In Vitro Fertilization (IVF) is a fertility treatment where an egg is fertilized by sperm outside the body, in a laboratory, before the resulting embryo is transferred to the uterus. While the primary goal of IVF is to achieve a successful pregnancy, understanding the likelihood of a twin pregnancy is a common consideration. This article explores the chances of conceiving twins through IVF and the factors that influence this probability.
Understanding the Likelihood
The likelihood of having twins with IVF has changed significantly due to evolving medical practices. In the past, IVF treatments had a substantially higher rate of multiple births, with up to 20-25% of deliveries being twins, compared to about 3.39% of naturally conceived births in 2014.
In the 1990s, approximately 28% of IVF births resulted in twins, but this figure has decreased significantly. By 2019, the multiple birth rate from IVF in the UK reached an all-time low of around 6%. In Australia and New Zealand, the multiple birth rate following assisted reproductive technology (ART) decreased to 2.7% in 2022. This reduction reflects a global shift towards safer practices in fertility treatment, aiming for single healthy pregnancies.
Key Influencing Factors
Several factors determine the chances of conceiving twins through IVF. The most impactful factor is the number of embryos transferred during an IVF cycle. Transferring more than one embryo directly increases the probability of a multiple pregnancy, with twin birth rates reaching 20-30% when two embryos are transferred. This practice was once common to improve overall pregnancy success rates.
Patient age also influences the likelihood of twins. Younger patients (under 35) often have higher quality eggs and embryos, leading to better implantation rates per embryo. If multiple embryos are transferred, the chance of both implanting and resulting in twins increases. Older patients may have more embryos transferred to compensate for lower viability, but advanced maternal age can be associated with a lower rate of identical twinning from a single embryo.
Embryo quality and its developmental stage also play a role. High-quality embryos are more likely to implant successfully. Transferring embryos at the blastocyst stage (day 5 or 6) generally yields higher implantation rates compared to cleavage-stage embryos (day 2 or 3). If multiple high-quality blastocysts are transferred, the chances of a twin pregnancy can increase.
The type of IVF cycle also plays a role. Some studies suggest that fresh embryo transfers might slightly increase the risk of identical twins compared to frozen embryo transfers. Freezing and thawing processes may alter the embryo, reducing its propensity for spontaneous splitting, which is the mechanism for identical twin formation.
Potential Considerations of Multiple Pregnancies
Multiple pregnancies carry increased medical risks for both the mother and the babies. Maternal risks include a higher incidence of pre-eclampsia, a serious condition involving high blood pressure, and gestational diabetes, which develops during pregnancy. Mothers carrying twins also face increased risks of preterm labor, necessitating an earlier delivery, and a higher likelihood of requiring a C-section.
For infants, the primary concern is prematurity, with about 60% of twins born prematurely. Premature babies are at higher risk of low birth weight, respiratory distress syndrome, and requiring a stay in the neonatal intensive care unit (NICU). There is also an increased risk of long-term health issues such as cerebral palsy and other developmental challenges. In identical twins sharing a placenta, Twin-Twin Transfusion Syndrome (TTTS) can occur, where blood flow is unevenly distributed between the fetuses.
Managing the Probability
To mitigate the risks of multiple pregnancies, fertility clinics increasingly recommend strategies to reduce the likelihood of twins. Elective Single Embryo Transfer (eSET) is a key approach, involving the transfer of only one high-quality embryo into the uterus, even when more viable embryos are available. The goal of eSET is to achieve a healthy singleton pregnancy, avoiding complications while maintaining good pregnancy success rates.
eSET is recommended for patients with a good prognosis for a successful pregnancy, such as younger women (under 35) or those with high-quality embryos. This strategy has been instrumental in the decline of multiple birth rates from IVF globally. The decision regarding the number of embryos to transfer is personalized, made in consultation with a fertility specialist. This discussion considers individual factors like age, embryo quality, previous IVF history, and potential risks and benefits to ensure the best possible outcome.