Genetic disorders, such as
Down syndrome or
cystic fibrosis, and birth defects occur more
frequently in multiple pregnancies than in pregnancies with one fetus. A group
of deformities of the ankles and/or feet (clubfoot) is an example of a birth
defect seen more commonly in multiple pregnancy.
Genetic disorders and birth defects occur more frequently in
identical fetuses than in fraternal ones. In rare cases, problems occur when
the genetic material (chromosomes) in the fertilized egg does not split evenly.
This can result in severe mental or physical defects in one or more
fetuses.
Conjoined (Siamese) twins occur rarely. This occurs when the egg does
not split completely and the fetuses are partially joined.
Most health professionals generally recommend screening for genetic
disorders for all pregnant women.