For the woman it is the most physiologic to bear one baby. But there are also polycarpous pregnancies, in this case twins or triplets are born. It not it the unusual occurrence, for example, is the share of each 40 childbirth one twins. To understand why twins are born, it is necessary to know, at least in general as there is a fertilization.
Approximately once a month, in the middle of a menstrual cycle, at the healthy woman occurs an ovulation. The ripened ovum is thrown out of an ovary an abdominal cavity. There she is captured a funnel of a uterine tube in which meets a spermatozoon. After fertilization the ovum gets into a uterus cavity where the germ begins to develop. But sometimes two ova which can be impregnated at the same time ripen. In this case each germ will independently develop in the fetal bubble and will be connected with mother's organism own placenta. Such twins often are not even similar at each other, they can be a different floor, they can have a different blood type. It occurs because each ovum is absolutely unique. Such children are called two-egg twins. Monoovular twins turn out at fertilization of one ovum with a big kernel and double or even threefold set of chromosomes. Such abnormal ova are called poliploidny. Similar to it, men's germinal cages can possess a double set of chromosomes also. Such spermiya are called poliploidny too. When meet poliploidny spermiya and a poliploidny ovum, after fertilization two germs develop. This kind of twins meets much less often. Monoovular twins always same-gender. They have an identical appearance, tendency to identical diseases. This similarity extends also to properties of mentality, characters at them too often similar. Such twins feel the soulmate even at distance. Often and destinies at such people identical. Many women dream to give rise to twins. Most often it happens if in a sort couple had cases of the birth of twins. At in vitro fertilization the polycarpous pregnancy often develops. With age an opportunity to conceive twins increases a little, especially if the woman already had a childbirth.