Twin and triplet births are on the decline. Here's how it breaks down by state.

In 2004, of the people younger than 35 who gave birth with the help of IVF, 32.7% delivered twins, and 4.9% delivered triplets, according to doctors at the Washington University School of Medicine. In 2014, 11.8% of those births were twin deliveries, and 0.4% were triplet deliveries. But there was an increase in the percentage of those undergoing IVF who had live births—from 36.6% to 48.7%—meaning more IVF babies were born, but a larger percentage were singles instead of multiples.

This decline in multiple births resulted from technological advancements in the embryo culture media (the solution that creates an environment for the cells to grow in) and controlled ovarian stimulation, wherein a pregnant person receives medication that encourages their body to produce more eggs. With more eggs to choose from and a better environment to grow embryos, IVF patients have had more success while implanting fewer embryos in one round.

In the greater population, this has contributed heavily to a decrease in triplet and higher-order births. This is good news for moms and their babies, as births of multiples can drive health complications such as premature labor, high blood pressure, and fetal growth restriction, according to the Cleveland Clinic.

The American Society for Reproductive Medicine has followed the science and recommends IVF patients limit the number of embryos they implant. Its 2021 guidance document states that most patients with a favorable prognosis should implant no more than one embryo at a time.

New parents across the country find themselves caring for twins, triplets, or more—whether through IVF, other ART, or unassisted conception. However, residents in some states give birth to doubles and higher more often—Michigan, Connecticut, and Alabama lead the way in the rate of multiple births, with several Midwestern states close behind.

Several factors can increase the odds of multiple births. Families with college educations and household incomes of at least $100,000 are more likely to spend more on IVF treatment to start or expand their families. Race and ethnicity also impact these chances, as white and Asian adults are more likely to say they or someone they know has used fertility treatments, according to the Pew Research Center.

Some states mandate that health insurance companies cover at least part of families' fertility journeys (with several caveats and nuances in each law), but others actively discourage the IVF industry as it currently operates. The Alabama Supreme Court ruled in February that surplus embryos, which are routinely frozen and stored as part of the IVF process, should be considered children. Therefore, if someone destroys an embryo, they could be held liable for wrongful death, according to CNN.

The Alabama legislature scrambled to protect the state's IVF practitioners, but the debate continues. In June, Southern Baptists, the largest Protestant denomination in the U.S., voted to condemn the current IVF process and encourage members to consider the moral and ethical questions surrounding the procedure.

As laws and public perception of ART evolve, they will continue to impact private conversations and decisions about fertility and family—as well as the rate of multiple births in the U.S.

Story editing by Mike Taylor. Additional editing by Kelly Glass. Copy editing by Paris Close. Photo selection by Ania Antecka.