Why Are Identical Twins More Common With Ivf?

Identical twins are more common with IVF because the laboratory procedures involved can cause a single embryo to split into two. This is a well-documented finding in fertility medicine. While the overall rate of identical twins in natural conception is about 1 in 250 births, studies show that rate is higher after IVF, though the exact increase varies by study and technique. The split happens for reasons that are not fully understood, but the process itself is a known biological event that occurs more often in the IVF setting.

What Is the Difference Between Identical and Fraternal Twins?

Identical twins come from one egg and one sperm. After fertilization, the embryo splits into two separate embryos. These twins share the same genetic material and are always the same sex.

Fraternal twins are completely different. They happen when two separate eggs are fertilized by two separate sperm. These twins are no more genetically similar than any other siblings. They can be different sexes.

IVF often involves transferring more than one embryo. That practice increases the chance of fraternal twins. But the rise in identical twins is a separate phenomenon. It happens even when only a single embryo is transferred back into the uterus.

Why Are Identical Twins More Common With IVF?

The leading explanation involves the embryo itself. In IVF, embryos are cultured in a laboratory dish for several days before transfer. Some research suggests this extended culture time may make the inner cell mass — the part that becomes the fetus — less stable. That instability may increase the chance of the embryo splitting.

Another factor is the zona pellucida. This is the protective outer shell of the embryo. In IVF, doctors sometimes use a technique called assisted hatching. A small hole is made in the zona pellucida to help the embryo “hatch” out and implant. Some studies suggest this procedure may increase the risk of the embryo splitting, though the evidence is not conclusive.

The process of freezing and thawing embryos may also play a role. Some research indicates that frozen embryo transfers have a slightly higher rate of identical twins compared to fresh transfers. The exact reason is unknown, but it may relate to changes in the embryo’s outer layer during the freeze-thaw process.

One important point: the split is not caused by the fertility medications themselves. The drugs stimulate the ovaries to produce multiple eggs, which can lead to fraternal twins. But they do not directly cause an embryo to split into identical twins.

How Much Higher Is the Risk?

Exact numbers vary across studies, and it is difficult to give a single precise figure. What researchers generally agree on is that the rate of identical twins after IVF is roughly 1 to 2 percent of all IVF pregnancies. That is higher than the natural rate of about 0.4 percent.

Some studies report higher rates, especially with certain techniques. For example, assisted hatching has been linked to rates of identical twinning that are higher than IVF without assisted hatching. But not every study confirms this, and the overall evidence is mixed.

The key takeaway is straightforward: IVF increases the chance of identical twins, but the absolute risk remains low. Most IVF pregnancies result in a single baby.

Does Transferring One Embryo Prevent Identical Twins?

No. Transferring a single embryo does not eliminate the risk of identical twins. The split happens before the embryo is transferred, during the culture period in the lab. By the time the embryo is placed into the uterus, the split may have already occurred.

This is why a woman can have a single embryo transfer and still end up pregnant with twins. The ultrasound will show two heartbeats, and the explanation is that the single transferred embryo divided into two.

Single embryo transfer does eliminate the risk of fraternal twins because only one egg is being used. But it does nothing to prevent identical twins. This is an important point for anyone undergoing IVF to understand before treatment begins.

Are Identical Twins From IVF Healthier or at Higher Risk?

Identical twins from IVF face the same general risks as identical twins conceived naturally. Twin pregnancies in general carry higher risks than singleton pregnancies. These include preterm birth, low birth weight, and pregnancy complications like gestational diabetes and preeclampsia.

Identical twins specifically have an additional risk called twin-to-twin transfusion syndrome. This condition occurs when blood vessels in the shared placenta connect, causing one twin to receive too much blood flow and the other too little. It is a serious condition that requires specialized care.

The risk of twin-to-twin transfusion syndrome is not higher because of IVF. It is a risk of identical twins themselves, regardless of how they were conceived. The same is true for most other twin-related complications.

What matters most is early and regular prenatal care. Women pregnant with twins after IVF should be monitored closely by an obstetrician experienced in multiple pregnancies.

What Should You Discuss With Your Fertility Doctor?

Before starting IVF, ask your doctor directly about the risk of identical twins. Specifically ask whether the clinic uses assisted hatching and whether embryos will be cultured to day five or day six, which is called blastocyst transfer. Both are associated with a possible increase in identical twinning, though the evidence is not definitive.

Ask about the clinic’s policy on single embryo transfer. Many clinics now recommend transferring one embryo at a time, especially for younger women with good-quality embryos. This reduces the risk of fraternal twins and the complications that come with any multiple pregnancy.

Also ask about how embryos are frozen and thawed. If you plan to freeze embryos for future use, understand that frozen transfers may carry a slightly higher identical twin rate. Your doctor can explain the specific data from their own clinic, if they track it.

The conversation should be honest and detailed. Fertility treatment involves many decisions, and understanding the risks — including the uncommon ones — helps you make informed choices.

Does the Type of IVF Matter?

Conventional IVF and intracytoplasmic sperm injection, or ICSI, both show an increased rate of identical twins compared to natural conception. ICSI involves injecting a single sperm directly into the egg. Some older studies suggested ICSI might have a higher identical twin rate, but more recent research does not consistently support that difference.

The same applies to the use of donor eggs. The risk of identical twinning appears to be related to the IVF process itself, not the source of the egg. Whether the eggs are from the patient or a donor does not appear to change the identical twin rate in a meaningful way.

What is clear is that the increased risk is tied to laboratory handling. The more the embryo is manipulated, the higher the theoretical chance of splitting. This is a biological observation, not a proven cause-and-effect relationship for every case.

What About the Long-Term Health of IVF Identical Twins?

Research on the long-term health of children conceived through IVF is ongoing. For identical twins specifically, the data is limited. What exists does not suggest that IVF identical twins are at higher risk for developmental problems compared to naturally conceived identical twins.

The larger concern is being a twin at all. Twins are more likely to be born early and have lower birth weights. These factors can influence health in childhood and beyond. But these are risks of twinning, not risks of IVF.

If you are pregnant with identical twins after IVF, focus on prenatal care and delivery planning. Your medical team will monitor growth and development closely. Most twin pregnancies result in healthy babies, especially with good medical support.