Normally a woman will release just one egg from her ovary each month (women are born with millions of eggs in their ovaries), which then travels through her fallopian tube and enters the uterus waiting to be fertilized.
Hyperovulation occurs when more than one egg is released by a woman during her monthly menstrual cycle. This release of multiple eggs increases the chance of conceiving fraternal twins or even triplets.
Some women carry a gene, popularly called the “twin gene,” for this pattern of hyperovulation and can pass it on to their daughters.
The number of multiple births in the United States is on the rise, in correlation with the growing number of women using fertility drugs. (Fertility drugs increase the odds that more than one of the eggs will be fertilized.) Since the 1970s, the number of twin births has more than doubled. According to the Centers for Disease Control (CDC), currently one in 35 US births are twins.