There is a wide range of noninvasive screening methods that aim to identify the subgroup of fetuses that are in a high risk of chromosomal defects. Invasive procedures should be offered to women in the high-risk group identified with the highest possible detection rate and the lowest false-positive rate.

The method of choice at 11 + 0 - 13 + 6 weeks is chorionic villus sampling. An early amniocentesis is much more dangerous and should be abandoned. CVS should be performed not earlier than at 11 + 0 weeks of pregnancy. Amniocentesis should be performed no earlier that at 15 + 0 weeks. Earlier procedure is associated with significantly higher rate of talipes equinovarius, amniotic fluid leakage and miscarriage. The umbilical cord insertion is a preferable site for fetal blood sampling. Care must be taken to distinguish between the vein and the artery, and the vein must be sampled, not the artery.

The operator's experience is very important issue. It has been postulated that to achieve a reasonable experience one should perform a minimum of 100 chorionic villus samplings, and a reasonable number of invasive procedures should be performed yearly.