There are two types of twins: identical and fraternal (non-identical). Scientists use the terms "monozygotic" and "dizygotic" because these terms are biologically meaningful. Monozygotic twins result from a single fertilized egg that, sometime during the first week following conception, divides into 2 embryonic discs, which develop into 2 separate embryos; dizygotic twins result from 2 eggs being fertilized during the same menstrual cycle.
Monozygotic (identical) twins possess characteristics that make them virtually indisguishable. Because they come from the same egg and sperm, monzygotic twins are genetically identical, similar in physical appearance, and the same sex. Physical differences are caused by environmental factors inside the womb.
Dizygotic (fraternal) twins arise from different eggs and sperm, look as different as siblings born at different times would look, and can be either the same or different sex. A dizygotic pregnancy does not always result from the same act of sexual intercourse, which means it is theoretically possible for dizygotic twins (as well as triplets and other higher-order pregnancies) to have different fathers.
Most twins, about two-thirds, are dizgotic, the other one-third monozygotic. The frequency of dizygotic twins varies widely among different populations. The monozygotic twin rate is remarkably constant among different ethnic groups and worldwide.
Occasionally, identical twins do not separate completely in the embryonic disc phase of devlopment and are born conjoined (i.e., attached to each other with shared organs). Conjoined twins are commonly known as "Siamese twins," named after conjoined twins who were born in Thailand (formerly Siam) in the early 1800s. Conjoined twins are described according to the body region where they are attached (e.g., thoracopagus twins are twins joined at the chest). In some cases, the twins are connected only by their skin. Some conjoined twins are successfully separated by surgery. It has been estimated that about 1 out of every 40 monozygotic twin pregnancies results in conjoined twins.
Other Multiple Births
If there are more than 2 ova, or a single ovum splits into 3 or more ova, higher-order pregnancies (e.g., triplets, quadruplets, and so on) result.
Triplets occur in the general population about once out of every 8100 pregnancies. They can develop in one of three ways:
- From a single zygote that splits into three (identical or monozygotic triplets)
- From 2 zygotes, one of which splits (identical twins and a singleton)
- From 3 separate zygotes
In the latter two cases, the triplets can be the same or different sexes. Similar combination occurs in quadruplets (4 fetuses), quintuplets (5), sextuplets (6), and septuplets (7).