Identical twins
Identical twins occur when a single egg forms a zygote, which is the initial stage of a new organism, and consists of only one cell, and the zygote then divides itself into two embryos. This division invariably happens within eight days of fertilization, but if it happens after this time, then conjoined twins are the usual result.
Identical twins which are referred to technically as Monozygotic twins, generally have the same physical appearances and traits, and are usually the same sex. They are often not exactly the same, but they do have almost the same DNA. This can be influenced inside and outside the womb, and may alter or switch off certain genes, thereby enhancing differences between the two. Identical twins are generally not a hereditary trait, but an anomaly that occurs in about three in every one thousand births. This can occur whatever the race.
Both embryos develop into foetuses which share the same womb. When one cell divides and separates due to fertilisation of one cell, the result is two identical cells. Most of the time the zygote splits after two days and the twins share not only a placenta, but two separate sacs. If the split is earlier than this, each cell is likely to develop its own placenta and sac.
Conjoined twins occur when the split is relatively late. Unfortunately, mortality rates are higher in conjoined twins due to the complications which arise from sharing organs. The increase of the use of fertility drugs has resulted in a rise in identical twin births, but the chances are still about one in ninety.
