Many women have successful, uncomplicated twin pregnancies. But, pregnancy involves some risk for every woman, and with each additional fetus the risks increase. The chance of having a multiple birth depends on many factors. Identical twins or triplets occur with the fertilization of a single egg that later divides into two or three identical embryos. Identical twins or triplets have the same genetic identity, are always the same sex, and look almost exactly the same.
Fraternal multiples develop from separate eggs which are fertilized by different sperm.
This can occur when the mother produces more than one ovum during ovulation. You have a higher chance of this happening if you are older, taller and heavier, if you are a twin yourself, or if having twins runs in the maternal side of your family. The use of drugs that increase fertility, such as Clomid and Pergenol, increases your chance of multiple birth. These procedures often involve the transfer of more than one fertilized eggs into the mother's womb to increase the odds of pregnancy occurring. In a pregnancy with triplets or more, the babies can be all identical, all fraternal, or a mixture of both.