About one in every 80 pregnancies results in twins. In the US identical twins are less common than what are called fraternal twins or non-identical twins. Geneticists prefer to call identical twins monozygotic because they arise from one fertilized egg. This makes fraternal twins dizygotic because they are just separate siblings who happen to gestate together. Twins are common enough to have inspired myths in every culture. Since the chances of being a singleton are so overwhelming, people ages ago had to explain something rare. They came up with some strange ideas. One was the belief that the woman had intercourse with her husband and with a deity, resulting in a demi-god and a mortal. If the deity was appreciated or feared, that could be a great benefit to the twins who would be given special favors or respectful distance. If the deity were an evil spirit it could be disaster for the twins (dumped into the river) and hard times for the mother who would be considered having practiced adultery with the evil spirit and thus she could be killed or banished.
These cultural traditions may have led to selection for or against twinning. Dizygotic twinning runs in the family but monozygotic twins are fairly constant around the world (about one in 400 pregnancies). A few years ago studies in mice showed identical twins were more likely to occur if the implanting preembryo was upside down (with the future embryo away from the uterine lining). We don’t know if this is true for humans. In Japan dizygotic twins are rarer than monozygotic twins and I suspect a past history of destroying twins in ancient Japanese culture.
In our century the incidence of twins is going up because infertile women are often given hormones to induce ovulation or, since the 1970s, they attempt in vitro fertilization and there’s about a 15 to 20 percent chance of having twins because three or four preembryos are implanted in this procedure to make sure that a pregnancy will occur. These twins are almost always dizygotic twins.
Identical twins have more risks during pregnancy than dizygotic twins. Dizygotic twins always have two separate membranes around each of the embryos or fetuses. Monozygotic twins in eighty percent of the cases, have a common outer sac and two inner sacs. The other twenty percent are indistinguishable from dizygotic twin membranes. Contrary to popular belief, most identical twins do not come from a divided egg that separates into two cells. They usually arise after implantation and involve the splitting of the preembryonic mass into two chunks. If this happens very late in the forming of the early embryo, conjoined twins may arise. We use the popular term, Siamese twins, for such children and they may be joined at the head, the abdomen, or the hips. If possible these conjoined twins are surgically separated. Identical twins may not look identical at birth. About 10% of them may be unequal in size, one looking ruddy and the other pale and emaciated. The smaller twin plays catch up and the two are usually the same size some six months after birth but in a few cases the difference remains for life. It should be no surprise that twin births have higher risks to mother and children than singletons.