Stepwise, Part 2

Prenatal screening provides information about a fetus’s chance of having trisomy 21 (Down syndrome), trisomy 18, or an open neural tube defect (ONTD). Prenatal diagnosis will tell whether or not the fetus has these disorders. A negative screen means it is unlikely the fetus has Down syndrome, trisomy 18, or an ONTD. But a negative screen does not guarantee the birth of a healthy baby. The screening test only screens for Down syndrome, trisomy 18, and ONTDs; it is not a diagnostic test.

The demographic information provided at the time of testing is used in calculating the patient’s risk of having a fetus with Down syndrome, trisomy 18, or ONTD. Please check the demographic information to ensure accuracy of calculated results.

A positive Down syndrome screen result means there is an increased risk for the fetus to be affected with Down syndrome or other chromosome abnormality. Ultrasound confirmation of gestational age is recommended. If there is a greater than 10 day difference between the estimated date of delivery (EDD) used in the screen and the ultrasound EDD, please contact the maternal serum screening laboratory to recalculate results. If the gestational age has been confirmed, the patient should be counseled regarding diagnostic testing options (eg, chromosome analysis of an amniocentesis sample) and/or noninvasive prenatal screening (NIPS; test code 92777 or 91933 [NY]). Guidelines recommend against repeating this test when the result is positive for Down syndrome.¹

A positive ONTD screen result means there is an increased risk for the fetus to be affected with an open neural tube defect or other congenital abnormality. If the patient’s AFP is less than 3.5 multiples of the median (MoM), if ultrasound dating confirms the gestational age, if this is a singleton pregnancy, and if the gestational age is less than 18 weeks, testing a newly collected sample can be considered. If this second sample confirms an elevated AFP MoM or if a second AFP test is not performed, genetic counseling, ultrasound examination, and possible amniocentesis would be appropriate.

Note that underestimation of gestational age is one of the most frequent causes of an elevated MoM.

A positive trisomy 18 result means there is an increased risk for the fetus to be affected with trisomy 18 or other chromosome abnormality. The patient should be counseled regarding a detailed fetal anatomic scan and diagnostic testing options (eg, chromosome analysis of an amniocentesis sample) 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, 15 weeks 4 days is reported as 15.6 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 4).

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. Similarly, if second trimester (but not first trimester) EDD is available and the gestational age used for screening is within the EDD ±10 day range, 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 15.0 to 22.9 weeks gestation, we can calculate and report new risks. If the revised gestational age is ≥14.0 weeks and <15.0 weeks, we can calculate and report new Down syndrome and trisomy 18 risks, but not ONTD risks (see Question 7). If the revised gestational age is <14.0 weeks, we cannot calculate a new risk. Consider submitting a second specimen for screening, collected when the woman is between 15.0 to 22.9 weeks gestation (preferably 16 to 18 weeks). If the revised gestational age is >22.9 weeks gestation, we cannot calculate a new risk, and a more accurate risk assessment cannot be provided.

The NTD detection rate is significantly lower at 14.0 to 14.9 weeks gestation than at 15.0 to 22.9 weeks gestation. Therefore, samples collected during the 14th week of gestation with an AFP value below 2.5 MoM (single gestation, nondiabetic) will be reported as screen negative without a patient-specific risk. For optimal NTD screening, collect samples when the woman is at 16 to 18 weeks gestation.

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.

A cutoff of 1 in 270, the risk of a 35-year old, is used to determine if a pregnancy is screen negative or screen positive for Down syndrome. This cutoff is used regardless of the woman’s age, since it’s the historical cutoff for offering diagnostic testing (amniocentesis).

When counseling a pregnant woman, it may be helpful to compare her age-related risk (ie, pre-test risk) with her screen-derived risk (post-test risk) and the general population risk (1 in 600-800 live births). This allows the woman and her partner to better understand her risk of carrying a Down-syndrome affected fetus and to weigh it against the risks and consequences of amniocentesis.

There is no consensus in the literature as to what constitutes a low unconjugated estriol (uE3) level. Therefore, no results will be flagged as low on the report. However, most centers consider the uE3 low if the MoM is <0.25. Some centers even consider anything <0.30 MoM as low.

A low uE3 MoM has been associated with fetal Smith-Lemli-Opitz syndrome (SLOS), steroid sulfatase deficiency (STSD), and some disorders of esterol biosynthesis. In about 60% of SLOS cases, the maternal serum uE3 is <0.3 MoM, while in STSD, the uE3 is often <0.1 MoM.

Diagnostic tests can be done using amniotic fluid samples. An SLOS test is performed at the Kennedy-Kreiger Institute (http://www.kennedykrieger.org/patient-care/patient-care-laboratories/genetics-laboratories/clinical-services/biochemical-testing). Quest Diagnostics offers a FISH test for STSD (FISH, X-Linked Ichthyosis Steroid Sulfatase Deficiency, test code 14607[X]).

Low or high analyte levels have been associated with the following pregnancy complications: low birth weight, intrauterine growth retardation, preterm labor, preeclampsia, and fetal demise.

Low uE3 values have been associated with Smith-Lemli-Opitz syndrome and steroid sulfatase deficiency. See Question 10 for more information.

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, 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. The AFP MoM is adjusted based on the number of fetuses to provide either an ONTD screen negative or screen positive result. An ONTD numerical risk estimate cannot be calculated due to insufficient data from affected twin pregnancies.

A trisomy 18 risk assessment is not calculated in a twin gestation due to insufficient screening marker data from affected twin pregnancies.