Two car seats, two jackets, two security blankets and two babies. No, you're not seeing double – they're twins. Take a look around a restaurant, a daycare or a pediatrician's office, and you may just see two children who look exactly alike. You may also see two children of the same age, perhaps of the opposite sex, who are dressed in the same colors or style. Are they twins, too? With one out of every 80 births being twins, they just may be.

There are several "types" of twins, but the two most common are fraternal and identical.

Fraternal twins – medically termed as dizygotic twins – occur when two offspring are born of the same pregnancy and develop from two separate ova that were released from the ovary and fertilized at the same time. Fraternal twins may be of the same or opposite sex and will differ genetically as well as in appearance.

"Fraternal twins are twins that come from different eggs," says Barbara Hanson, a professor of genetics at Canisius College in New York. "There are women who are known as 'multiple ovulators.' These women do not ovulate one egg each month – they ovulate multiple eggs each month. As two eggs are released at the same time, women who release more than one egg a month are 10 times as likely to give birth to twins than those who release only one. Multiple ovulation is one of the main reasons behind the birth of fraternal twins."

The occurrence of fraternal twins varies, and there are several factors that contribute to the presence of these twins, including ethnic origin and maternal age. The highest occurrence of fraternal twins is found in black women, while the lowest is found in Asian women. Women between the ages of 35 to 39 tend to give birth to fraternal twins twice as many times as those not within this age group.

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