A single embryo transferred during in vitro fertilization (IVF) can split into two genetically identical individuals, a phenomenon known as monozygotic twinning. This process differs from the conception of fraternal twins, which results from two separate eggs fertilized by two different sperm. The specific laboratory procedures used in IVF treatments appear to inadvertently increase the likelihood of this early embryonic division compared to natural conception. The resulting pregnancies require specialized monitoring due to the unique risks associated with the shared environment in which the twins develop.
The Biology of Identical Twinning
Identical, or monozygotic, twins originate from a single fertilized egg, or zygote, that splits into two separate embryos early in development. The precise timing of this cellular division determines the degree to which the developing twins share a placenta and amniotic sac, a characteristic known as chorionicity and amnionicity.
If the split occurs within the first three days after fertilization, the twins will typically develop with their own separate placentas and sacs. If the division happens later, between four and eight days, the twins will share a single placenta but will have their own amniotic sacs, a condition called monochorionic-diamniotic. The latest splits, occurring eight to thirteen days post-fertilization, result in the twins sharing both a placenta and an amniotic sac. This is the rarest and highest-risk form of twinning.
Increased Twinning Rates in IVF
The rate of identical twinning is notably higher following IVF compared to natural conception, where the incidence is consistently low at approximately 0.4% of all births. With assisted reproductive technology, the reported incidence of monozygotic twinning can range from 1.2% to 8.9% of pregnancies, representing a two-to tenfold increase over the natural rate.
The move toward transferring only one embryo, known as elective single embryo transfer (eSET), was intended to decrease the high rates of fraternal twins seen when multiple embryos were transferred. However, this practice did not eliminate the risk of multiple births, as the single transferred embryo still retains the potential to split.
Specific Factors Influencing Embryo Splitting
The increased rate of monozygotic twinning in IVF is closely linked to specific laboratory practices that may impose stress on the developing embryo. One of the most frequently cited factors is the transfer of embryos at the blastocyst stage (Day 5 or Day 6 of development), rather than the earlier Day 3 cleavage stage. Extended culture to the blastocyst stage is associated with a significantly higher splitting rate, possibly due to the physical stresses of the rapid expansion required for blastocyst formation.
Another procedure implicated in splitting is assisted hatching (AH), which involves creating a small breach or thinning the zona pellucida. The manipulation of this outer layer is thought to make the inner cell mass more susceptible to physical division as it attempts to hatch and implant. Furthermore, the overall culture environment, including specific components of the media and the duration the embryo spends outside the uterus, may stress the embryo and affect cellular adhesion. Intracytoplasmic Sperm Injection (ICSI), where a single sperm is injected directly into the egg, has also been suggested as a possible contributor.
Outcomes of Monochorionic Pregnancies
The splitting event in an IVF embryo most often occurs after the fourth day of development, which means the resulting twins usually share a single placenta, resulting in a monochorionic pregnancy. This shared placental environment significantly increases the risk of specific complications not typically seen in twins with separate placentas.
The connections between blood vessels within the shared placenta can lead to an unbalanced distribution of blood, resulting in conditions such as Twin-to-Twin Transfusion Syndrome (TTTS). In TTTS, one twin receives too much blood while the other receives too little, threatening the health of both fetuses. Other risks include selective fetal growth restriction and a higher risk of preterm birth. Specialized monitoring with frequent ultrasounds is immediately initiated to detect and manage these unique complications.