In Vitro Fertilization (IVF) is a widely used fertility treatment. A common question concerns the possibility of a single embryo, transferred during an IVF cycle, splitting to form identical twins. This phenomenon, while less common, can indeed occur, adding a unique aspect to IVF pregnancies.
Understanding Different Types of Twins
Twins arise from two distinct biological processes: monozygotic or dizygotic. Monozygotic, or identical, twins develop from a single fertilized egg that splits into two embryos. This division results in two individuals who share nearly identical genetic material and are always of the same sex. The timing of this split influences whether they share a placenta and amniotic sac; an early split typically leads to separate placentas and sacs, while a later split may result in shared structures.
Dizygotic, or fraternal, twins originate from two separate eggs, each fertilized by a different sperm. These twins are genetically distinct, sharing about 50% of their DNA, similar to any other siblings. They can be of different sexes and always develop with their own placentas and amniotic sacs. Dizygotic twinning is often influenced by factors like genetics or fertility treatments that stimulate the release of multiple eggs.
Why IVF Embryos Might Split
The rate of monozygotic twinning is slightly higher in IVF pregnancies compared to natural conception, with estimates ranging from 1% to 2% in IVF transfers versus approximately 0.4% in natural pregnancies. Several factors associated with the IVF process contribute to this increased incidence. One factor is the extended culture of embryos to the blastocyst stage before transfer. Blastocyst transfers, involving embryos developed for five or six days, are associated with a higher risk of monozygotic twinning compared to earlier-stage embryo transfers.
Assisted hatching, a procedure making a small opening in the embryo’s outer shell to facilitate implantation, has been linked to an increased risk of embryo splitting. Embryo biopsy, performed for genetic testing, involves removing a few cells from the embryo. This manipulation, particularly when performed at the blastocyst stage, has been shown to increase monozygotic twinning.
The process of freezing and thawing embryos (cryopreservation) is another factor. Frozen-thawed embryo transfers carry a higher risk of monozygotic twinning, possibly due to the stress induced by the cryopreservation process. Additionally, the specific culture medium and conditions in the laboratory where embryos are grown are considered potential influences on embryo development.
What Identical Twin Pregnancies Mean for IVF Patients
When a single IVF embryo splits to form identical twins, it can be a significant surprise for patients who anticipated a singleton pregnancy. Despite the intention of a single embryo transfer to result in one baby, this outcome means a shift in pregnancy management. Medical providers will emphasize specialized monitoring throughout the pregnancy. Early and accurate determination of chorionicity (whether the twins share a placenta) and amnionicity (whether they share an amniotic sac) is crucial.
The timing of the embryo’s split dictates these characteristics, which in turn inform the level of medical oversight required. For instance, twins sharing a placenta (monochorionic) require more frequent monitoring due to potential complications like twin-to-twin transfusion syndrome. Identical twin pregnancies from a single IVF embryo are managed within established obstetric guidelines for multiple gestations. Healthcare teams provide tailored care, focusing on the specific needs arising from the shared or separate placental and amniotic environments.