What is First Trimester Screening?

First trimester screening is a pregnancy screening which can be performed during the first trimester of pregnancy, between 11 and 13 weeks, to identify increased risks of chromosomal abnormalities and some other complications which can occur during pregnancy. This screening is not diagnostic in nature; a positive result does not mean that the developing fetus has a chromosomal abnormality. It only identifies pregnancies where an increased risk is present, providing information which can be used to make decisions about testing and other options as the pregnancy moves forward.

A doctor may recommend first trimester screening for the purpose of determining whether or not additional tests need to be conducted. The screening is not painful or dangerous, and can provide useful information which can be utilized during the ongoing pregnancy to make decisions about which diagnostic and screening tests might be beneficial.

In first trimester screening, a sample of blood is taken from the mother for the purpose of checking the levels of hormones associated with pregnancy. An ultrasound examination is performed to measure nuchal transparency, the amount of fluid which builds up under the skin behind the baby's neck. The mother's age and ethnicity are also taken into account, along with other risk factors such as a family history of genetic conditions. All of this information is used to determine whether or not there is an increased risk associated with the pregnancy.

When positive results are returned, the parents can discuss them with the doctor and decide whether or not to move forward with additional, more invasive screenings which do carry risks. Some parents find it helpful to consult with a genetic counselor after receiving a positive result on a first trimester screening, as genetic counselors have experience and detailed information which can be beneficial to parents.

False positives do occur on a first trimester screening. Sometimes the test reveals a positive and additional testing shows that no anomalies are present. For this reason, it is important to discuss the results with an experienced professional, and to weigh the potential costs and benefits of moving forward with additional testing. Each pregnancy is different, and recommendations for one pregnancy may not hold true in another. For example, if a young and healthy mother with no family history of disease receives a positive result, it will be interpreted very differently than a similar result in a woman over 40 having her first child.