The child's birth – great happiness in life of each family, and emergence at once two – double joy. Earlier twins were born with a frequency of 1 time at 80-100 childbirth, but recently, in connection with implementation of reproductive technologies, it happens much more often.
In medicine the development in an organism of the woman of two and more fruits is called polycarpous pregnancy, and the children born in its result - twins. Naturally mnogoplody it can be formed according to the following scenarios: - in the woman's ovaries two ova which are impregnated by spermatozoa at the same time ripen therefore two embryos – dvuyaytsovy twins turn out; - in ovaries one ovum with two kernels ripens, and after fertilization two embryos – uniovular twins are formed;
- the impregnated ovum is divided into 2 independent parts, in each of which the fruit – also uniovular twins develops.
Dvuyaytsovy twins meet three times more often uniovular. Each of them the placenta, can have them a different floor and are similar no more, than ordinary brothers and sisters. Uniovular develop in the general placenta, have one blood type, as a rule, same-gender and are similar at each other, as like as two peas.
Usually twins are born in families in which there are twins in different generations. And the hereditary predisposition needs to be considered not only on maternal, but also on the fatherly line. Besides, the woman is more senior, the probability of polycarpous pregnancy, especially, is higher if childbirth not the first. It is possible to conceive twins also after the end of reception of oral contraceptives: in an organism there is a reorganization therefore two and more ova can ripen. At the women who are receiving medical treatment for infertility with application of hormonal stimulation of an ovulation, twins are born in 30-40% of cases. At last, the main growth of frequency of the birth of twins is connected with development of in (artificial) vitro fertilization: to the patient appoint medicines which stimulate formation of several ova (in certain cases up to 20), take them from ovaries a puncture, impregnate, and then transfer 3-4 embryos to a uterus to increase birth probability at least of one child. Very often from them take root 2-3 that at successful combination of circumstances guarantees to family appearance of twins.