where available embryos are considered lower quality and each individual embryo has a relatively low chance of implantation.

Even in these situations, the potential benefits must be carefully balanced against the increased risks associated with twin pregnancy.

Research also suggests that for women aged 35 years and older, transferring two blastocysts often provides little or no meaningful improvement in live birth rates while still increasing the chance of twins.

Interestingly, some studies indicate that transferring one excellent-quality embryo may produce better outcomes than transferring one excellent embryo together with a poorer-quality embryo.

For this reason, the decision should always be individualised based on age, embryo quality, previous IVF history, and whether embryos have undergone PGT-A screening.

Conclusion

Choosing between single and double embryo transfer isn't simply about increasing the odds of pregnancy. It's about achieving the healthiest possible outcome for both mother and baby.

For most IVF patients, elective single embryo transfer offers an excellent chance of success while dramatically reducing the risks associated with twin pregnancies. Thanks to advances in embryo freezing and IVF technology, transferring embryos one at a time provides a similar overall chance of taking home a healthy baby from a complete treatment cycle.

If you're deciding how many embryos to transfer, the right choice depends on your age, embryo quality, fertility history, and whether your embryos have been genetically tested. A personalised consultation with your fertility specialist can help determine the safest and most effective approach for your treatment journey.

Book a consultation with MMC IVF today to discuss the embryo transfer strategy that's right for you.