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Twins are two offspring produced by the same pregnancy.[1] Twins can either be monozygotic ("identical"), meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic ("fraternal"), meaning that they can develop from two different eggs, each are fertilized by separate sperm cells.[2]

In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple
The twin birth rate in the United States rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births.[4] (The first successful 'test tube baby' was born in 1978.) The Yoruba have the highest rate of twinning in the world, at 45–50 twin sets (or 90–100 twins) per 1,000 live births,[5][6][7] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.[8][9]

In Central Africa there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.[10] In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6–9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.[10]

Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.[11]

Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-ovulate. There is no known genetic link for identical twinning.[12] Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births
Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning.

The three most common variations are all dizygotic (fraternal):[14]

Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
Female–female dizygotic twins (sometimes called "sororal twins").
Male–male dizygotic twins.
The other two variations are monozygotic ("identical") twins:[14]

Female–female monozygotic twins.
Male–male monozygotic twins (less common).
Male–female monozygotic twins (without chromosomal abnormalities) (Very rare, only few cases known).
Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,[15] while it is 1.07 males/female in Italy.[16] However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.
Dizygotic (DZ) or fraternal twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular. Fraternal twins are, essentially, two ordinary siblings who happen to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings. Instead of fraternal twins, this situation can also occasionally result in a chimera, someone colloquially described as their own fraternal twin.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Even if they happen to have the same chromosome profile, they will always have different genetic material on each chromosome, due to chromosomal crossover during meiosis. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other.

Studies show that there is a genetic basis for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.[8]

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[17] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.