Multiple pregnancies include twins, triplets, and higher-order pregnancies (four or more babies). When more than one egg is released during the menstrual cycle and each is fertilized by a sperm, more than one embryo may implant and grow in the uterus. Fraternal twins result from this type of pregnancy (non-identical). Splitting a fertilized egg results in multiple identical embryos. Pregnancies of this type result in identical twins (or more). Twins who are identical are less common than twins who are fraternal.
The use of fertility drugs to induce ovulation can result in multiple eggs being released from the ovaries, resulting in twins, triplets, or more. Multiple pregnancies may result from in vitro fertilization if more than one embryo is transferred to the uterus. Identical multiples can also occur when a fertilized egg splits during transfer. Older women are more likely to release two or more eggs during a single menstrual cycle than younger women. Consequently, they are more likely to become pregnant with multiples than younger women. Other reasons are if the patient has a family history of twins on her mother’s side or if she has already had multiple pregnancies.