This is purely due to the use of fertility drugs like Clomid , which stimulate the ovaries to release more than one egg at a time. Without using fertility-enhancing drugs, twins occur naturally in about 1 in 60 births. About two-thirds of those are fraternal twins, those born from two separate eggs. The only way to be really sure that your twins are fraternal is if one is a boy and the other's a girl. But half of fraternal twins are the same gender.
Some people think that having twins runs in families or skips a generation.
Fraternal twinning does tend to run in families, carried by the tendency for some women to release more than one egg at a time. In the days before ultrasound was used as widely as it is today, it was not at all uncommon for twins to be diagnosed in the delivery room. Nowadays, most twins are diagnosed in the second trimester when ultrasounds are usually performed. The mother's belly is usually pretty similar in size to women carrying singletons until six months or so. The main problem with twins is that the uterus is designed to carry only one baby.