A study published Dec. 5, 2013 in the New England Journal of Medicine detailed the growth in multiple births resulting from assisted reproduction methods. It also revealed a surprising finding:  fertility drugs are the culprit behind most multiple births, not in vitro fertilization (IVF), as was previously thought.

“Fertility clinics have become a lot better at reducing the number of embryos that are put back in a woman’s uterus after IVF, a trend that has contained the number of multiple pregnancies after IVF, especially triplets and higher-order multiples,” says epidemiologist Maurizio Macaluso, MD, DrPH. “By contrast, little can be done to reduce the number of embryos that result from combining ovarian stimulation and intrauterine insemination, a very popular form of infertility treatment.”

Macaluso, who is Director of Biostatistics and Epidemiology at Cincinnati Children’s, joined with colleagues at institutions around the country in the study. Their goal was to understand and try to halt the upward trend in multiple births. Even with advances in pre- and post-natal care, multiple births are risky for both moms and babies.

“The size of a healthy full-term baby is almost at the anatomical limits of the mother’s womb, and to accommodate a multiple pregnancy the babies have to be smaller and must be delivered earlier,” Macaluso says. “This results in increased risk of adverse health outcomes for both mother and children.”

The researchers compared data on multiple births from a period before and after fertility treatments became available in this country. Among their findings: by 2011, 36 percent of twins and 77 percent of triplets, quads and quintuplets were born as a result of fertility treatments, including fertility drugs.

The researchers found that multiple births from IVF dropped after 1998 following guidelines limiting embryo transfers in IVF issued by the Society for Assisted Reproductive Technology. However, non-IVF treatments such as fertility drugs continued to cause multiple births. Non-IVF twin births increased by 3 percent and triplet births increased from 36 percent to 45 percent between 1998 and 2011.