[Clinical approach to prenatal diagnosis of chromosome abnormalities]

In the case of prenatal diagnosis of chromosomal aberrations, the obstetrician has to determine the high risk situation for which a fetal karyotype examination has to be done. Before pregnancy, the karyotyping indications are defined by genetic consultation in order to analyze classical data such as the parents' age, repeated spontaneous abortions, previous child with a genetic defect, fetal malformations or intrauterine death... During pregnancy, clinical warning signs, in particular, anomalies in the amniotic fluid and fetal growth retardation should be recognized, but most abnormal karyotypes are detected by ultrasonographic examination (average: 11.3%). Down's syndrome diagnosis is presently evaluated by some more specific ultrasonographic signs such as fetal nape or femur length measurements and by new biological parameters such as hCG assay. The sampling method for karyotyping, early or late amniocentesis, first trimester but also second and third trimester placental biopsy or cordocentesis will be chosen in accordance with the importance of the chromosomal aberration expected, gestational age, procedure complication risks and the obstetrician's experience. The following examinations could be proposed: in high risk cases determined before pregnancy, a chorionic villus sampling should be done between the 9th and 11th weeks of gestation; in low risk cases such as advanced maternal age, a first trimester chorionic villus sampling or a second trimester amniocentesis could be chosen; in the case of Down's syndrome, warning signs, for example ultrasonographic or biological parameters, a second trimester placental biopsy to relieve the parents' anxiety; in high risk cases such as ultrasonographic malformations, late placental biopsy or cordocentesis.