About one in a hundred births are twin births. Trillingefødsler is even more rare, around one in ten thousand. Pregnancies of more than one embryo is called multiple pregnancy. Multiple gestations are more frequently through artificial insemination, which often set up more than a fertilized egg.
Two-thirds of twin pregnancies aretveæggedetwins, ie. who have been released two eggs in a woman's ovulation, and both were fertilized with their sperm. The two fetuses are no more similar than siblings who were born at different times.
Very soon after fertilization can cluster of dividing cells divide into two, and evolve into two embryos, despite the fact that there is only one egg that has been fertilized. This is calledenæggedetwins. They have the same inheritance material, and is therefore of the same sex, and will resemble each other more than tveæggede tvilligner. Depending on how late this split happens, the twins will either have their own placenta and amniotic membrane, parts placenta, but each have their amniotic membrane, or parts amniotic membrane and placenta.
All risks are over parts of pregnancy and childbirth are increased by multiple gestations. This includes:
- Pregnancy poisoning.
- Conciliation Degressive placenta.
- Bleeding in the town after the period.
- Premature birth.
Multiple gestations discovered during routine checks when the woman becomes ultrasound scanned.