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Screening for fetal and genetic abnormalities.

Screening for genetic abnormalities is an integral part of obstetrics. Prior to initiating screening, however, several prerequisites must be met: (i) capacity to alter clinical management, (ii) cost effectiveness, (iii) reliable means (usually assays) of assessment, and (iv) capacity to handle problems. In all pregnancies one should determine in systematic fashion whether family history places a pregnant woman at increased risk over the background risk of 2-3% congenital anomalies. All women over age 35 years at delivery should be offered prenatal cytogenetic testing, and women of all ages should be offered maternal serum alpha-fetoprotein screening for neural tube defects. Screening ostensibly normal populations is appropriate in certain ethnic groups to determine heterozygosity for selected disorders: Blacks for sickle-cell anaemia, Mediterranean people for beta-thalassaemia, Southeast Asians and Filipinos for alpha-thalassaemia, Ashkenazi Jews and perhaps French-Canadians for Tay-Sachs disease. Cystic fibrosis screening (delta F508 mutations) is not currently recommended for the general populations, but should be offered to relatives of an individual having delta F508 cystic fibrosis. Irrespective of the extent of screening programmes for Mendelian traits, the mutant allele will remain in the general population because by far the greatest genetic load lies in clinically normal heterozygotes, affected contributing far less to the load despite the obvious clinical effect.