In Vitro Fertilization (IVF) is a widely recognized fertility treatment. While IVF presents a higher chance of multiple pregnancies compared to natural conception, this outcome is not an unavoidable aspect of the treatment itself. It often stems from specific practices employed during the IVF process, which aim to maximize the chances of a successful pregnancy. This article explains the reasons behind the common occurrence of twins in IVF.
The Role of Multiple Embryo Transfer
One primary reason IVF has been associated with a higher rate of twin pregnancies is the practice of transferring more than one embryo into the uterus during a single cycle. Historically, fertility clinics transferred multiple embryos to improve the chance of at least one implanting successfully. Each embryo transferred carries the potential to implant and develop, directly increasing the probability of a multiple gestation. When two distinct embryos are transferred and both implant, the result is dizygotic, or fraternal, twins. While the transfer of multiple embryos has been a common strategy, there has been a growing shift towards single embryo transfer (SET) to reduce the risks associated with multiple pregnancies. This aims to achieve a singleton pregnancy, which is generally considered healthier for both the parent and the babies.
Influencing Factors in Embryo Transfer Decisions
The decision to transfer multiple embryos is influenced by several patient-specific and clinical factors. A patient’s age is a significant consideration; younger patients typically have higher implantation rates per embryo, while older patients might receive more embryos to compensate for potentially lower implantation rates. The quality of the embryos, assessed by their morphology and genetic testing results, also plays a role; higher quality embryos may reduce the need for multiple transfers. The number of previous unsuccessful IVF cycles can additionally sway the decision, with more embryos sometimes transferred after repeated failures to improve success odds. Clinic protocols and national guidelines also shape embryo transfer practices, balancing the desire for high pregnancy success rates with minimizing the health risks associated with multiple gestations. For instance, the American Society for Reproductive Medicine (ASRM) now recommends single embryo transfer for patients with a good prognosis. This approach, known as elective single embryo transfer (eSET), is designed to mitigate the risk of twins while striving for favorable pregnancy outcomes, particularly in patients with promising prognoses.
Types of Twin Pregnancies in IVF
IVF can result in both fraternal and identical twin pregnancies, though the mechanisms for each differ. Fraternal, or dizygotic, twins are the most common type of multiple birth resulting from IVF. This occurs when two separate embryos, transferred simultaneously, successfully implant and develop. These twins originate from two distinct eggs fertilized by two distinct sperm, leading to genetic differences similar to any other siblings. Identical, or monozygotic, twins, on the other hand, occur when a single embryo splits into two separate embryos after implantation. While less common than fraternal twins, there is a slightly increased incidence of monozygotic twinning in IVF compared to natural conception. The exact reasons for this increased rate are not fully understood, but potential contributing factors include specific culture conditions in the laboratory or techniques like assisted hatching, which involves making a small opening in the embryo’s outer shell before transfer.