First Trimester Screening

What is First Trimester Screening?

First Trimester Screening is a prenatal test to determine the likelihood that a fetus (unborn baby) has a chromosome abnormality such as trisomy 21 (Down syndrome) or trisomy 18. Chromosome abnormalities occur more frequently as a woman ages. If the test indicates an increased risk for one of these disorders, further testing such as chorionic villus sampling (CVS) or amniocentesis is available. First Trimester Screening does not screen for birth defects.

When and how is First Trimester Screening performed?

First Trimester Screening is performed between 10 and 14 weeks of pregnancy. A blood sample is collected from the pregnant woman (between 10 and 13 weeks of pregnancy) and a special fetal ultrasound exam (between 11 and 14 weeks of pregnancy) is performed at our office to measure the nuchal translucency (fluid collection in the back of the fetal neck). Two hormones are measured in the woman’s blood sample; these results are combined with the nuchal translucency measurement and the woman’s age to calculate the risk that the fetus has trisomy 21 (Down syndrome) or trisomy 18.

The blood sample requires a special lab form, which the patient receives from her prenatal care provider.  If the patient has her blood drawn at least 7 business days before her ultrasound appointment, then she will most likely receive her first trimester screening results immediately after her ultrasound exam. This process is called Instant Risk Assessment. Otherwise the patient will receive her first trimester screening results from her prenatal care provider.

Does First Trimester Screening detect all chromosome abnormalities?

No. There are many different chromosome abnormalities; approximately half include trisomy 21 (Down syndrome) or trisomy 18. First Trimester Screening identifies approximately 75 % of fetuses with trisomy 21 (Down syndrome) and 59% of fetuses with trisomy 18. Diagnostic testing (CVS or amniocentesis) will identify 99 % of all chromosome abnormalities. These procedures are associated with a small risk of pregnancy loss.

What if my First Trimester Screening result is screen positive (abnormal)?

If the result is positive, it does not mean that the fetus has a problem. It does mean, however, that the risk of the fetus having a chromosome abnormality is increased and that diagnostic testing (CVS or amniocentesis) should be considered. Our genetic counselors are available to discuss first trimester screening results and options for further testing.

What if my First Trimester Screening result is screen negative (reassuring)?

If the result is negative, the risk the fetus has a chromosome abnormality is estimated to be less than 1%. It is important to realize that First Trimester Screening does not rule out all chromosome abnormalities and it does not detect birth defects.

Click here for more information on prenatal testing options.

To schedule an appointment:

Please speak with your healthcare provider to obtain a referral to our office.