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 doctors advise pregnant women to be screened for genetic disorders.