Fraternal Twins

Twins are two children who share the same pregnancy and are born during the same birthing process. Fraternal twins, or nonidentical twins, are twins who are the [Page 689]products of two separate eggs fertilized by two separate sperm in contrast to identical twins who are formed from one sperm and one egg. Fraternal twins result in two independent zygotes that both implant in the uterus. Thus, fraternal twins are also known as dizygotic twins. In regards to genetics, fraternal twins are exactly like any other pair of siblings. They are extremely unlikely to have identical genes, can be the same or opposite gender, and can appear similar or different.

The rate of fraternal twins in the general population is estimated to be around two to 14 per 1,000 births.

As with all twins, fraternal twins are more likely to be born prematurely due to the size restriction of the female uterus. Such premature newborns are susceptible to more complications that full-term newborns. With current medical technology and the capability to successfully care for premature babies, twins generally do as well. The rate of fraternal twins in the general population is estimated to around two to 14 per 1,000 births, but is higher among couples who undergo infertility treatments. Fraternal twins are rare because women usually release one ovum per menstrual cycle. However, those who are undergoing in vitro fertilization may be implanted with multiple embryos to increase the chances of at least one taking hold in the uterus and those taking drugs such as clomifene are intentionally stimulated to release multiple eggs from their ovaries.

Therefore, these women have a higher chance of having fraternal twins. In recent years, there has been an increase in the rate of fraternal twin births. Some attribute this to changing practices in society where some women are having children at later ages. Such older woman are more likely to need fertility treatment and thus are more inclined to give birth to multiple babies.