Twins are always something special: Among 1,000 births in our country only about 17 are twins. More than two thirds of them are fraternal twins. They are more similar to normal siblings. For fraternal twins to occur two eggs mature simultaneously, both of which are independent of each and are fertilized by two sperms.
In some families fraternal twins are born very often. Therefore, it is believed that there might be a hereditary system – probably in the female line that plays an important role.
In recent years the number of dizygotic twins has increased – due to hormonal treatments for infertility, according to which often ripen several eggs. In artificial insemination of women often more implantions of a fertilized egg occur.
Another reason is also the increasing age of the mothers. Because increases in a woman with age, the concentration of the ovulation hormone (FSH), often several fertilizable oocytes are present.
Pretty sure today is that fraternal twins can also arise from an egg cell, which divides shortly before conception. Each half is then fertilized by different sperm. The genes of the mother are identical in this case, the father differently.
As twin girls, they are sometimes mistaken for same. Because identical twins get identical genes at conception and therefore the same sex.
In identical twins only one egg is fertilized. These shares within ten days in two nuclei. A phenomenon that occurs worldwide with the same frequency: 1,000 births around four times
Divide the egg shortly after fertilization, then each twin has its own placenta and own amniotic sac. If the division, however only four to seven days after fertilization, then the twins share the placenta. Sometimes it comes into so-called “feto-fetalen transfusion syndrome”: a twin to the other takes away the food. Therefore it is important to let the growth of the twins regularly by ultrasound. Even less common are twins with only a gestational sac and a placenta are rare.
By the way, can be in adult monozygotic twin pairs to determine some differences in the genome. This stunning revelation made recently by an international research team that studied 19 pairs of twins. Each of these pairs showed so-called “copy number variations” on – even a part of the chromosome was missing in one twin, at times, some even several times. The scientists speculate that these variations will be earned during the course of life of cell divisions. They hope to be able to identif adorn the future by using this finding genes that are responsible for certain diseases: ill only one of identical twins, they could then locate the cause in which any of the other twin of the different genes.
Even more rarely it happens that a woman is pregnant with more than two children. These kids are definitely fraternal. Previously, multiple births were common in the artificial insemination – but the children were often born prematurely, do not always have all lived. Therefore, a maximum of three embryos are implanted into the uterus in Germany. The doctors now tend even to women under 38 years not more than use two fertilized ova.