First Trimester Screening

Now that you're pregnant, your doctor will be prescribing a variety of screenings and tests in order to determine the health of your baby. The first trimester is a particularly precarious one, with the highest rate of miscarriage occurring in these initial 13 weeks. However, with good medical support and a healthy lifestyle, you will likely sail through the first trimester without much difficulty - save the various initial pregnancy symptoms that go along with the territory.

First Trimester Screening

First trimester screening is an optional set of non-invasive evaluations that combine both blood tests and ultrasound to identify if your baby is at risk for specific chromosomal abnormalities that include both Down syndrome Trisomy-21 and Edward's syndrome Trisomy-18. Additionally, one part of the ultrasound screening, called nuchal translucency is usefully in identifying other potential problems with the baby such as cardiac disorder. These screenings do not detect neural tube defects such as spina bifida.

The nuchal translucency screen is usually performed between 10 weeks and 13 week. It is an ultrasound measurement of the space between the baby's spine and the skin at the back of the neck. Although it is not a diagnostic test, it can reveal if there is an increased amount of space which could indicate Down syndrome. These tests require a specialist with the training to both perform and interpret the screen.

Down Syndrome Check

The goal of first trimester screening is to screen for Down syndrome and Edward's syndrome during pregnancy. They may point toward an increased likelihood of the baby carrying the abnormal chromosomes that are consistent with both Down and Edward's syndromes. Diagnostic testing may be done if the screening indicates abnormalities. Diagnostic testing during pregnancy involves taking fluid and tissue samples from around the baby and doing an evaluation to determine if there is an extra trisomy-21 or trisomy-18 chromosome. The diagnostic testing carries a risk of infection or miscarriage.

First trimester screening has been around for several years in the US, however, it is only in recent times that it has proven to be of real value in determining Down syndrome. In November of 2005 a study that was published in the New England Journal of Medicine supported first trimester screening as the most accurate non-invasive way to determine chromosomal abnormalities. The accuracy rate of the two screenings, when combined, resulted in an 85 percent accuracy rate for detecting chromosomal abnormalities with a five percent false positive rate. The false positive rate means that five percent of all normal pregnancies will receive a test result of positive or an abnormal level when, in fact, it is normal. If a positive result is received, it means there is a 1/100 to 1/300 chance of the baby having one of the abnormalities. A positive result does not mean the situation exists. It does mean the potential is higher and there's room for discussion about how to proceed in terms of testing.

The blood screening is done to measure the levels of the pregnancy hormones hCG (human chorionic gonadotropin) and PAPP-A. Levels of these hormones that are either abnormally high or low can indicate a chromosomal abnormality.

Not Diagnostic Testing

It is important to remember that these are screenings and they are not diagnostic - that is, they do not confirm a problem. They look at the results of the two blood tests and the ultrasound and compare a number of various factors like age of the mother; ethnicity, other blood tests results, and additional information, then estimate the chances of having an abnormality. They are a signal that further testing may be a consideration.

If there are abnormal screening results it indicates the mother is at risk for carrying a baby with a genetic disorder. Many women who have had the screening and have received abnormal test results later find the test was false and they go on to have a perfectly normal baby.

Learn More

The first trimester is filled with all kinds of experiences. To find out more about what to experience in those initial weeks, check our article on The First Trimester.