Fraternal and Identical Twins

In perhaps 5 to 10 percent of cycles, two eggs are ovulated, one from each ovary or both from the same one. If both eggs are fertilized and then implant and develop, which occurs in less than 1 percent of cycles, two-egg, or nonidentical or fraternal twins result. Since two eggs and two sperms are involved, the twins are no more genetically alike than single-born siblings. One could say that fraternal twinning is not true twinning, since “twin” comes from the Old English “getwin,” dividing into two. Fraternal twins are more common among certain populations. In the United States, twin births occur naturally in about 1 in every 73 births among African Americans and 1 in every 93 among Caucasian women. Twins also reflect a family tendency on the mother’s side; a female relative of a woman who has fraternal twins has an increased chance of having fraternal twins herself.True, or identical, twinning is an even rarer event, accounting for only about 30 percent of twins. Identical twins are born approximately once in 250 to 300 pregnancies. There appear to be no geographic, racial, or hereditary factors in the occurrence of identical twins. It appears to be an accidental event. Identical twins develop when a single ovum, fertilized by a single sperm, divides into two. The genetic material contained in each twin is thus exactly the same-hence the term identical twins. This division can occur any time between two days and two weeks after fertilization. If the division occurs later in this time frame, it is likely to be incomplete and cause the twins to be conjoined.