New Jersey, Massachusetts, and Connecticut have the greatest rates of twin births. The national rate of twins in 2016 was 33.4 per 1,000 live births. That's about one in every 30 babies born. The number of twins has been increasing since the 1980s. This may be because more women are choosing to delay marriage and have children later in life. It may also be because more couples are staying together long enough to have more than one child. Another reason for the increase in twin births is error. Sometimes doctors make mistakes when performing procedures on single babies but not twins. These errors cause people to think that twins are born together when they aren't. About 10% of twins are born with different birth defects or problems. Finally, there are more twins being born today than ever before. This shows that twins are now becoming common rather than rare. Twins are the product of a single ovum that divided into two cells instead of one. The cells then developed into twins who share some genes but not all of them. This means that twins tend to have traits of both parents. For example, their eyes may be different colors or one of them might have blond hair while the other has dark hair. They also have a higher chance of having health problems later in life.