How Common Are Twins? For Beyoncé And Other Women, Genetics And Fertility Drugs Play A Role

Twins have always been a captivating occurrence, appearing in different mythologies throughout history and across the globe. With superstar Beyoncé announcing her multiple pregnancy in the beginning of February, the birth phenomenon has not failed to intrigue the public and beg the question: How common are twins?

Twins are born when a woman has two or more embryos in her uterus at the same time, eventually giving birth to fraternal, identical or conjoined babies. Twin births are more common than expected: Recent U.S. Census data from 2015 found the chances of having twins was 33.5 per 1,000 live births, while triplet or higher order births were born at a rate of 103.6 per 100,000 live births. Identical twins specifically occur at a rate of 3 to 4 per 1,000 live births around the world.

The odds of a woman having twins vary based on several factors such as environmental influences and genetics. In vitro fertilization (IVF) and fertility drugs used to conceive also increase the chance of twins.

IVF is a process that involves the conception of sperm and eggs in a laboratory instead of in a woman’s reproductive system. After an embryo has formed, it is then inserted by a medical professional in a woman's uterus.

"There is a risk of multiple pregnancies when more than one embryo is placed into the womb. Carrying more than one baby at a time increases the risk of premature birth and low birth weight," according to MedlinePlus, which is produced by the National Library of Medicine.

The chances of conjoined twins are also possible but much rarer: About one in every 200,000 live births produces twins whose internal organs and skin merged together during the mother’s pregnancy. The odds of conjoined twins surviving birth are equally rare, with around 40 to 60 percent stillborn. Nearly 35 percent do not live past the first day.