In Vitro Fertilization (IVF) is a fertility treatment where eggs are fertilized by sperm outside the body. This process involves stimulating the ovaries to produce multiple eggs, retrieving them, and combining them with sperm. Resulting embryos are cultured for several days before one or more are transferred into the uterus to establish a pregnancy. While IVF helps many achieve parenthood, a consideration is the potential for multiple pregnancies like twins or triplets. Reproductive medicine advancements aim to optimize IVF protocols for a healthy singleton pregnancy, minimizing associated risks.
How Multiple Births Occur in IVF
Multiple pregnancies in IVF primarily arise from two mechanisms. Historically, transferring multiple embryos into the uterus during a single IVF cycle was common. This approach aimed to increase pregnancy chances, as not all transferred embryos implant. If more than one implanted, a multiple pregnancy resulted.
Even with a single embryo transfer, a small chance of identical (monozygotic) twins remains. This occurs if the single embryo spontaneously splits after implantation, leading to two genetically identical fetuses. This can happen in any pregnancy, natural or IVF. While the exact cause of increased monozygotic twinning in IVF is not fully understood, it is influenced by factors like extended embryo culture to the blastocyst stage and certain laboratory procedures.
Key Methods to Minimize Multiple Births
Modern IVF strategies prioritize reducing multiple pregnancies, primarily through Single Embryo Transfer (SET). This involves transferring only one embryo per cycle, the primary method for preventing twins or higher-order multiples. Elective Single Embryo Transfer (eSET) is when a patient chooses to transfer a single embryo even if other viable embryos are available. This decision is often made with high-quality embryos, significantly lowering multiple birth risks while maintaining favorable success rates.
Preimplantation Genetic Testing for Aneuploidy (PGT-A) facilitates successful SET. PGT-A tests a small biopsy from an embryo to determine if it has the correct number of chromosomes. Identifying chromosomally normal (euploid) embryos increases confidence in transferring a single, healthy embryo, reducing the need for multiple transfers to compensate for abnormalities. This allows for precise embryo selection and contributes to higher implantation rates for single transfers.
Optimized embryo selection also contributes to SET’s effectiveness. Advances like extended embryo culture to the blastocyst stage (day 5 or 6) allow embryologists to observe development longer. This helps identify embryos with the highest developmental potential and implantation capability. Selecting the best quality embryo maximizes success chances with a single transfer.
Personalized Approach to Embryo Transfer
The number of embryos to transfer in an IVF cycle is highly individualized, considering various patient-specific factors. Patient age is an important consideration, as embryo quality generally decreases with advancing maternal age. Younger patients, typically under 35, often have embryos with higher implantation potential, making SET a recommended option. For older patients, the decision may involve a more nuanced discussion, though eSET remains beneficial.
Embryo quality and developmental stage are also important factors. Embryos are graded based on appearance and development rate, with higher-quality embryos having a greater chance of successful implantation. If a patient has one or more high-quality blastocysts, a single embryo transfer is often recommended. A patient’s reproductive history, including previous IVF cycles, successful pregnancies, or recurrent implantation failures, also guides this decision.
A patient’s overall medical history and pre-existing health conditions are carefully evaluated. Conditions exacerbated by multiple pregnancy, such as gestational diabetes or hypertension, may indicate a single embryo transfer to minimize maternal and fetal risks. Clinics also adhere to professional guidelines, like those from the American Society for Reproductive Medicine (ASRM), which recommend the maximum number of embryos to transfer based on age and prognosis to reduce multiple birth rates.
Success Rates and Outcomes with Single Embryo Transfer
A common patient concern is whether transferring fewer embryos reduces pregnancy chances. However, advancements in IVF technology, including improved embryo culture and widespread PGT-A, mean SET can achieve comparable or even higher success rates per embryo transferred, especially with high-quality embryos. Studies indicate SET can result in live birth rates of 50-60% per cycle for patients under 35. Some research even suggests transferring multiple embryos of varying quality might reduce pregnancy chances.
The objective of modern IVF is to achieve a healthy singleton live birth, not just a pregnancy. SET significantly improves this outcome by reducing risks associated with multiple pregnancies, such as preterm birth, low birth weight, and maternal complications like pre-eclampsia and gestational diabetes. The rate of twins after eSET is very low, around 1-2%, comparable to spontaneous pregnancies.
Even if an initial SET cycle does not result in pregnancy, cryopreserved embryos allow for subsequent frozen embryo transfers. This often leads to high cumulative success rates over multiple cycles, offering a similar or better chance of a healthy live birth over time compared to transferring multiple embryos in a single fresh cycle, but with reduced risks. Focusing on cumulative live birth rates emphasizes long-term IVF success while prioritizing mother and baby safety.