Penn Ob/Gyn Care

First Trimester Prenatal Screening

Giving birth to a child can be one of the most exciting times in a woman's life. However, it can also be a time filled with concern and reservation if the health of the baby is in question.

To help reduce some of the anxiety related to the health of the baby, pregnant women now have the option of first trimester screening for genetic or chromosonal defects. This screening method is a non-invasive way to inform physicians and their patients earlier in the pregnancy of the risk for the fetus to have Down syndrome.

First trimester screening for Down syndrome is available to pregnant women who start prenatal care early in their pregnancy. The American College of Obstetrics and Gynecology still recommends genetic counseling, for patients who are 35 years or older at the time of delivery since the risk for having a child with a chromosome abnormality increases with maternal age. In addition to first trimester screening, older mothers have the option of amniocentesis or chorionic villus sampling for the detection of fetal chromosome abnormalities.

Not A Diagnostic Test

First trimester prenatal screening is not a diagnostic test – it is intended for women interested in determining whether their baby is at "increased risk" for Down syndrome. It will also assess your risk for Trisomy 18. The first trimester screening must be performed between 10 and 13 weeks of pregnancy, and involves the following:

  • Ultrasound – The ultrasound is performed to measure nuchal translucency (NT), or the area of fluid at the back of the baby's neck. The accumulation of fluid is normal; however, if an increased amount is found, this may indicate an increased risk for chromosome abnormalities, congenital heart defects and other genetic disorders. A fetal echocardiogram will be performed if the NT measurement is increased.

  • Blood test – The blood test measures two levels of proteins, HCG and PAPP-A, which are normally found in pregnant women.

The levels of these two proteins combined with the NT measurement and the age of the mother provide a risk assessment for Down syndrome and Trisomy 18. First trimester screening cannot be used as a screening test for spina bifida. It is also important to note that women seeking this early screening should see healthcare professionals with appropriate ultrasound training.

Advantages of First Trimester Screening

If results are negative, it may help reduce the parents anxiety earlier in the pregnancy; however, it is not a diagnostic test. A negative screen only indicates that the risk of having a child with Down syndrome is reduced.

If results are positive, prenatal diagnosis can be performed in the first trimester by chorionic villus sampling (CVS) at 10 to 12 weeks, or by an amniocentesis in the second trimester (after 15 weeks) to determine if the fetus is affected.

Determining problems earlier in the pregnancy allows women to prepare for a child with health problems. If the mother decides to terminate the pregnancy, she can do so earlier with the utmost privacy and confidentiality since her pregnancy may not yet be visible.

How Accurate is First Trimester Screening?

Screening tests can give false positive results. A positive result (showing an increased risk) does not mean that your baby definitely has a health problem, but indicates that you may wish to pursue further testing options.

The first trimester detection rates for Down syndrome are 80 to 85 percent. A negative screening test is usually reassuring. It is important to understand that screening tests will not detect all cases of Down syndrome; therefore, it is still possible that a woman with a negative screen result can give birth to a child with Down syndrome.

Are There Other Screening Tests for Down Syndrome?

Screening for Down syndrome in the second trimester (15 to 20 weeks) is still available. This is a blood test, which measures four proteins (HCG, AFP, estriol and inhibin) and uses the levels to determine your risk for having a child with Down syndrome, Trisomy 18 and spina bifida.

A combination of first and second trimester screening is also available. This offers the highest detection rate (90 to 95 percent) for Down syndrome. Women who screen positive then have the option of having an amniocentesis to determine if the fetus is affected. Ultrasound in the second trimester is a less sensitive screening test for Down syndrome. However, if an abnormality is seen, your physician may recommend genetic counseling and/or genetic testing.

If you have questions about first trimester screening or would like more information, talk to your obstetrician. To make a prenatal appointment with a Penn Ob/Gyn Care physician, call 800-789-PENN (7366) or request an appointment online.