Twin

The term twin referring to people refers to two individuals who have shared the same uterus (womb), usually born on the same day but not necessarily. There are two different kinds of twins: identical and fraternal.

Identical twins occur when a single egg is fertilized and then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb, and generally the same amnion. These twins are identical genetically unless there has been a mutation in development, and are necessarily the same gender.

Fraternal twins occur when two eggs are released by the mother at the same time, within the same menstrual cycle, or within one menstrual cycle of each other. These twins are no more similar genetically than any siblings and develop in separate amnions, with separate placentae. They may be of mixed gender or the same gender.

Twin studies refers to the practice of assessing identical twins for medical, genetic, or psychological studies to try to find innate similarities and differences. Twins that have been separated early in life and raised in separate households are the most sought-after for these studies.

Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Historically, triplets have been quite rare and more than triplets so rare as to be almost unheard of. However, modern fertility drugs often stimulate release of multiple eggs by the mother, sometimes resulting in multiple fetuses.