Carrying a twin pregnancy is not an uncommon event. It occurs when the uterus carries two fetuses simultaneously rather than one. Having twins runs in some families and age also appears to be a contributing factor.
Women in their late 30s have a higher chance of having twins, usually due to release of more than one egg during the menstrual cycle. But with increasing use of assisted conception, there is a trend towards a disproportionate number of twin pregnancies.