The decision of how many embryos to transfer is one of the most significant choices made during an in vitro fertilization (IVF) cycle. This choice balances the desire for a successful pregnancy with the risks associated with multiple gestations. The options are Elective Single Embryo Transfer (eSET), implanting one embryo, and Double Embryo Transfer (DET), implanting two. Medical guidelines increasingly favor eSET due to safety concerns, but DET remains an option based on personalized factors. This article explores live birth rates, the medical consequences of carrying multiples, and the clinical criteria guiding this decision.
Comparing Live Birth Rates
Patients often assume transferring two embryos doubles the chance of pregnancy compared to one. However, data comparing DET versus eSET suggests the increase in live birth rate (LBR) per single transfer cycle is often marginal, especially with high-quality embryos. While the LBR for a single fresh DET cycle may be higher than for a fresh eSET cycle, the difference is not proportional to the number of embryos transferred. For instance, transferring two embryos might result in an LBR of around 42%, compared to approximately 27% for one embryo in a single fresh cycle.
The overall chance of having a baby over multiple cycles, known as the cumulative live birth rate, changes the picture. If an initial eSET is followed by a subsequent frozen embryo transfer (FET) if the first fails, the cumulative LBR is often comparable to, or higher than, the LBR from a single DET cycle. Modern cryopreservation techniques, such as vitrification, allow frozen embryos to survive thawing at high rates. This makes sequential eSET an effective strategy to pursue a successful pregnancy while maintaining a lower multiple birth rate.
The eSET approach achieves similar overall success rates to DET without the substantial increase in health risks associated with multiple pregnancies. Clinics utilizing higher rates of eSET often report similar live birth rates as those that transfer more embryos. For patients with a good prognosis, transferring a single embryo significantly decreases the risk of multiples without compromising the chance of eventually having a baby.
Health Consequences of Multiple Gestation
Medical organizations advocate for eSET to mitigate the substantial health risks that multiple gestation poses to both the mother and the infants. The human uterus is optimized for a singleton pregnancy, and carrying more than one fetus significantly increases the chance of complications. The risk of delivering twins is higher with DET, with studies reporting multiple birth rates as high as 34% compared to approximately 8% for eSET.
For the infants, the most significant risk is preterm birth (delivery before 37 weeks of gestation). Twins are more likely to be born prematurely than singletons, often leading to low birth weight and extended care in the neonatal intensive care unit (NICU). Prematurity can result in long-term health issues for the child, including respiratory distress syndrome, developmental delays, and cerebral palsy. DET has been linked to 406 grams less birth weight compared to eSET, highlighting the strain on fetal development.
The expectant mother also faces increased health risks when carrying multiples, with maternal complications nearly double in twin pregnancies compared to singletons. A twin pregnancy increases the likelihood of a Cesarean section delivery, which carries a higher risk of surgical complications and recovery time. Other potential maternal issues include:
- Preeclampsia, characterized by high blood pressure.
- Gestational diabetes.
- Postpartum hemorrhage due to the overstretched uterus.
- Anemia.
- Placental abruption.
Criteria for Choosing One or Two Embryos
The decision between eSET and DET is guided by clinical and patient-related factors. The goal is to maximize the chance of a healthy, singleton live birth while minimizing the risk of a multiple pregnancy. The most significant factor influencing the recommendation is the patient’s age when the embryos were created, as this relates directly to egg and embryo quality.
Patients with favorable prognoses are advised to pursue eSET. This group typically includes women under 35 years old undergoing their first or second IVF cycle, especially if high-quality embryos are available. Embryo quality is assessed by two characteristics: the embryo’s developmental stage (Day 5 or 6 blastocysts have a higher implantation chance) and its chromosomal status.
If an embryo has undergone Preimplantation Genetic Testing for Aneuploidy (PGT-A) and is confirmed to be euploid (having the correct number of chromosomes), the recommendation is to transfer only one, regardless of the patient’s age. The high implantation potential of a euploid embryo makes transferring a second one an unnecessary risk for multiples. DET is considered for patients with less favorable prognostic factors, such as advanced maternal age (over 38-40 years old), or those who have experienced multiple failed transfers with high-quality embryos. Other factors include the health of the uterine lining and the number of previously retrieved eggs, which indicates overall ovarian reserve.