One believes that the multiple pregnancies happen of two ways. The most frequent thing is that the woman liberates two ova in a brief space and both are fertilized. The resultant babies are twins heterozigóticos (two ova), commonly called twins. But also it can happen that the only fertilized ovum splits into two, a species of "accident" in the prenatal development. The babies who result from this cellular division are twins homozigóticos (an ovum), often known like identical twins. The triplets, fourfold and other multiple pregnancies can be a consequence of these two processes.

The twins homozigóticos have the same hereditary composition and they are of the same sex but they differ in certain aspects partly due to his prenatal and postnative experience. For example they can be different as for the temperament (disposition or way of docking and of reacting to the situations). In some physical characteristics as the curls of the hair, the teeth... they are usually identical. The twins heterozigóticos, created by different sperms and generally different ova are so similar as any other pair of brothers and they even can be of different sex.

The twins homozigóticos constitute a third part of the whole of the multiple pregnancies. It is something that happens equally in all the cultures The twins heterozigóticos are more frequent in the African Americans, the white persons of the north of Europe and the native ones to the east of the indies, being very little frequent in other Asians. It is probable that hormonal differences between women of different ethnic groups are the cause.

In societies like that of the USA, the incidence of multiple pregnancies has increased vertiginously. Enter 1980 and 1997, the births of alive twins increased 52 % and the number of triplets and other multiple pregnancies it was quadrupled. Meanwhile, the number of the only fetuses alone increased 6 %. In Spain they produce the double of multiple pregnancies to themselves that the European average.