In vitro fertilization (IVF) is a medical procedure where an egg is fertilized by sperm outside the body to create an embryo. While twins occur naturally in about one in 250 pregnancies, IVF has historically been associated with a significantly higher chance of multiple births, often exceeding 25% of pregnancies. This increased likelihood was tied to the procedures developed in early IVF practice.
The Primary Cause of Multiples in IVF
The main reason IVF historically led to twin and higher-order multiple pregnancies was the practice of transferring more than one embryo into the uterus. This procedure, known as Multi-Embryo Transfer (MET), was the standard approach in the early days of assisted reproductive technology. Doctors performed MET to compensate for the relatively low rate at which a single embryo would successfully implant in the uterine lining.
A single embryo transfer in the past carried a lower chance of pregnancy per cycle. This prompted the transfer of two or more embryos to increase the odds of at least one implanting. If two transferred embryos implanted, the result was a twin pregnancy; if three implanted, it resulted in triplets. This calculated risk aimed to maximize the patient’s chance of achieving pregnancy in a single, costly IVF cycle.
The Two Types of Twins Resulting from IVF
Twins conceived through IVF can be classified into two distinct biological categories, known as fraternal and identical twins. The most common type of multiple birth resulting from IVF is fraternal, or dizygotic, twins. This occurs when two separate embryos, which were transferred during the same procedure, both successfully implant and develop independently.
Fraternal twins are genetically distinct, sharing approximately 50% of their DNA, similar to any other siblings. The second, less frequent type is identical, or monozygotic, twins, which result from a single transferred embryo splitting into two separate embryos after implantation. This splitting event is biologically random and results in two genetically nearly identical individuals.
The risk of identical twins in IVF is slightly elevated compared to natural conception, occurring in about one to three percent of IVF embryos. This increase may be linked to laboratory procedures, such as extended embryo culture time or certain manipulations like assisted hatching. Therefore, even when only one embryo is transferred, a small possibility of an identical twin pregnancy remains.
How Modern IVF Minimizes Twin Pregnancies
Modern fertility medicine has largely shifted its focus to achieving a single, healthy pregnancy, primarily by implementing the practice of Single Embryo Transfer (SET). This change was driven by the understanding that multiple pregnancies, whether twins or higher-order, carry increased health risks for both the mother and the babies, including preterm birth and low birth weight. The goal is now a “singleton at term.”
SET has been made highly effective through advancements in laboratory technology that allow for better selection of the most viable embryo. Techniques like culturing embryos to the blastocyst stage (Day 5 or 6) and preimplantation genetic testing (PGT) help identify embryos with the highest potential for implantation. This improved selection process gives a single embryo a much greater chance of success, lessening the need to transfer more than one.
The widespread adoption of SET has dramatically reduced the incidence of multiple births from IVF. For example, the twin rate in the United States fell from 8.4% in 2011 to 2.3% in 2020. Similarly, the multiple birth rate in the United Kingdom decreased from around 28% in the 1990s to approximately 6% in 2019. These figures underscore the success of current protocols in minimizing twin pregnancies without compromising the overall live birth rate.