Stepwise, Part 1

This means the Down syndrome and trisomy 18 risks did not exceed the Part 1 cutoffs. To complete the test and obtain a final result, please submit a second trimester sample (test code 16465 [16466 for NY]) within the date range provided on the Part 1 report.

A High risk for Down syndrome result means there is an increased risk for the fetus to be affected with Down syndrome or other chromosome abnormality. The demographic information provided at the time of testing is used in calculating the patient’s Down syndrome risk. Please check the demographic information to ensure accuracy of calculated results. Ultrasound confirmation of gestational age is recommended. If a first trimester ultrasound estimated date of delivery (EDD) is available and the gestational age used for screening is within the EDD ±7 days, the gestational age should not be changed for screening purposes. If the gestational age used for screening is outside the ultrasound EDD range, it may be appropriate to change the gestational age used for screening. If you want to change the EDD/gestational age used for a specific patient’s screening test, please contact your local Quest Diagnostics laboratory or call 866-GENE-INFO.

Guidelines recommend counseling women with a positive screening test. Such counseling may include a discussion of the significance of the screening results and diagnostic testing options (eg, chorionic villus or amniocentesis testing) and/or noninvasive prenatal screening (NIPS; test code 92777 or 91933 [NY]). Guidelines recommend against repeating this test when the result is high risk for Down syndrome.¹

A High risk for trisomy 18 result means there is an increased risk for the fetus to be affected with trisomy 18 or other chromosome abnormalities. The demographic information provided at the time of testing is used in calculating the patient’s trisomy 18 risk. Please check the demographic information to ensure accuracy of calculated results.

Guidelines recommend counseling women with a positive screening test. Such counseling may include a discussion of the significance of the screening results and diagnostic testing options (eg, chorionic villus or amniocentesis testing) and/or noninvasive prenatal screening (NIPS; test code 92777 or 91933 [NY]). Revising the EDD/gestational age or repeating the test when it is screen positive for trisomy 18 is not recommended. The potential for intrauterine growth restriction in fetuses with trisomy 18 can negatively affect the accuracy of the gestational age.

The gestational age is calculated from the crown rump length (CRL) provided, using criteria from the Fetal Medicine Foundation. If the CRL is not provided, the gestational age is derived from the estimated date of delivery

(EDD) and the collection date provided and is an exact calculation by calendar days. Gestational wheels may be inaccurate by several days or more.

Note that the gestational age is reported in decimal weeks. For example, 11 weeks 4 days is reported as 11.6 weeks.

It is appropriate to change the gestational age or EDD when the data used for screening are substantially above or below that determined by ultrasound. In case of a positive screen for trisomy 18, however, guidelines recommend against changing the gestational age or EDD (see Question 3).

The earliest EDD calculated by ultrasound should be used for dating purposes.² An ultrasound derived EDD is most accurate when determined in the first trimester. Accuracy decreases with advancing gestational age. For example, accuracy of an ultrasound EDD is ±7 days in the first trimester and ±10 days in the second trimester.

If a first trimester ultrasound EDD is available and the gestational age used for screening is within the EDD ±7 days, the gestational age should not be changed for screening purposes. If the gestational age used for screening is outside the ultrasound EDD range, it may be appropriate to change the gestational age used for screening.

If you want to change the EDD/gestational age used for a specific patient’s screening test, please contact your local Quest Diagnostics laboratory or call Quest Genomics Client Services at 866-GENE-INFO. If the revised gestational age is between 10.0 to 13.9 weeks’ gestation, we can calculate and report new risks. If the revised gestational age is <10.0 weeks, we cannot calculate new risks. Consider submitting a second specimen for screening, collected when the patient is between 10.0 to 13.9 weeks’ gestation. If the revised gestational age is >13.9 weeks’ gestation, we cannot calculate new risks and a more accurate risk assessment cannot be provided.

Please call Quest Genomics Client Services at 866-GENE-INFO to discuss this case with a genetic counselor. Documentation of the abnormality in the family may enable a more specific risk assessment, or indicate whether additional studies should be performed.

Prenatal screening in twin pregnancies is complex. The serum markers can be measured in a patient with a twin gestation and then divided by corresponding medians for unaffected singleton pregnancies to calculate multiple of medians (MoMs). These MoMs are then adjusted for twins in order to provide a pseudo-risk for Down syndrome. This calculation accounts for the presence of two fetuses and also takes into account the chorionicity of the pregnancy and the nuchal translucency measurement of each specific fetus. The result is a pregnancy-specific pseudo-risk, rather than a fetus-specific risk.

Prenatal screening in twin pregnancies is complex. A trisomy 18 risk assessment is not calculated for twin gestations due to insufficient screening marker data from affected twin pregnancies.