In Vitro Fertilization (IVF) is a medical process that assists with conception, involving the fertilization of an egg outside the body. After eggs are retrieved and fertilized in the laboratory, the resulting embryos are monitored for several days before the final stage: embryo transfer. This transfer stage introduces the primary decision point regarding the quantity of embryos, which directly impacts the safety and outcome of the treatment. The overarching goal for modern reproductive medicine is to accomplish the birth of a single, healthy baby.
The Medical Standard: Single Embryo Transfer (SET)
The current consensus among major organizations, including the American Society for Reproductive Medicine (ASRM) and the European Society of Human Reproduction and Embryology (ESHRE), defines the safest outcome as a singleton pregnancy. This philosophy has driven a significant shift toward the elective Single Embryo Transfer (eSET) as the preferred treatment strategy. Transferring just one embryo greatly reduces the chance of multiples, which are associated with serious medical complications. This approach prioritizes a healthy pregnancy over the riskier, per-cycle pregnancy rate offered by transferring two or more embryos. Guidelines strongly advocate for eSET, especially in patients with a favorable prognosis, such as those under 35 or those with high-quality embryos.
Factors Determining the Optimal Number
The optimal number of embryos to transfer is a highly personalized decision based on several clinical variables. Patient age is a significant factor, as the likelihood of an embryo being genetically normal decreases with the age of the woman providing the eggs. For women under 35, ASRM guidelines strongly recommend transferring only a single embryo.
Embryo quality is another element, with embryos typically transferred at the blastocyst stage (Day 5 or 6) due to their higher implantation potential. Preimplantation Genetic Testing for Aneuploidy (PGT-A) allows specialists to identify euploid embryos, which have the correct number of chromosomes. Transferring a single PGT-A tested euploid embryo is considered the gold standard, yielding high success rates while minimizing the risk of multiples.
For women in older age groups, such as those 38 to 40, recommendations are more flexible to account for lower success rates per embryo. A doctor might consider transferring two embryos if they have not been genetically tested. The patient’s history, including previous successful or failed IVF cycles, also informs the decision. The decision to transfer two embryos is generally reserved for specific, higher-risk profiles where the chance of a successful singleton pregnancy with one embryo is low.
Understanding the Risks of Multiple Embryo Transfer
While transferring multiple embryos may seem like a way to boost pregnancy chances, it significantly increases the risk of multiple gestation pregnancy and serious medical complications. When two embryos are transferred, the chance of having twins can be as high as 38%, compared to about 1% with a single embryo transfer. Carrying multiples places substantial stress on both the mother and the developing infants.
For the infants, the primary concern is the increased probability of being born prematurely (before 37 weeks of gestation). Approximately 60% of twins are born preterm. This prematurity is directly linked to an elevated risk of low birth weight, respiratory distress syndrome, and long-term developmental issues such as cerebral palsy.
The mother also faces heightened risks, including a greater likelihood of developing gestational diabetes or preeclampsia, a serious condition characterized by high blood pressure. Multiple gestation often necessitates a Cesarean section delivery, which presents surgical risks and a longer recovery time. The goal of a single, healthy baby remains the safest and most desirable outcome due to these compounding health issues.
Success Rates and Cumulative Outcomes
The effectiveness of IVF treatment is not measured solely by whether the first transfer results in a pregnancy. Specialists now focus on cumulative success rates, which represent the chance of achieving a live birth from all embryos created during a single egg retrieval cycle, including subsequent transfers of frozen embryos.
Research consistently demonstrates that transferring one embryo at a time across two sequential cycles often results in a cumulative live birth rate that is comparable to, or even better than, a single transfer of two embryos. Combining a fresh elective SET with a subsequent frozen SET can yield a similar overall success rate as a single double embryo transfer. This strategic, staged approach achieves the ultimate goal of a healthy baby while drastically minimizing the risk of twins or higher-order multiples.