The twin birth rate in the United States is close to 1 in 31, a nearly 40 percent increase since the early 1980s.

That’s because there are growing numbers of older moms (hormonal changes are believed to be responsible for the release of more than one egg at ovulation) and more successful fertility treatments.

Here are some other facts about twin pregnancies that you might not know, from Pregnancy Day by Day editor-in-chief Maggie Blott, M.B., B.S.; and consultant editor Paula Amato, M.D. (Dorling Kindersley):

• Research shows that 10 percent to 15 percent of all singleton births may have started off as twins; often one is lost early in pregnancy in a phenomenon known as “vanishing twin syndrome.” 
• The odds of having identical twins (the result when a single fertilized egg splits in two) are about 3 1⁄2 in 1,000. 
• If you’ve had nonidentical, or fraternal, twins (the result when two eggs are released at ovulation and both are fertilized) without having taken fertility drugs, your chance of having a second set is about 1 in 3,000. 
• More than half of twins are born before 37 weeks; plan your maternity leave accordingly so you don’t leave your employer in the lurch. 
• Twins conceived naturally (i.e., without <a href="https://glowing.com/glow-fertility-program">fertility treatment</a>) are most common in the African-American population, followed by Caucasians, Hispanics and Asians. Taller women are also more likely to have twins. 
• For most women pregnant with twins, gaining 24 pounds by week 24 is advised; a total weight gain of 35-45 pounds is generally desirable.