The final step of in vitro fertilization (IVF) is the embryo transfer, which determines the patient’s immediate chance of pregnancy. Deciding the number of embryos to transfer is the most consequential choice in the IVF process. This decision balances maximizing the probability of success with upholding patient and fetal safety. The number of embryos transferred directly influences the likelihood of a multiple gestation, which carries significant medical risks for both the parent and the children. This choice is made after evaluating individual medical history, embryo viability, and clinical guidelines.
Prioritizing Single Embryo Transfer
The medical community now strongly advocates for elective Single Embryo Transfer (eSET) as the preferred course of action for many patients. The rationale is rooted in achieving a healthy, singleton pregnancy, rather than simply achieving pregnancy itself. A singleton birth is recognized as the safest outcome for both the parent and the baby, resulting in fewer complications.
Multiple Embryo Transfer (MET) significantly increases the rate of twin and higher-order multiple births. Although some patients may desire twins, the associated health risks are substantial, leading medical guidelines to prioritize reducing this outcome. Clinics now aim to transfer the minimum number of embryos that offers a high chance of success, leveraging advances in laboratory techniques and embryo selection. This strategy allows for similar cumulative success rates over multiple cycles while reducing the dangers inherent in multiple gestations.
Patient Specific Factors Influencing the Decision
The determination of whether to transfer one or two embryos is an individualized process guided by clinical criteria and national guidelines. Maternal age is consistently cited as the most important factor influencing the maximum number of embryos recommended for transfer. For patients under 35 years of age with a favorable prognosis, medical societies like the American Society for Reproductive Medicine (ASRM) encourage a single embryo transfer.
As a patient’s age increases, the quality of oocytes and resulting embryos declines, leading to a higher rate of chromosomal abnormalities. For women aged 38 to 40, ASRM guidelines allow for the transfer of up to two blastocysts or three untested cleavage-stage embryos. Patients over 40 may be considered for the transfer of up to three blastocysts or four cleavage-stage embryos, reflecting the lower expected implantation rate.
The patient’s previous IVF history also plays a considerable role in the decision. Patients who have had a prior successful IVF pregnancy or those undergoing their first attempt typically have a favorable prognosis for eSET. Conversely, a history of multiple failed IVF cycles may lead clinicians to consider transferring two embryos, provided the patient is counseled on the increased risks of multiples.
Furthermore, if a patient has a coexisting medical condition, such as heart disease or uterine anomalies, a multiple pregnancy would significantly increase morbidity. In these cases, a single embryo transfer should be strictly adhered to.
How Embryo Quality is Assessed
The viability and quality of the available embryos are central to the transfer decision, often superseding other factors in high-prognosis patients. Embryos are assessed by embryologists using a morphological grading system that evaluates their physical appearance and developmental stage. This assessment helps predict which embryos are most likely to implant successfully.
The developmental stage at the time of transfer is classified as either the cleavage stage (typically Day 3) or the blastocyst stage (usually Day 5 or 6). A Day 3 embryo is generally a 6- to 8-cell structure. A Day 5 blastocyst is a more advanced embryo characterized by a distinct inner cell mass and an outer layer called the trophectoderm. Blastocysts have a higher implantation potential because they have survived a longer period of laboratory culture, allowing for a more rigorous selection process.
The grading system examines specific characteristics like the number of cells, the rate of cell division, and the degree of cellular fragmentation. For blastocysts, grading includes assessing the expansion of the fluid-filled cavity, the quality of the inner cell mass (which becomes the fetus), and the quality of the trophectoderm (which forms the placenta). An embryo of the highest quality, such as a top-grade blastocyst, is often the deciding factor in recommending eSET, as its superior viability offers a high success rate while minimizing the need for a second embryo.
Maternal and Fetal Risks of Multiple Gestation
The primary reason for the medical push toward eSET is the substantially increased health risk associated with twin and triplet pregnancies. Multiple gestations are strongly linked to preterm birth (delivery before 37 weeks), which is the most common and severe complication. For twins, the risk of preterm birth can be as high as 65%, compared to a much lower rate for singleton pregnancies.
Preterm delivery often leads to low birth weight, with twins having a nearly 57% chance of weighing less than 5.5 pounds at birth. Babies born prematurely or with low birth weight face an increased risk of long-term developmental issues, chronic lung disease, and cerebral palsy. Neonatal complications are also more likely, with nearly a third of all twins requiring admission to a neonatal intensive care unit (NICU).
Carrying multiples elevates the risk of several serious maternal health complications for the expectant parent. Preeclampsia (characterized by high blood pressure and organ damage) and gestational hypertension are more frequent in multiple pregnancies. The risk of gestational diabetes is also higher. Multiple gestations increase the likelihood of needing a Cesarean section delivery and can lead to complications like postpartum hemorrhage.