ABSTRACT
Prenatal screening in the first or second trimester to identify pregnancies that may be complicated by neural tube defects (NTDs) or chromosomal abnormalities is an established part of obstetric practice in many parts of the world. Such programs aim to identify a subset of women within the general pregnant population, whose pregnancy may be at increased risk of being affected by such a problem. These women may then be offered a suitable diagnostic test. In the case of NTDs, the definitive diagnosis is usually accomplished with a detailed spinal ultrasound scan, which in most centers has superseded the invasive withdrawal of amniotic fluid and its subsequent biochemical analysis. However, fetal chromosomal abnormalities can only be diagnosed by fetal karyotyping from cultured amniotic fluid cells or chorionic villi, using either conventional techniques, or the recently advocated more rapid techniques of fluorescent in situ hybridization or quantitative polymerase chain reaction (Q-PCR). Thus, an invasive test is currently the only method of diagnosis for chromosomal abnormalities. Such procedures carried out in singleton pregnancies carry risks to both the fetus and the mother. In particular, the fetal loss rate post-amniocentesis is often quoted as 0.5-1.0% above the background rate, and that of chorionic villus sampling (CVS) some 1-2% above the background rate. As with all invasive procedures, the fetal loss rates vary tremendously between operators, based on skill, training and experience.
