Can One Embryo Split Into Twins After Transfer?

Short answer: Yes. A single embryo transferred during IVF can occasionally split and develop into identical twins. This is uncommon, but it means that single-embryo transfer greatly reduces - rather than completely eliminates - the chance of a twin pregnancy.

How can one embryo become two babies?

When one fertilized egg divides into two embryos, the result is a monozygotic, or identical, twin pregnancy. The twins usually share nearly all the same inherited DNA because they developed from the same original embryo.

This is different from fraternal twins, which usually result when two separate eggs are fertilized. In IVF, fraternal twins are more likely when more than one embryo is transferred, although a second pregnancy can rarely arise from conception outside the transfer procedure if ovulation and intercourse occur around the same time.

How common are identical twins after a single embryo transfer?

They are uncommon. The American Society for Reproductive Medicine notes that single-embryo transfer reduces multiple pregnancy to a background risk of about 1% to 2% for monozygotic twins in assisted reproduction. A large 2023 study of 15,567 pregnancies after single-embryo transfer reported monozygotic twins in about 1.6% of pregnancies.

These figures describe groups, not an individual prediction. Rates vary across studies because researchers use different populations, embryo stages, laboratory methods and ways of confirming whether twins are identical.

Does IVF increase the chance that an embryo will split?

Research has found that monozygotic twinning may be somewhat more common after assisted reproduction than after unassisted conception. However, the reason is not fully understood, and studies have not reached identical conclusions about every possible factor.

Some research has associated blastocyst-stage transfer with a higher observed rate of monozygotic twinning. Studies have also examined assisted hatching, embryo biopsy, ICSI, fresh versus frozen transfer and other laboratory procedures. These associations do not prove that any one procedure directly causes an embryo to split, and they should not be used to choose or reject a treatment without clinical context.

Does transferring one embryo prevent twins?

No, but it substantially reduces the chance of twins compared with transferring two embryos. This is why professional guidance favors single-embryo transfer for many patients and strongly recommends it in gestational-carrier cycles.

Transferring two embryos can lead to fraternal twins if both implant. It can also, very rarely, lead to more than two fetuses if one of the embryos splits. A single-embryo transfer avoids the added multiple-pregnancy risk created by transferring a second embryo, while preserving the small possibility that the one embryo may divide.

If you are planning surrogacy, see our related guides on transferring two embryos to a surrogate and how many embryos you may need for surrogacy.

Are identical-twin pregnancies higher risk?

All twin pregnancies require closer prenatal monitoring than singleton pregnancies. Compared with carrying one baby, twins are associated with higher risks of preterm birth, low birth weight and pregnancy complications.

The exact risk also depends on whether the twins share a placenta or an amniotic sac. The timing of the embryo split influences this arrangement. Twins that share a placenta can face additional complications, so early ultrasound is important for identifying chorionicity and planning appropriate monitoring.

A twin finding on an early scan does not by itself show whether the twins are identical, and ultrasound interpretation should come from the treating clinic or obstetric provider.

What should intended parents or a gestational carrier ask the clinic?

• How many embryos does the clinic recommend transferring, and why?
• What is the clinic's own multiple-pregnancy rate after single-embryo transfer?
• At what point will ultrasound confirm the number of gestational sacs, embryos and heartbeats?
• If twins are identified, when will the care team determine whether they share a placenta or amniotic sac?
• Would the pregnancy need referral to a maternal-fetal medicine specialist?

For a gestational-carrier journey, embryo-transfer decisions should be addressed before treatment and reflected consistently in medical consent, the parties' expectations and the legal agreement. The surrogate remains the patient and must give informed consent to the treatment plan.

What do professional sources say?

The ASRM committee opinion on multiple gestation explains why single-embryo transfer is central to reducing multiple pregnancy while acknowledging the remaining background risk of identical twins. ASRM's gestational-carrier recommendations strongly recommend single-embryo transfer in gestational-carrier cycles.

For the research evidence, see the 2023 single-embryo-transfer study and a systematic review of monozygotic-twinning risk factors after IVF.

Bottom line

One embryo can split into identical twins after transfer, but this is uncommon. Single-embryo transfer remains the most effective way to reduce the chance of a multiple pregnancy caused by IVF, even though it cannot make that chance zero. Your fertility specialist can explain how your embryo, treatment plan and medical history affect the recommendation.

If you are comparing embryo-transfer and surrogacy options, Expecting.ai can help you explore available surrogate profiles and understand the questions to raise with your clinic and surrogacy professionals.

This article is for educational purposes only and is not medical or legal advice. Treatment and pregnancy decisions should be made with qualified healthcare and legal professionals who know your circumstances.