The question of how many embryos to transfer during In Vitro Fertilization (IVF) is one of the most important decisions in the fertility process, balancing the desire for a successful pregnancy with the need for a healthy outcome. The Embryo Transfer (ET) procedure is the final step, involving the precise placement of one or more resulting embryos into the uterus using a thin, flexible tube called a catheter. The primary objective of modern reproductive medicine is not simply to achieve pregnancy, but to maximize the likelihood of a single, healthy live birth.
The Primary Goal Single Embryo Transfer
Modern fertility practice has shifted significantly toward promoting Elective Single Embryo Transfer (eSET), which involves choosing to transfer only one embryo even when multiple high-quality embryos are available. This approach is driven by the understanding that a singleton pregnancy is the safest outcome for both the mother and the child. Transferring a single embryo virtually eliminates the risks associated with twin, triplet, or higher-order multiple gestations.
By adopting eSET, medical professionals prioritize the non-maleficence principle—the duty to do no harm—over simply maximizing the immediate pregnancy rate. Even when a twin pregnancy might seem like a welcome outcome to patients, the significant increase in health complications makes a single pregnancy the ultimate goal. For patients with a favorable prognosis, eSET yields similar overall live-birth rates compared to transferring two embryos, especially when considering the option of using cryopreserved embryos in a subsequent cycle.
Patient Factors Guiding the Number of Embryos
The decision regarding the number of embryos to transfer is highly individualized. Maternal age is often the single most significant factor, as the quality and chromosomal health of eggs decrease as a woman gets older. Younger patients, particularly those under 35, have a higher likelihood of producing chromosomally normal embryos, meaning a single embryo has a higher chance of leading to a successful pregnancy.
Embryo quality also plays a substantial role, with embryos that have developed to the blastocyst stage (typically five or six days after fertilization) generally having a higher implantation potential than earlier-stage cleavage embryos. Furthermore, Preimplantation Genetic Testing (PGT) status is a powerful determinant; if an embryo has been confirmed as chromosomally normal, the recommendation is almost universally to transfer only one, regardless of the patient’s age. A euploid embryo has a strong probability of implanting, and transferring two would unnecessarily double the risk of a multiple pregnancy.
A patient’s previous IVF history is another factor, particularly the number of prior failed cycles. For patients who have not succeeded after multiple attempts despite transferring high-quality embryos, the medical team may consider transferring an additional embryo, although this is done with caution and extensive counseling. The overall prognosis for the cycle—which considers the number of high-quality embryos available for freezing—helps classify the patient as having a favorable or less favorable outlook, directly influencing the final number considered for transfer.
Professional Guidelines and Maximum Limits
To standardize practice and reduce the rate of multiple births, professional organizations provide specific numerical guidelines for the maximum number of embryos that should be transferred. The American Society for Reproductive Medicine (ASRM) and the Society for Assisted Reproductive Technology (SART) provide guidance based on age and prognostic factors. These limits are designed to be a ceiling, not a target, with the goal of minimizing the risk of high-order multiple pregnancies.
For patients under 35 years old who have a favorable prognosis, the recommendation is strongly for the transfer of a single embryo. For women aged 35 to 37, a single embryo transfer is still strongly considered, but the maximum limit may increase to two embryos if the prognosis is less favorable. As age increases, the maximum number permitted for transfer rises to account for the diminished likelihood of implantation per embryo.
For example, a patient between 38 and 40 years old with a favorable prognosis may be limited to a maximum of two blastocysts, or three cleavage-stage embryos. For women aged 41 to 42, the maximum may increase to three blastocysts or four cleavage-stage embryos in cases with a less favorable prognosis. Regardless of age, if a patient is transferring an embryo that has been confirmed to be euploid through PGT, the guideline consistently limits the transfer to one embryo.
Risks Associated with Multiple Embryo Transfer
The primary reason for the strict limitations on the number of embryos transferred is the significant increase in medical risks associated with multiple gestations. A twin or triplet pregnancy is categorized as high-risk, carrying elevated dangers for both the mother and the babies compared to a singleton pregnancy. The most common and serious risk is preterm birth, which affects nearly 60% of twins and 90% of triplets.
Babies born prematurely often have low birth weight, which increases the likelihood of neonatal intensive care unit (NICU) admission and long-term developmental, respiratory, and learning issues. For the mother, carrying multiples significantly raises the risk of complications such as preeclampsia, a condition characterized by high blood pressure and organ damage. Other maternal risks include gestational diabetes, a higher rate of Cesarean section, and an increased risk of postpartum hemorrhage. The shift to eSET is a direct medical response to mitigate these well-documented and potentially severe complications.