1st Trimester Screening

FIRST TRIMESTER SCREENING FOR TRISOMIES 13, 18, 21

A method for first trimester screening has been developed and proven to be 85-90% effective in screening for the following chromosome abnormalities:

 Trisomy 21 (Down Syndrome)

 Trisomy 13 (Patau Syndrome)

 Trisomy 18 (Edwards Syndrome)

These three chromosome abnormalities account for approximately 50% of the chromosome abnormalities seen at birth.

The first trimester screening protocol goes by various names (e.g. Ultrascreen by NTD Labs, Early Screen by Genecare). The protocol consists of two parts

  • Obtaining blood spots (fingerstick or blood draw) on sample card
  • Ultrasound measurements of
    • The fetal length (Crown Rump Length (CRL)), and
    • The thickness of an area in the fetal neck (Nuchal Translucency (NT)).
    These measurements and samples must be obtained when a fetus is between 11 weeks 3 days to 13 weeks 6 days. These dates correspond to Crown Rump Lengths of between 45 and 84 mm.

These measurements and blood sample results are then combined with the mother’s age in a computer program to estimate the risk for the three chromosome abnormalities (Trisomy 21, Trisomy 13, Trisomy 18). These risks are then reported (examples 1:50, 1:10,000) for each of the three chromosome abnormalities.

Decisions to proceed to additional screening tests (second trimester serum screening or targeted ultrasound) or to invasive diagnostic tests (Amniocentesis or Chorionic Villus Sampling) are then made after considerations of the:

  • The reported risk of a chromosome abnormality(ies)
  • The risk of invasive diagnostic procedures
  • The importance of obtaining diagnostic information for the mother and family

What First Trimester Screening Does Provide

An early method of determining risk for the most common chromosome abnormalities

  • Can be obtained at 11 weeks
  • Screens for Trisomies 21, 13, 18 which make up about 50% of chromosome abnormalities at birth

An effective method of screening for Trisomies 13, 18, 21 with a detection rate of 85-90%

A method for effectively screening all pregnancies regardless of maternal age


What First Trimester Screening Does NOT Provide

It is NOT A DIAGNOSIS.

  • It does NOT say the fetus has a chromosome abnormality.
  • It does NOT say the fetus does not have a chromosome abnormality.

It does NOT screen for all chromosome abnormalities

  • Just Trisomies 13, 18 and 21.
  • 50% of chromosome abnormalities seen at birth are NOT looked for by this test.

It does NOT screen for Spina Bifida

It does NOT screen for organ or structural abnormalities (e.g. heart, kidney, gastrointestinal, brain, lung arm, leg etc. abnormalities)

WHO should be offered First Trimester Screening

Women under the age of 35 years without a family history of genetic or chromosome problems. Based upon individual preference, the mother will then decide

  • Not to pursue any additional information or testing or
  • To progress to other screening tests (2nd trimester serum screening and/or targeted ultrasound) or
  • To progress to invasive studies (amniocentesis or Chorionic Villus Sampling).

Women over the age of 35 years and/or with a family history of genetic or chromosome abnormalities must have genetic counseling prior to the first trimester screening. The screening results will then increase or decrease this individual’s risks. Based upon individual preference, the mother will then decide

  • Not to pursue any additional information or testing or
  • To progress to other screening tests (2nd trimester serum screening and/or targeted ultrasound) or
  • To progress to invasive studies (amniocentesis or Chorionic Villus Sampling).

NOTE: Insurance may or may not cover the screening. Both MPA and the Laboratory WILL bill insurance. If insurance declines full payment for either the blood tests or the ultrasound, the patient will be responsible for any and all of the amount not covered.