An analysis of three decades of twin births (2009 data are the most recent year included) finds large increases in twin birth rates for all age groups, among all racial and ethnic groups and in the U. S. The overall U.S. twin birth rate increased 76%, from 18.9 in 1980 to 33.2 per 1,000 births in 2009.
And for mothers age 35 and over, those twin rates have skyrocketed. Among women 35-39, rates rose by nearly 100%, and among those 40 and over, rates rose more than 200%, finds the report from the National Center for Health Statistics, part of the Centers for Disease Control and Prevention.
While the chance of having twins does increase with a mother's age, the report suggests that fertility treatment provides the backdrop for this baby bonus, says CDCstatistician Joyce Martin.
"We found that about one-third of the increase over three decades was attributed to older maternal age and two-thirds likely the result of fertility-enhancing therapies — both drugs and technologies like in-vitro fertilization," she says.
As for women in their 20s who had twins, Martin says a small proportion of women in that age group do receive fertility treatment.
The analysis finds that although the pace of these increases in twin birth rates slowed from 2005 to 2009, twins now make up 3.3% of all births, up from 1.9% in 1980. The number of twin births increased from 68,339 to more than 137,000 in each year between 2006 and 2009.
As pregnancy rates with reproductive technologies have improved, we've been working hard at Reproductive Partners over the past several years to reduce the multiple birth rate. We can do that by returning fewer embryos to the woman's uterus. In many cases it's possible to achieve IVF success by transferring one embryo in women with a favorable prognosis.