In Vitro Fertilization (IVF) is a medical process designed to assist conception by uniting sperm and egg outside the body. The goal of modern IVF is to achieve a healthy, single pregnancy, often by transferring just one embryo into the uterus. Despite this focus, a small percentage of IVF pregnancies result in an unexpected phenomenon: the embryo splits after implantation, leading to identical twins. This outcome is referred to as monozygotic twinning, where a single fertilized egg (zygote) divides into two genetically identical embryos. This differs from dizygotic or fraternal twinning, which occurs when two separate eggs are fertilized by two separate sperm, resulting in two distinct embryos.
Prevalence of Embryo Splitting in IVF
The rate at which embryos spontaneously divide into identical twins is significantly higher in IVF pregnancies compared to those conceived naturally. In the general population, the baseline rate of monozygotic twinning is low, typically occurring in about 0.3% to 0.4% of all births. This natural rate is consistent across different populations and remains largely independent of race or heredity.
Following IVF procedures, the incidence of monozygotic twinning is elevated, resulting in a rate that is often cited as being two to five times higher than the natural baseline. Across various studies, the reported rate of embryo splitting after IVF ranges widely, from approximately 0.9% to as high as 2.5% per transferred embryo or clinical pregnancy.
When analyzed per single embryo transfer (SET), which is the standard of care to prevent multiple gestations, the monozygotic twinning rate averages around 1.5%. This rate highlights a persistent biological interaction between the laboratory environment or specific procedures and the embryo’s development. The risk is elevated whether measured per transferred embryo or per established pregnancy, demonstrating a clear correlation with the assisted reproductive technology itself. The measurement of this rate is important for counseling patients, as the unexpected splitting of a single transferred embryo means the intended single pregnancy has become a twin pregnancy.
Procedural Factors That Increase Splitting Risk
The increased rate of monozygotic twinning in IVF is associated with specific laboratory techniques thought to induce stress on the developing embryo.
Extended Culture to Blastocyst Stage
The most significant factor identified is the extended culture of embryos to the blastocyst stage before transfer. Embryos are typically cultured for five or six days to reach this stage, which allows for better selection and higher implantation rates compared to transferring them at the cleavage stage (Day 3). This prolonged period of in vitro culture may cause subtle changes in the environment surrounding the embryo, leading to a higher likelihood of the inner cell mass (ICM) splitting in two. Studies suggest that blastocyst transfer is associated with nearly a two-fold increase in the risk of monozygotic twinning compared to cleavage-stage transfers. The exact mechanism remains unclear but may relate to changes in the embryo’s cellular adhesion or metabolism.
Assisted Hatching (AH)
Another procedural factor linked to increased splitting is Assisted Hatching (AH), a technique where a small opening is created in the zona pellucida, the embryo’s outer shell, just before transfer. This manipulation is performed to help the embryo “hatch” and implant into the uterine lining. Breaching the zona pellucida, whether chemically or with a laser, may destabilize the embryo’s structure and encourage the division of cells that leads to identical twins. Data indicates that the use of Assisted Hatching can increase the odds of monozygotic twinning by three to four times compared to cycles where it is not used.
Intracytoplasmic Sperm Injection (ICSI), where a single sperm is injected directly into the egg, has also been implicated in some analyses, with one meta-analysis suggesting a 2.25 times higher risk of splitting with ICSI. These findings suggest that any manipulation of the embryo or extended time outside the womb may contribute to unexpected identical twinning.
Clinical Considerations for Monozygotic Twins
Once embryo splitting occurs, the resulting monozygotic twin pregnancy carries specific medical risks that necessitate specialized monitoring and management. The primary concern is determined by the timing of the split, which dictates the chorionicity (number of placentas) and amnionicity (number of amniotic sacs) of the pregnancy.
The majority of monozygotic twins resulting from IVF are monochorionic-diamniotic. This means the twins share a single placenta but have separate amniotic sacs. This configuration arises when the embryo splits between four and eight days after fertilization. Sharing a single placenta poses a significant risk because the twins’ circulatory systems are often connected through vascular anastomoses within the placenta.
The most serious complication arising from this shared circulation is Twin-to-Twin Transfusion Syndrome (TTTS), which affects approximately 10% to 15% of monochorionic pregnancies. In TTTS, blood flow becomes unbalanced, with one twin (the donor) giving blood to the other (the recipient), leading to severe growth discrepancies and potentially fatal outcomes. The donor twin can become dehydrated with low amniotic fluid, while the recipient twin can develop fluid overload and heart failure.
Because of the heightened risk of TTTS and other complications such as selective growth restriction, a monozygotic twin pregnancy requires more frequent and specialized ultrasound surveillance throughout the pregnancy. Early diagnosis of chorionicity, typically in the first trimester, is a priority for any twin pregnancy conceived via IVF. This intensive monitoring is necessary to detect and potentially intervene in complications like TTTS, which can be treated with fetal laser surgery in severe cases.