Integrated prenatal screening is a combination of three different tests used to determine the probability of a child being born with Down syndrome, Trisomy 18, or open neural tube defects. Testing can begin at 11 weeks gestation and results are typically available by 16 to 17 weeks gestation. While integrated prenatal screening is not a diagnostic tool, it can give pregnant women and doctors a good idea of whether or not further testing is necessary.
The first test in this screening is an ultrasound, which can be done either vaginally or with an abdominal scan. It is typically performed between week 11 and 13 of a woman's pregnancy and is often one of the first tests administered to pregnant women. For the purpose of this screening, the ultrasound looks at the amount of amniotic fluid behind the fetus's neck; this test is known specifically as a nuchal scan. Fetuses with a higher than normal amount of amniotic fluid behind their neck are at greater risk for Down syndrome, also known as Trisomy 21, Trisomy 18, and different open neural tube defects.
Between 10 and 13 weeks gestation, a blood test is carried out to determine the pregnancy associated plasma protein A (PAPP-A) levels in an expectant mother's blood. This is the second portion of integrated prenatal screening. All pregnant women have this specific protein in their blood; if levels are low, it could also indicate that the fetus is at greater risk for Trisomy 21, Trisomy 18, and open neural tube defects.
The final test in integrated prenatal screening is another blood test, typically done between 16 and 17 weeks gestation. This test measures several different protein levels: inhibin A, unconjugated estriol, alpha fetoprotein, and human chorionic gonadotropin. Once this testing is complete, the results of the entire integrated prenatal screening become available.
The information gathered from these three tests can estimate the probability of a fetus having Down syndrome by a little over 90%. It can also determine the chances of Trisomy 18 in 90% of cases and the chances of an open neural tube defect, such as anencephaly or spina bifida, in 80% of cases. These tests only provide doctors and pregnant women with the probability of the fetus having these issues; it is not a diagnostic tool.
Out of every 30 women, one will receive abnormal results on the integrated prenatal screening. An abnormal test result simply means that the fetus is at greater risk than average for developing these defects. If a woman receives abnormal test results, she will typically have the option of undergoing additional diagnostic testing.