Clinical Practice Guidelines Antenatal care - Module I

9.1 Background

Page last updated: 02 April 2013

In recent years an increasing number of non-invasive biochemical screening tests and ultrasound techniques have been developed that can significantly increase the identification of pregnancies at risk of chromosomal abnormalities such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome)(see Glossary). These conditions may result in the death of the fetus or baby, some are associated with long-term serious morbidity and some require neonatal investigation or treatment. A positive screening test leads to the offer of a diagnostic test (chorionic villous sampling, amniocentesis, or ultrasound). If an anomaly is diagnosed, the woman and her partner may, after counselling, choose to continue with or terminate her pregnancy.

The suitability of any screening test depends on the gestation of pregnancy. Any screening test must include extensive pre- and post-test information and counselling, with consideration also being given to the woman’s preferences, availability of testing facilities, costs to the woman and, for ultrasound, operator expertise. All women should be made aware of the options for antenatal screening, with both verbal and written explanations being given which contain best current evidence.

Screening in the first trimester for trisomy 21 and trisomy 13/18 must be done between 11 weeks and 13 weeks 6 days pregnancy (when the fetus has a crown-rump length of 45–84mm). Tests to assess risk are:

  • maternal serum testing of pregnancy-associated placental protein-A (PAPP-A) and free beta-human chorionic gonadotrophin (-hCG); combined with
  • ultrasound measurement of fetal nuchal translucency thickness.
The results of these tests are combined with the background risks of maternal age and gestation of pregnancy, maternal weight, ethnic background and smoking status to provide a risk value.

Later in pregnancy (14 to 20 weeks), the triple test (maternal serum testing of α-fetoprotein [AFP], free hCG [or total hCG] and unconjugated estriol) or the quadruple test (which also includes inhibin A) is used to assess the risk of fetal chromosomal abnormality.

Chromosomal abnormalities in Australia

The National Perinatal Statistics Unit reported on congenital anomalies in Australia in 2002–03 (Abeywardana & Sullivan 2008). Trisomy 21 was the most commonly reported chromosomal condition at birth (11.1 per 10,000 births), but there was a high proportion (60%) of fetal deaths and terminations. When terminations were included, the estimated rate was 26.3 per 10,000 pregnancies. Trisomies 18 and 13 were associated with a large number of fetal deaths or terminations. All conditions were more common among women aged 40 years or older.

Analysis of the WA birth defects register between 1980 and 1996 (Leonard et al 2000) found the prevalence of Down syndrome to be similar among Aboriginal and Torres Strait Islander women and non-Indigenous women of the same age.