Prenatal Diagnosis and Ultrasound

Screening for Down's syndrome
Our genes determine who we are, and our genes are stored in chromosomes (46 in 23 pairs). Occasionally during reproduction an extra chromosome is added, making one pair of chromosomes into a triple (trisomy). Down's syndrome is where there are three of the 21st pair of chromosomes (Trisomy 21). There are lots of errors made in the reproduction process, but trisomy 21 remains the most common chromosome anomaly found after the first trimester, when the majority of these problems are dealt with by miscarriage (see Early pregnancy and Threatened miscarriage). A chromosomal abnormality can only be diagnosed with an invasive procedure (amniocentesis, chorion villus sampling, cordocentesis), which carry a 1% risk of miscarriage. Naturally, many women do not wish to take this risk unless they believe the risk of finding a chromosomal abnormality is high.

Screening tests, such as nuchal translucency measurement or placental hormone assay, or both, refine the calculation of risk. With this information the woman, usually in discussion with her partner, can decide if they are happy to live with risk of trisomy 21, or take the risk of an invasive procedure. There is no right and wrong answer to this dilemma. Every woman/couple have to weigh up the risks and decide for themselves. (Continued next page)