In addition to Down syndrome, screening tests also assess the risk of another chromosomal abnormality called trisomy 18. Babies with this condition have more severe mental and physical abnormalities than babies with Down syndrome and seldom survive beyond a year. The condition is rare, with each year around 1 in 6000 babies born with trisomy 18, compared to 1 in 800 babies born with Down. Screening can also detect neural tube defects like spina bifida and anencephaly, which occur in about 1 in 1000 pregnancies.

The "combined" test

The recommended screening test, and the one most commonly offered, for Down is the "nuchal translucency screening," performed between 11 and 14 weeks. This test has a high accuracy rate and the results are produced quickly.

The test uses a combination of a maternal blood test and an ultrasound of your baby. The ultrasound measures the thickness of the skin at the back of your baby's neck, called the nuchal fold (see Nuchal translucency screening). The blood test looks at the levels of two chemicals: pregnancy associated plasma protein A (PAPP-A) and one of the pregnancy hormones, human chorionic gonadotrophin (hCG). The results of the blood test is combined with your age, and the measurement of the nuchal fold. A mathematical formula is then used to calculate your baby's risk of Down syndrome.

If you have your blood test before your ultrasound, you will usually receive your results immediately following your ultrasound. If you have your blood drawn at the time of the ultrasound you will receive your results a few days later. Your risk based on the combined test may be higher than your risk based on your age alone, lower, or stay the same.

The integrated test

This test is done in two stages using results from a range of screening tests done in the first and second trimesters. Your age will also be taken into account when calculating your risk. In the first trimester, a blood test at 12 weeks measures the PAPP-A and hCG. You will also have a nuchal translucency screening. At between 15 and 22 weeks, another blood test measures the levels of hCG, AFP, inhibin A, and estriol. The integrated test is the most sensitive screening test, but it has the disadvantage that the results are not available until later in pregnancy. It is not available everywhere.

Triple and quadruple tests

These screening tests are done in the second trimester, usually between 15 and 20 weeks gestation, and assess your baby's risk of Down syndrome from blood tests alone. The triple test measures levels of the hormones hCG, AFP and estriol. The quadruple test measures levels of inhibin-A in addition to these three hormones. If your doctor or hospital doesn't offer a nuchal translucency screening, then a triple or quadruple test will be offered instead.