Identifying risk

Pregnancies at risk of fetal abnormality may be identified in
various ways. A pregnancy may be at increased risk of Down
syndrome or other chromosomal abnormality because the
couple already have an affected child, because of abnormal
results of biochemical screening, or because of advanced
maternal age. The actual risk is usually low, but prenatal testing
is often appropriate, since this allows most pregnancies to
continue with less anxiety. There is a higher risk of a
chromosomal abnormality in the fetus when one of the parents
is known to carry a familial chromosome translocation or when
congenital abnormalities have been identified by prenatal
ultrasound scanning. In other families, a high risk of a single
gene disorder may have been identified through the birth of an
affected relative. Couples from certain ethnic groups, whose
pregnancies are at high risk of particular autosomal recessive
disorders, such as the haemoglobinopathies or Tay–Sachs
disease, can be identified before the birth of an affected child
by population screening programmes. Screening for carriers of
cystic fibrosis is also possible, but not generally undertaken on a
population basis. In many mendelian disorders, particularly
autosomal dominant disorders of late onset and X linked
recessive disorders, family studies are needed to assess the risk
to the pregnancy and to determine the feasibility of prenatal