The increase in the twin birth rate, however, does seem to be leveling off. “The 2012 twin birth rate was 33.1 per 1,000 total births, and was essentially unchanged from 2009-2011,” another CDC report found.
That is, in part, because assisted reproductive technology procedures have changed. For example, the number of embryos implanted during IVF has a direct relationship with the number of multiple births. So, implanting two (or even three) embryos increases the chance that at least one of them will grow and develop, but simultaneously ups the risk of a plural pregnancy.
In 2000, three or more embryos were implanted in two-thirds of IVF cycles. During the 2000s, though, the Society for Assisted Reproductive Technology and American Society for Reproductive Medicine published guidelines that recommended fewer embryo transfers, particularly for younger women. So, from 2003 until 2010, one study found that the average number of embryos transferred had fallen from 2.6 to 2. In 10 percent of cases, the prospective parents chose to go with a single embryo transfer, too, which was unheard in earlier decades.
These changes have slowed the increase in the twin birth rate, but have not reversed it yet. Still, every year, more than 50,000 extra twins are still born, joining the million others driving their parents crazy.
Article Appeared @http://www.theatlantic.com/health/archive/2014/04/1-million-extra-twins-have-been-born-in-the-last-31-years/360849/