Twins are two individuals who have come from the same womb at the same time. Almost every kind of species on this earth can produce multiple births at the same time. Twins are quite rare, accounting for around 1.2% of all human births. In short, one in every eighty births is a twin birth. The birth of identical twins is even more rare, rating at one in every 250 births. Twin pairs may be both boys, both girls, or one boy and one girl.

Birth of twins may be different based on several factors. Some ethnic groups and people in certain places were found to have a greater number of twins. Use of fertility drugs is also supposedly one cause for the birth of twins, as these drugs lead to hyperovulation (release of multiple eggs). Thus, women undergoing fertility treatments are more likely to have multiple pregnancies.

Genetic factors are also believed to play a part in multiple pregnancies. Women who are between the ages of thirty and forty and who have an above-average body height and weight are also likely to give birth to twins. Women who had several previous pregnancies and those who have recently stopped oral contraceptives may also give birth to multiple babies.

Twins can be categorized into many kinds: fraternal twins or dizygotic twins (non-identical), identical or monozygotic twins (from one egg that splits), vanishing twins (when one fetus fails to survive), conjoined twins (monozygotic twins with bodies joined at some place), parasitic twins (one fetus depends upon the other for survival), mirror twins (mirror image of each other), and miscarried twins. Twins have a shorter gestation period of around thirty-six weeks as compared to forty weeks for a single baby.

Twins, even identical ones, have different personalities; there would be at least a slight difference in them. However, they may have identical brainwave patterns. Some people also believe that twins have their own twin language through which they communicate with each other.

Twin pregnancies are generally more difficult, due to greater weight as well as extra placenta and amniotic fluid. One or both of the twins may be found in an imperfect position inside the womb due to limited space. One or both of them can come out with their bottom or feet first. Twins are also smaller and weaker as compared to normal babies at the time of birth. Most twin births are problem free.

These days, there are magazines, shows, and whole websites dedicated to twins. These contain useful information, facts, and trivia about twins that are also useful for the mothers who are expecting twins.