The decision to transfer one or two embryos during In Vitro Fertilization (IVF) is a significant choice for patients and their fertility teams. This choice balances the desire for a quick pregnancy with the need to ensure the safest possible outcome for both the parent and the future child. The core dilemma is maximizing the chance of a successful live birth while minimizing the risks associated with multiple gestations. Modern advancements in reproductive technology have made this balance achievable, but the choice remains highly individualized.
Defining Single and Double Embryo Transfer
Single Embryo Transfer (SET) involves placing only one embryo into the uterus, while Double Embryo Transfer (DET) involves transferring two embryos. Historically, multiple embryo transfer was the standard practice because implantation rates were lower, and transferring more embryos was seen as the most direct way to increase the chance of success. This practice, however, led to an increase in twin and higher-order multiple pregnancies, which carry substantial health risks.
The field of reproductive medicine has shifted its focus toward achieving a healthy singleton pregnancy, leading to the widespread adoption of SET, often referred to as elective Single Embryo Transfer (eSET). This change was enabled by technical improvements, such as culturing embryos to the blastocyst stage (day 5 or 6). Another element is Preimplantation Genetic Testing (PGT-A), which allows for better selection of the most viable embryo. The goal of modern IVF is to deliver a single, healthy baby, and eSET is the strategy designed to meet that objective.
Comparing Live Birth Success Rates
Whether transferring two embryos significantly increases the chance of a live birth compared to transferring one is a central question for many patients. While DET generally offers a slightly higher chance of pregnancy per transfer attempt, particularly in cycles using fresh embryos, the difference in live birth rates is often marginal when only high-quality embryos are involved. Studies have shown that for patients with good prognosis, the live birth rates are similar between SET and DET, especially when frozen embryo cycles are considered.
The concept of cumulative success rate is important here, as it considers the outcome of using all available embryos, including those frozen for later use. When a fresh SET is followed by the transfer of any remaining frozen embryos, the cumulative live birth rate is frequently comparable to or even higher than that achieved with a single DET cycle. Transferring a single, high-quality embryo has been shown to be as effective as transferring two embryos when the embryos have been genetically tested. Transferring a poor-quality embryo alongside a good-quality one in a DET may not increase the chance of success and could potentially decrease the outcome compared to a single transfer of the high-quality embryo alone.
Assessing the Risks of Multiple Gestation
SET is favored in modern practice due to the significant increase in medical risks associated with twin pregnancies resulting from DET. Twin pregnancies are associated with a five to tenfold increase in complications for both the mother and the babies. For the parent, these risks include elevated rates of preeclampsia, gestational hypertension, and gestational diabetes. There is also an increased likelihood of needing a Cesarean section and experiencing postpartum hemorrhage.
The risks to the infants are particularly pronounced, with approximately 60% of twins being delivered preterm, compared to about 10% of singletons. Preterm birth can lead to neonatal complications, including low birth weight, respiratory distress syndrome, and extended stays in the neonatal intensive care unit (NICU). In the long term, premature birth can lead to developmental delays and conditions such as cerebral palsy. These severe health consequences underscore the clinical preference for eSET, as the slight potential gain in pregnancy rate from DET is often outweighed by the severity of the maternal and neonatal risks.
Patient and Embryo Factors Guiding the Decision
The decision to transfer one or two embryos is not universal; it is a personalized choice guided by specific patient and embryo characteristics. Clinicians use factors like maternal age as a strong predictor of success, often recommending eSET for younger patients, typically those under 35, who have a higher chance of success with a single embryo. For women over 37, or those with a history of multiple unsuccessful cycles, the recommendation may shift to considering two embryos, depending on the quality of those embryos.
Embryo quality is a significant determinant, with the highest quality embryos being the best candidates for eSET. The stage of development, such as a blastocyst (Day 5 or 6), is important, as is the result of Preimplantation Genetic Testing for Aneuploidy (PGT-A). If an embryo is confirmed as chromosomally normal (euploid) through PGT-A, a single transfer is recommended because the success rate per transfer is maximized. Other personal factors, such as a history of recurrent implantation failure or specific uterine conditions, are also considered to tailor the final number of embryos transferred.