Twins are two people who have shared the same womb at the same time. They may also have come from the same egg. Twin births are becoming more and more common due to several factors such as increasing use of fertility treatments, child births at an older age, and even genetic factors. Women who are slightly above-average in build are also likely to have multiple births or twins. Certain ethnic groups are also found to have a greater twin population. One in every 250 births in the world could be an identical twin birth.

Identical twins are also known as monozygotic twins. They are from a single egg that forms a single zygote that splits into two embryos. Two fetuses emerge from the two embryos in the same womb. The exact reason for this splitting hasn’t been identified yet. Sometimes, identical twins also share the same amniotic fluid, making them monoamniotic. Otherwise, they are diamniotic. Twins sharing the same placenta are monochorionic. Those who don’t are dichorionic. This happens only in the case of identical twins. All monoamniotic twins are also monochorionic. This depends on the stage at which the zygote divides. Twinning at the earliest stages would result in diamniotic and dichorionic twins. Twinning that occurs four to eight days after fertilization and eight to twelve days after fertilization results in monochorionic-diamniotic and monochorionic-monoamniotic, respectively. Twinning twelve days post-fertilization may result in conjoined twins. Conjoined twins are attached to each other at some place. They may be more dependent on each other physically.

Since identical twins share the same sac, amniotic fluid, and placenta, there could be some complications in pregnancy. These could be a result of the entanglement of the umbilical cords or twin-to-twin transfusion syndrome because of the shared placenta. This may cause one or both the babies to be underdeveloped.

Identical twins are generally of the same sex. They have identical DNA and shared genes. They may be very similar to each other, almost like mirror images. Some twins can be told apart only by their fingerprints, teeth, handwriting, or any other intricate observations. However, they have individually different personalities and character traits. Twins are generally emotionally attached to each other and are also believed to live longer because of this attachment. Some people even believe that identical twins have their own language through which only the two of them can communicate. Some identical twins are also mirror twins—they are exact mirror images of each other. About one-fourth of all identical twins are mirror twins. Sometimes, one mirror twin may have a condition where some or all the internal body organs will be on the opposite side of the body. However, these are considered to be birth defects and are extremely rare.