Prenatal screening for chromosome abnormalities.

An abnormal chromosome complement (aneuploidy) contributes significantly to fetal loss during pregnancy, as well as to perinatal morbidity and mortality. The contribution of chromosomal abnormalities to fetal loss decreases as pregnancy continues with an estimated 50% of first trimester spontaneous abortions due to chromosomal abnormalities, but only 5% of stillbirths (after 28 weeks). Prenatal screening for aneuploidy (in particular Down syndrome) can be undertaken using maternal serum biochemistry, fetal ultrasound or a combination of both. In this chapter the advantages and disadvantages of screening programmes currently in use, or undergoing evaluation, will be reviewed.