Deciding how many embryos to transfer into the uterus during an In Vitro Fertilization (IVF) cycle is a significant choice. This decision balances maximizing the chances of pregnancy success against minimizing the serious medical risks associated with carrying multiple fetuses. The primary goal of modern reproductive medicine is to achieve a healthy, single-fetus pregnancy.
The Primary Concern: Risks of Multiple Pregnancies
Medical guidelines recommend limiting the number of transferred embryos due to the significant health risks associated with twin or higher-order multiple pregnancies. The human uterus is designed to carry a single baby to term, and introducing multiples dramatically increases the chance of complications for both the mother and the infants.
For the mother, carrying multiples elevates the risk of severe pregnancy complications like preeclampsia and gestational diabetes. Maternal death rates are nearly four times higher with twins compared to singleton pregnancies. Twin pregnancies also carry a higher likelihood of requiring a Cesarean section and experiencing postpartum hemorrhage.
The most substantial risk for the babies is premature birth, occurring in nearly 60% of twin pregnancies and over 90% of triplet pregnancies. Prematurity often leads to low birth weight, requiring extended stays in the neonatal intensive care unit (NICU). Long-term health issues linked to premature birth can include chronic lung conditions, cerebral palsy, and developmental delays.
Guidelines and Factors for Determining Transfer Numbers
Professional organizations, such as the American Society for Reproductive Medicine (ASRM), publish guidance to help clinicians determine the appropriate number of embryos to transfer. While these standards prioritize safety, the final decision is always highly individualized, based on several patient-specific factors.
Patient age is the most significant factor influencing recommendations, as it correlates with egg and embryo quality. For women under 35 with a favorable prognosis, the transfer of a single embryo is strongly advised. Older patients, especially those aged 38 to 40, may be candidates for transferring two embryos to compensate for the lower implantation potential.
Embryo quality is another determining variable, assessed through two methods. Embryo grading involves examining the embryo’s appearance and rate of development, where higher-quality embryos have a greater chance of implantation. The ploidy status, determined by Preimplantation Genetic Testing (PGT), identifies if an embryo has the correct number of chromosomes. A “euploid” embryo should only be transferred one at a time due to its high success rate.
The stage of development also affects the number transferred. Embryos transferred on Day 5 or Day 6, known as blastocysts, are more developed than Day 3 cleavage-stage embryos and have a greater implantation potential. Therefore, fewer blastocysts are transferred. A patient’s previous IVF history, specifically the number of failed cycles, may also justify transferring one additional embryo, though this requires careful clinical documentation and counseling.
The Shift Towards Elective Single Embryo Transfer
Elective Single Embryo Transfer (eSET) is the practice of transferring only one high-quality embryo when multiple viable embryos are available. This approach is rapidly becoming the standard of care because it addresses the medical risks of multiples while maintaining high rates of live birth.
Advancements in the IVF laboratory have enabled eSET to become a highly viable option. Modern culture systems allow embryos to grow to the blastocyst stage, which provides a better measure of viability and helps identify the single best embryo for transfer. Improved embryo freezing techniques, specifically vitrification, ensure that unused high-quality embryos can be safely preserved for future frozen embryo transfer cycles.
The use of Preimplantation Genetic Testing (PGT) further supports eSET by accurately identifying euploid embryos, which have the highest likelihood of leading to a successful singleton pregnancy. When a euploid embryo is transferred, success rates are comparable to those achieved by transferring two untested embryos, but the risk of a multiple gestation is dramatically reduced. This focus on transferring one carefully selected embryo prioritizes the health and safety of both the mother and baby.