[Sonographic features of chromosomal defects in the second trimester of pregnancy]

Each chromosomal defect has its own syndromal pattern of detectable abnormalities. The authors describe the sonographic features of trisomy 21 and other major chromosomal defects in the second trimesters of pregnancy. It is recommended that, when an abnormality/marker is detected at routine ultrasound examination, a thorough check is made for the other features of the chromosomal defect known to be associated with that marker; should additional abnormalities be identified, the risk is dramatically increased. In the case of apparently isolated abnormalities, the decision whether to carry out an invasive test depends on the type of abnormality. Minor fetal abnormalities or soft markers are common and they are not usually associated with any handicap, unless there is an underlying chromosomal defect. Routine karyotyping of all pregnancies with these markers would have major implications, both in terms of miscarriage and in economic costs. It is best to base counseling on an individual estimated risk for a chromosomal defect, rather than the arbitrary advice that invasive testing is recommended because the risk is 'high\ The estimated risk can be derived by multiplying the a priori risk (based on maternal age, gestational age, history of previously affected pregnancies and, where appropriate, the results of previous screening by NT and/or biochemistry in the current pregnancy) by the likelihood ratio of the specific abnormality or marker.