When an abnormality suggestive of a chromosomal defect is detected a thorough check should be made for the other features of the chromosomal defect known to be associated with that marker.

• Multiple abnormalities:
  The risk for chromosomal defects is substantially increased and fetal karyotyping should be considered.
• Isolated abnormalities:
  The decision of whether to carry out an invasive test depends on whether the abnormality is major or minor.
  • If there is a major abnormality it is advisable to offer fetal karyotyping, even if these abnormalities are apparently isolated. This is because: first, if the abnormalities are either lethal or they are associated with severe handicap (e.g. holoprosencephaly), fetal karyotyping constitutes one of a series of investigations to determine the possible cause and thus the risk of recurrence, and second, if the abnormality is potentially correctable by intrauterine or postnatal surgery (e.g. diaphragmatic hernia), it is important to exclude an underlying chromosomal defect – especially because, for many of these conditions, the usual defect is trisomy 18 or 13.
  • If there is a minor abnormality the estimated risk can be derived by multiplying the a priori risk (based on the results of previous screening) by the likelihood ratio of the specific abnormality or marker. The likelihood ratio for trisomy 21 is about 1 (therefore the a priori risk is not increased in the case of choroid plexus cysts, echogenic endocardiac focii, mild hydronephrosis and short femur) and about 10 (therefore there is a 10-fold increase in the a priori risk for nuchal or prenasal edema and hypoplastic nasal bone).