In vitro fertilization (IVF) is a fertility treatment where eggs are fertilized by sperm in a laboratory. This process involves several steps, with embryo transfer being a crucial stage where embryos are placed into the uterus to establish a pregnancy. Deciding how many embryos to transfer is crucial, especially for those considering multiple pregnancies like twins. This balances the desire for success with potential health considerations for both the pregnant individual and the developing fetuses.
The Science of Embryo Transfer for Multiple Pregnancies
The biological mechanism of embryo transfer directly influences the likelihood of a multiple pregnancy. When only a single embryo is transferred, known as Single Embryo Transfer (SET), the typical outcome is a singleton pregnancy. However, even with SET, there is a small chance (approximately 1-3%) of identical twins occurring if the transferred embryo spontaneously splits after implantation. This phenomenon results in monozygotic twins, sharing the same genetic material.
Conversely, Multiple Embryo Transfer (MET) significantly increases the probability of conceiving fraternal (dizygotic) twins or higher-order multiples. Fraternal twins develop when two separate embryos implant and grow independently, each originating from a different egg and sperm. MET also carries a higher chance of identical twins if one transferred embryo splits, in addition to potential fraternal twins from other implanted embryos.
Factors Guiding Embryo Transfer Decisions
Several factors influence the decision on how many embryos to transfer. A patient’s age is key, as younger individuals often have embryos with higher implantation potential, leading to better success with fewer transfers. Embryo quality also plays a central role; high-quality embryos are more likely to implant successfully.
The stage of embryo development is another determinant. Embryos can be transferred at the cleavage or blastocyst stage. Transferring at the blastocyst stage allows for more natural selection of viable embryos, as only the most robust typically reach this advanced stage. A patient’s previous IVF cycle history can also guide the decision, with unsuccessful cycles sometimes leading to consideration of more embryos, though risks are carefully weighed. Patient preferences and their tolerance for multiple pregnancy risks are also integrated into this individualized process.
Health Considerations with Multiple Embryo Transfer
Transferring multiple embryos introduces health implications for both the pregnant individual and the fetuses. Preterm birth, occurring before 37 weeks, is a common complication, affecting about 60% of twin pregnancies. This can lead to low birth weight, long-term health issues, and higher neonatal intensive care unit (NICU) admission rates.
For the pregnant individual, multiple pregnancies increase the risk of preeclampsia (high blood pressure and organ damage) and gestational diabetes (elevated blood sugar). Placental complications and a higher incidence of Cesarean section deliveries (often exceeding 70% for twins) are additional concerns. These highlight why the number of transferred embryos is carefully managed, even with a desire for twins.
Current Clinical Approaches and Recommendations
Clinical guidelines increasingly favor Single Embryo Transfer (SET) to mitigate multiple pregnancy risks. Organizations like the American Society for Reproductive Medicine (ASRM) and the Society for Assisted Reproductive Technology (SART) recommend SET for many patients, especially those with favorable prognoses, such as younger individuals with high-quality embryos. This aims for a healthy singleton pregnancy, considered the safest outcome.
The focus is on optimizing live birth chances with the lowest risk, not just maximizing implanted embryos. Shared decision-making is central to this approach, with reproductive endocrinologists discussing all factors with patients. These include specific circumstances, embryo quality, and multiple gestation risks, to reach an informed, personalized decision on embryo transfer numbers.