The aim of prenatal screening tests is to determine if the fetus has an increased risk of a chromosomal abnormality (such as Down syndrome) and to check for a physical birth defect. The tests are offered at 11 to 14 weeks, 15 to 17 weeks and 18 to 20 weeks of pregnancy.
The results of these tests provide detailed information about risk and depend on accurate knowledge of the duration of the pregnancy (gestation).
Age: the risk of having a baby with Down syndrome increases according to the woman’s age. The risk of some other rare chromosomal abnormalities also increases with age, for example, Trisomy 18, in which an extra copy of chromosomal 18 occurs.
Neural Tube Defect (NTD): the risk of having a baby with a NTD is about one in 1,000, provided the parents have no family history of a NTD or other predisposing factors (such as medications used to treat epilepsy).
The benefits of having an ultrasound scan in the first trimester are:
From 11 weeks to 13 weeks and six days of pregnancy, an ultrasound scan can measure the thickness of fluid-filled tissue at the back of the fetal neck. This is known as the ‘nuchal translucency’ (NT). The thicker the NT, the greater the risk of Down syndrome.
Combined Test for Down syndrome
The Combined Test is the use of both ultrasound (for measurement of NT) and a blood test called first-trimester maternal serum screening (MSS).
When both NT ultrasound and first-trimester MSS are performed, the assessment of risk for Down syndrome is more accurate than using either test alone.
Ultrasound scans at 18 to 20 weeks
Although an ultrasound can be performed at any time during pregnancy, the usual practice is to offer women an ultrasound scan at 18 to 20 weeks of gestation. The operator assess the following:
• Size of the fetus
• Development of the fetus
• Location of the placenta
• Amount of amniotic fluid
• Well-being of the fetus
Screening tests are not perfect and cannot identify all fetuses with chromosomal abnormality or other problem. A low-risk result does not mean that the fetus has no risk of a disorder being present. Similarly, an increased-risk result does not mean that the fetus is certain to have a disorder.
Although a screening test may indicate an increased risk of an abnormality, most women are still likely to have a normal baby. This is best explained in terms of ‘level of risk’. Your level of risk is best discussed with, and explained by, your doctor, ultrasound specialist, a genetic counsellor or midwife.