What tests should be ordered at a 23-week obstetrics (OB) visit?

Tests to Order at a 23-Week OB Visit

At 23 weeks gestation, the primary focus should be on completing second-trimester screening if not already done, with maternal serum alpha-fetoprotein (MSAFP) screening being the most critical test, along with routine anatomic ultrasound evaluation and glucose screening preparation.

Essential Laboratory Tests at 23 Weeks

Maternal Serum Screening

• MSAFP screening should be offered if not already completed, optimally performed between 16-18 weeks but can be interpreted through 22-25 weeks gestation 1.
• If the patient had first-trimester screening or CVS, MSAFP screening should still be offered for detection of open neural tube defects (ONTDs) and anencephaly 1.
• Multiple marker screening (quad screen: AFP, hCG, uE3, and inhibin-A) should be offered if the patient did not have first-trimester aneuploidy screening, though at 23 weeks this window is closing 1, 2.
• The quad screen has a detection rate of approximately 80% for Down syndrome with a 5% false-positive rate 2.

Glucose Screening Preparation

• While the standard glucose challenge test is performed at 24-28 weeks, at 23 weeks you should prepare for imminent glucose screening (typically scheduled for the next visit at 24-28 weeks) 2.
• For high-risk women (obesity, previous gestational diabetes, strong family history), consider ordering the glucose challenge test at this 23-week visit rather than waiting 2.

Ultrasound Evaluation

Anatomic Survey

• A detailed fetal anatomic ultrasound (CPT code 76811) should be performed or confirmed as completed between 18-23 weeks gestation 1.
• At 23 weeks, this is the tail end of the optimal window for the standard second-trimester anatomic survey 1.
• The anatomic survey should systematically evaluate all major organ systems including cardiac anatomy, central nervous system, spine, face, abdomen, kidneys, and extremities 1.

Soft Marker Evaluation

• If isolated soft markers are identified on ultrasound and the patient had negative prior aneuploidy screening (serum or cell-free DNA), no further aneuploidy evaluation is recommended 1.
• For patients without previous aneuploidy screening who have isolated soft markers detected, counseling should be provided regarding probability of trisomy 21 and options for cell-free DNA or quad screen 1.
• Isolated echogenic intracardiac focus after negative screening requires no further evaluation and is considered a normal variant 1.

Fetal Growth Assessment

• Fetal biometry should be documented to establish baseline growth parameters, particularly estimated fetal weight (EFW) and abdominal circumference (AC) 1.
• If fetal growth restriction (FGR) is suspected (EFW or AC below 10th percentile), umbilical artery Doppler assessment should be initiated 1.

Additional Considerations Based on Risk Factors

For Women with Chronic Hypertension

• Baseline laboratory assessment should include liver enzymes, renal function (creatinine, BUN), and uric acid levels if not already obtained 2.
• Monitor for development of superimposed preeclampsia 1.

For Women with Pre-existing Diabetes

• Hemoglobin A1C should be checked if not recently obtained 2.
• Capillary blood glucose profiles or continuous glucose monitoring if HbA1c ≥6.5% and/or fasting plasma glucose ≥7.0 mmol/L 1.

For Women After Bariatric Surgery

• Monthly fetal growth monitoring ultrasound should be performed from viability (which includes this 23-week visit) 1.
• Serum indices should be checked including full blood count, serum ferritin, iron studies, serum folate, and vitamin B12 1.
• Additional micronutrient monitoring including serum vitamin A, zinc, copper, and selenium 1.

Common Pitfalls to Avoid

• Failing to offer MSAFP screening to women who had first-trimester screening is a critical error, as MSAFP specifically screens for neural tube defects which first-trimester screening does not adequately detect 1.
• Missing the window for second-trimester serum screening (quad screen), which becomes less reliable after 22-23 weeks 1.
• Not following up on incomplete anatomic surveys from earlier scans—if structures were not adequately visualized at 18-20 weeks, a repeat detailed scan should be scheduled 1.
• Ordering diagnostic testing (amniocentesis) solely for isolated soft markers when the patient has had negative serum or cell-free DNA screening, as this is not recommended 1.
• Delaying glucose screening in high-risk women until the standard 24-28 week window when earlier testing is indicated 2.
• Not reinterpreting serum screening results if gestational age was revised by ultrasound by 2 or more weeks 1.

Management of Abnormal Results

• Positive second-trimester serum screening should prompt genetic counseling and offer of amniocentesis 1.
• If major fetal anomalies are detected, a detailed obstetrical ultrasound examination should be performed and diagnostic testing with chromosomal microarray (CMA) should be offered 1.
• For unexplained isolated FGR diagnosed before 32 weeks, prenatal diagnostic testing with CMA should be offered 1.