Assisted Reproductive Technology (ART), particularly In Vitro Fertilization (IVF), has provided a path to parenthood for millions. The final step is the embryo transfer, where the developed embryo is placed into the uterus. A significant question is how many embryos should be transferred to achieve a pregnancy. This decision involves balancing the chance of a successful live birth against minimizing the medical risks associated with carrying multiple fetuses. The goal of modern reproductive medicine is a healthy singleton live birth, requiring a careful, personalized assessment.
The Standard Approach: Single Embryo Transfer
The current medical consensus, supported by organizations like the American Society for Reproductive Medicine (ASRM), is that transferring only one embryo (Single Embryo Transfer, or SET) is the preferred standard of care. This approach promotes the birth of a single, healthy baby rather than twins or higher-order multiples. The shift reflects the understanding that a multiple pregnancy carries substantial health risks for both the mother and the infants.
Physicians now actively encourage elective SET for patients who have a favorable prognosis, such as those who are younger or who have high-quality embryos available. This movement has successfully reduced the rate of multiple births resulting from ART. The goal of SET is a full-term, healthy, singleton delivery.
The development of advanced techniques like improved embryo culture and cryopreservation (vitrification) has made SET a highly effective strategy. A high-quality embryo that is not transferred fresh can be safely frozen and used in a subsequent cycle with a similar chance of success. This capability allows clinics to pursue a single-embryo strategy without significantly compromising the overall chance of a live birth.
Understanding Live Birth Success Rates
Data comparing the live birth rate (LBR) of single versus multiple embryo transfer shows that while transferring two embryos (Double Embryo Transfer, or DET) may result in a slightly higher initial pregnancy rate, it primarily results in a dramatic increase in the risk of a multiple gestation pregnancy. DET does not necessarily double the chance of a successful outcome.
Studies show that performing two consecutive cycles of SET achieves a cumulative live birth rate (CLBR) comparable to, or sometimes higher than, a single DET cycle. This sequential approach capitalizes on the high success rates of frozen embryo transfers, which are now nearly equal to fresh transfers. For instance, two sequential SET cycles have resulted in a CLBR of around 41%, compared to 36% for a single DET cycle, while eliminating the multiple birth risk.
The effectiveness of guidelines promoting SET is evident in national data, where the multiple birth rate decreased significantly while the cumulative live birth rate remained stable. For patients with a favorable prognosis, the overall chance of taking home a baby from a single egg retrieval cycle is essentially the same whether they choose DET or a sequence of SETs. The key difference is safety, as the multiple birth rate for DET can be as high as 29% to 37%, compared to less than 2% for SET.
Health Risks Associated with Multiple Transfers
The most significant concern with transferring multiple embryos is the resulting multiple gestation pregnancy, which presents elevated health risks compared to a singleton pregnancy.
Maternal Risks
For the expectant mother, carrying twins or triplets more than doubles the risk of developing gestational hypertension, which can lead to preeclampsia. The risk of gestational diabetes is also increased. Mothers have a higher likelihood of complications such as anemia, placental abruption, and postpartum hemorrhage. Consequently, multiple gestations often necessitate a Cesarean section delivery, which carries its own surgical and recovery risks.
Infant Risks
The increased risk to the infants is even more pronounced, with the majority of multiples being born preterm, often before 37 weeks gestation. Prematurity is the largest cause of infant complications and mortality, frequently leading to low birth weight (less than 2,500 grams). These vulnerable infants often require an extended stay in the Neonatal Intensive Care Unit (NICU) for support.
Infants from multiple gestations face higher risks of serious long-term health issues, including chronic lung disease, developmental delays, and cerebral palsy. Specific to twins, there is also the risk of twin-to-twin transfusion syndrome (TTTS). These medical consequences underscore why the medical community strongly advocates for SET.
Key Factors Guiding the Transfer Decision
The determination of how many embryos to transfer is a highly individualized decision made jointly by the medical team and the patient, based on several prognostic factors.
Patient Age
Patient age is a primary consideration. Younger patients (typically under age 35) with a better prognosis are strongly advised to undergo a single embryo transfer due to their higher implantation potential. As a woman’s age increases, the likelihood of chromosomal abnormalities in the embryos rises, which may lead the medical team to consider transferring more than one embryo.
Embryo Quality and Testing
The quality of the embryo is another determining factor, assessed through a standardized grading system that evaluates morphology and stage of development. The transfer of a euploid embryo, confirmed to have a normal number of chromosomes through preimplantation genetic testing (PGT-A), is almost universally performed as a single embryo transfer because its implantation success rate is significantly higher. If the available embryos are of lower quality or have not been genetically tested, the medical team may discuss the option of transferring two.
Previous Reproductive History
The patient’s previous reproductive history, including the number of prior failed IVF cycles, also influences the decision. A history of multiple unsuccessful transfers, despite the use of high-quality embryos, might lead to a discussion about transferring two embryos in a subsequent cycle to improve the odds per procedure. Finally, the receptivity of the uterus and other underlying medical factors must be considered as part of the overall risk-benefit analysis.