The birth of more than one baby at a time, twins, triplets, etc.
Two or more babies developing during a single pregnancy. Approximately one out of three multiple births develop from a single zygote (fertilized egg), which divides after conception; these twins (or other multiples) are called monozygotic, monovular, or identical. Because they have the same genetic material, they are always of the same sex and look alike, though one is generally larger than the other at birth. Identical twins also share the same placenta and chorion in the uterus, though with individual umbilical cords and generally separate sacs of amniotic fluid. About one out of four pairs of identical twins are mirror twins, meaning that they have the same features but on opposite sides, sometimes even including internal organs, such as the heart. If division of the single egg is not fully complete, the two may be joined in some part of the body, often the chest, and need to be separated surgigally after birth, where possible; these so-called Siamese twins occur about once in very 50,000 births.
A pregnancy in which a woman is carrying more than one fetus. It occurs when two or more eggs are released simultaneously from the ovaries and are fertilized or when a single fertilized egg divides into two eggs early in development. A pregnancy with two or more fetuses is considered a higher risk pregnancy because multiple fetuses have greater nutritional needs than a single baby and place extra strain on the mother’s body.