Adult dizygotic twins. These healthy, twenty-one year old, male twins are sitting on a garden bench. They are non-identical (dizygotic). In their mother's womb they were conceived simultaneously but developed from two separate eggs (ova). Dizygotic twins have their own placenta. The twins may be of the same or different sexes and may look quite different. Twins occur in about one in eighty pregnancies. Dizygotic twins are more likely to occur in older women, in women who have had many previous pregnancies, women who have a family history of twins in the family and those on fertility treatment. Photographed in Sweden.