A routine prenatal ultrasound is offered to all pregnant women around 18-20 weeks of pregnancy. The purpose is to measure the length of the pregnancy, find out if there are twins or other multiples, look for abnormalities and to locate the placenta.

The examination is voluntary.

The first trimester combined screening test is a blood test (biochemical test) combined with an ultrasound. Foetuses with Down's syndrome tend to have an increased amount of fluid around the neck, nuchal translucency. This can be seen at an ultrasound from week 12-13.

Research shows that in pregnancies where the fetuses have chromosomal abnormalities such as trisomy 13, 18 and 21, the levels of the substances PAPP-A and free beta-hCG found in the blood are different from those found in a normal pregnancy. With the combined results from ultrasound, blood-test and the pregnant woman's age, one estimates the risk for Down's syndrome. The probability is explained as a quota; for example 1:34 or 1:1768 or 1:512. Figures over 1:200 are considered low probability for trisomy 13, 18 and 20. If the quota is 1:200 or less, the pregnant woman is offered an additional blood test, NIPT (non-invasive). If the quota is 1:50 or less, the pregnant woman is offered a chorionic villius sampling (CVS) or an amniocentesis (invasive).

These tests can tell you whether your baby has Down's syndrome or other abnormalities. The set of chromosomes are counted. The analysis is made with a technique called QF-PCR where you count certain chromosomes. Most often number 13, 18, 21 and gender chromosomes. Tests can be taken from both the amniotic fluid and the placenta,  to look for congenital diseases. This is done only  if there is a high risk for a specific abnormality. The probability to give birth to a child with Down's syndrome increases with age. A woman in her twenties has a probability of 1:1500 while a woman that is 40 years old has a probability of 1:100. Amniocentesis carries a risk for miscarriage calculated to 1 miscarriage per 100-200 examinations.