The most common and under-appreciated risk of infertility treatments is multiple gestation pregnancies. Women who undergo infertility treatments are much more likely to become pregnant with more than one fetus than women who conceive naturally.
In assisted reproductive technologies (ART), multiple births are largely due to the practice of transferring multiple embryos back into the uterus during in vitro fertilization (IVF). They are preventable by limiting the number of embryos transferred.
When reproductive medicine was in its infancy, many fertility clinics transferred multiple embryos during each cycle, attempting to increase the rate of pregnancy. This technique resulted in many twin, triple, and even higher-order multiple births. Unfortunately, multiple births—including twin births—greatly increase the chances that a woman and her children will have poor health outcomes.
In 2010, 46 percent of all babies conceived through Assisted Reproductive Technologies were twins, triplets or more. The most common problems in the short term are that multiples are often preterm and the babies often have a low birth weight.
Premature babies are more likely to need intensive care and longer stays at the hospital and are at higher risk of dying shortly after birth. Birth impairments are also more common among multiples, although they affect a minority of the infants.
The longer-term consequences of preterm delivery and low birth weight include developmental disabilities such as cerebral palsy (CP). Some studies have found that the rate of cerebral palsy is five to ten times higher among multiple births than among singleton births. These risks are not limited to multiple births: babies that are born singletons but were part of a multiple pregnancy have some of them as well. Multiple pregnancies also carry a higher risk of health complications for the mother during pregnancy and birth.
And there are effects on the health of the family as well: studies show that divorce is much more common among couples that have had twins than among couples who have had singletons.
Finally, there are consequences for society at large, because the costs of the medical care for multiple pregnancies and infants born in multiple births are huge compared to the costs associated with caring for singletons.
Because of these risks, many experts in the field now encourage the strategy of transferring only one embryo at a time. A 2010 meta-analysis of clinical trials comparing the outcomes of double embryo transfers with the outcomes of a sequence of one single-embryo transfer followed by a thawed embryo transfer if the first cycle has not succeeded found single-embryo transfer more likely to lead to the birth of a single, healthy baby.
Though the first strategy—double-embryo transfer—leads to pregnancy faster, it is associated with multiple births and the related adverse health outcomes. The second strategy — single-embryo transfer — may require more attempts but almost invariably yields singleton births and better health outcomes for both mother and baby. The chance of a woman giving birth to a single full-term baby (over 37 weeks) following single-embryo transfer is almost five times greater than her chance of doing so following double-embryo transfer.
In the past few years, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology have issued increasingly stricter guidelines for the numbers of embryos that should be transferred back into the uterus. The fertility clinics have been responsive: in 1996, more than 60 percent of ART procedures entailed the transfer of four or more embryos, whereas in 2008 that proportion was reduced to 14 percent.
Currently, the largest number of procedures entails the transfer of two embryos. This trend has had an important impact on the number of high-order pregnancies. In 1996, seven percent of ART births were triplets or higher-order multiples, and this proportion came to slightly less than two percent in 2008. That is the main reason why the number of high-order multiples has declined nationally since 2003.
What has not happened, however, is a decrease in the number of twin births in ART: this can be accomplished only by promoting single-embryo transfer, and the United States has been slow at adopting this standard of care. The percentage of single-embryo transfers increased from six percent in 1996 to 12 percent in 2008. In contrast, in Sweden about 75 percent of embryo transfers are now single-embryo transfers. In Japan, all women under 37 years of age who have experienced no previous failure with ART must transfer only one embryo.
The main barrier to elective single-embryo transfers is cost. Many families are unable to pay for multiple IVF cycles, so they opt to have multiple embryos transferred in a single cycle. The first insurance company to address this problem, Aetna, recently marketed a benefit that offers a free “rescue” IVF cycle to women who choose elective single-embryo transfer if the first cycle fails.