If as a result of one pregnancy two or more children are born, then call them twins. Development of germs in a womb of mother can happen differently, there are monoovular also nonidentical twins who differ in that how many ova gave them life.
In the wild nature at many animals many cubs at the same time often are born, but at people one case of birth of twins is the share of 250 cases of the birth of one child. Scientists distinguish monogerminal and dizygotic twins, and in informal conversation call them monoovular and raznoyaytsevy or twins and twins.
Monoovular twins
Monoovular twins develop from one impregnated ovum which was divided into two or more parts which began to develop separately. Thus, all germs receive an identical set of genes and develop approximately equally. Such children always have one floor and are very similar at each other, even parents cannot sometimes distinguish them. Though sometimes they have mirror lines: for example, at one birthmark on the right side, at another with left, one right-handed person, and other lefthander. They have similar traits of character, almost identical temperaments, predispositions to diseases and often even destinies.
Cases when the monoovular twins separated in the childhood who were not knowing about each other lived almost identical lives are known.
Cases of the birth of monogerminal twins several times more rare, than raznoyaytsevy as for normal pregnancy the division of an ovum into several parts is uncharacteristic.
Nonidentical twins
Nonidentical twins have a different set of genes as various spermatozoa and ova took part in their conception. In certain cases at women for one menstrual cycle two or more ova ripen at once if all of them are impregnated, then polycarpous pregnancy will begin, and two or several children will be born. These are quite widespread cases, they meet at some nationalities very often – it is considered that mnogoplody has the genetic nature. Genotypes at nonidentical twins who are called still twins various therefore children can be a different floor and in general differ from each other as ordinary brothers and sisters. It is known that with age the probability of polycarpous pregnancy at the woman increases.
In the countries where it is accepted to give birth after thirty-year age, for example, in the USA or the European states, it is more cases of the birth of nonidentical twins.
One more type of twins which is called transitional between described by two – polar or semi-identical meets. It occurs when two different spermatozoa impregnate an ovum and the polar little body formed together with it (usually it dies off). As a result at embryos the different set of genes identical from mother turns out half.