In Vitro Fertilization (IVF) is a medical process where an egg is fertilized by sperm outside the body in a laboratory setting. This procedure involves several distinct stages, culminating in the Embryo Transfer (ET), which is the physical placement of the resulting embryo into the patient’s uterus. Determining the precise number of embryos to transfer (NET) is a fundamental decision within the IVF cycle, as this choice directly impacts both the probability of a successful pregnancy and the potential risks involved. The ultimate goal of modern fertility treatment is to achieve a healthy pregnancy resulting in a single, live birth. The decision about embryo quantity balances maximizing conception chances per cycle and minimizing serious health complications associated with multiple gestations.
Current Medical Guidelines for Embryo Transfer
Major professional organizations, such as the American Society for Reproductive Medicine (ASRM), favor transferring the fewest embryos necessary for a successful pregnancy. This approach is codified in the recommendation for Elective Single Embryo Transfer (eSET) for most patients with a favorable prognosis. The widespread adoption of eSET has fundamentally shifted clinical practice, leading to a significant reduction in the rate of multiple births resulting from IVF.
For any patient, the transfer of an embryo confirmed to be chromosomally normal (euploid) through Preimplantation Genetic Testing (PGT) should be limited to one. This single euploid embryo transfer offers a high chance of success while virtually eliminating the risk of twins or higher-order multiples. For patients under 35 years of age, who generally have the highest success rates, a single embryo transfer is strongly encouraged, even if the embryo has not been genetically tested.
As patients age, the guidelines allow for an increase in the number of transferred embryos due to declining egg quality. For instance, patients between 38 and 40 years old may receive two blastocysts or up to three cleavage-stage embryos if they have not undergone PGT. This increase is a concession to the lower implantation potential of embryos in older age groups. The overall trend remains focused on limiting the transfer number to prevent the complications of twin or triplet pregnancies.
Key Patient Factors Guiding the Decision
The number of embryos transferred is a personalized decision that customizes general guidelines to the patient’s biological and medical profile. Patient age is the single most influential factor affecting embryo viability. As a woman ages, the likelihood of her eggs containing chromosomal abnormalities (aneuploidy) increases, which significantly lowers the implantation rate.
For women in their early 30s, the high implantation rate strongly supports the choice of eSET. Conversely, for women over 40 using their own eggs, the higher rate of aneuploidy may lead a clinician to consider transferring two untested embryos to maintain a reasonable chance of pregnancy. The decision is not based solely on age, but on a combination of prognostic data.
The quality and developmental stage of the embryo also provide crucial information. Blastocysts (cultured for five to six days) are considered higher quality than cleavage-stage embryos (cultured for three days) because they have demonstrated greater developmental capacity. Transferring a single, high-quality blastocyst is often preferred because it offers a higher per-embryo implantation potential compared to a less mature embryo.
A patient’s history with IVF is another significant variable. If a patient has experienced multiple failed embryo transfers, the medical team may consider transferring an additional embryo to overcome unexplained factors related to repeated implantation failure. Additionally, the health of the patient’s uterus, including the thickness and appearance of the endometrial lining, is assessed. A less receptive uterine environment may prompt a discussion about transferring two embryos.
The Risks of Transferring Multiple Embryos
The primary reason medical practice has shifted toward single embryo transfer is the substantial health risk posed by multiple gestations. The human uterus is optimally designed to carry a single fetus, and the presence of twins or triplets elevates the chance of complications for both the mother and the babies. Premature birth, defined as delivery before 37 weeks of gestation, is a major concern.
In a singleton pregnancy, the risk of preterm birth is approximately 14%, but this rate jumps to about 65% for twins. Premature infants face numerous complications, including respiratory distress syndrome, developmental delays, and long-term disabilities. The risk of a baby being born with a low birth weight (under 5.5 pounds) rises from 9% in singletons to 57% in twins, compounding neonatal health challenges.
Maternal health risks are heightened in multiple pregnancies. Women carrying multiples have a higher incidence of gestational diabetes and preeclampsia, a condition characterized by high blood pressure and organ damage. Additionally, multiple gestation increases the likelihood of a necessary Cesarean section delivery and the possibility of fetal reduction. Fetal reduction is a difficult medical and ethical decision sometimes required to improve the survival chances of the remaining fetus. The goal of modern IVF is to avoid these complications by prioritizing the transfer of one high-quality embryo.