What does a low PAPP‑A (pregnancy‑associated plasma protein‑A) multiple‑of‑the‑median in a dual‑marker first‑trimester screen indicate, and what are the recommended next steps for management?

Low PAPP-A in First Trimester Dual Marker Screening

A low PAPP-A (<0.5 MoM) in first trimester dual marker screening indicates increased risk for both fetal aneuploidy (particularly Down syndrome and trisomy 18) and adverse pregnancy outcomes including preeclampsia, intrauterine growth restriction (IUGR), preterm birth, and fetal loss.

What Low PAPP-A Means

Low PAPP-A (pregnancy-associated plasma protein-A) is typically reduced in Down syndrome pregnancies and serves as a key component of first trimester combined screening alongside hCG and nuchal translucency (NT) measurement 1. However, even when aneuploidy is excluded, persistently low PAPP-A carries significant implications for pregnancy complications.

Aneuploidy Risk Assessment

The primary purpose of measuring PAPP-A is aneuploidy screening. First trimester screening combining NT measurements with PAPP-A and free beta-hCG or intact hCG achieves detection rates comparable to second trimester quad screening 2. When the combined first trimester screen is positive (typically using a risk cutoff of 1:200 for a 35-year-old in the first trimester), offer genetic counseling and discuss options for cell-free DNA screening or diagnostic testing via CVS 1.

Pregnancy Complication Risk

Beyond aneuploidy, unexplained low PAPP-A (<0.4 MoM) after gestational age confirmation by ultrasound is independently associated with increased risk of adverse obstetrical outcomes 3:

• Preeclampsia: 2-5 fold increased risk, particularly early-onset preeclampsia 4, 5, 6, 7
• IUGR: 2.5-fold increased risk 5, 6
• Preterm birth: 7-8 fold increased risk for spontaneous preterm delivery 5, 3
• Fetal loss/miscarriage: 4-fold increased risk 5, 3

Recent evidence demonstrates that PAPP-A levels ≤0.4 MoM are associated with a 25.5% rate of combined adverse outcomes versus 1.9% in women with normal PAPP-A 6.

Recommended Management Algorithm

Step 1: Confirm and Interpret Results

• Verify gestational age with ultrasound (crown-rump length in first trimester). If gestational age changes by ≥2 weeks, reinterpret results with corrected dating 2
• Define low PAPP-A as <0.5 MoM for general risk assessment, or <0.4 MoM for highest risk stratification 3
• Assess the combined first trimester screen result (PAPP-A + hCG + NT)

Step 2: Address Aneuploidy Risk

If combined screen is positive (high risk):

• Provide genetic counseling to estimate probability of trisomy 21 and trisomy 18
• Offer cell-free DNA (NIPS) as next step, or proceed directly to diagnostic testing (CVS or amniocentesis) based on patient preference 1, 8
• Cell-free DNA has superior sensitivity for common trisomies and should be made accessible to all patients 8

If combined screen is negative (low risk) but isolated low PAPP-A:

• No further aneuploidy evaluation is recommended based on isolated low PAPP-A alone 9
• Focus shifts to pregnancy complication surveillance

Step 3: Pregnancy Surveillance Protocol

For unexplained low PAPP-A (<0.4 MoM), implement enhanced surveillance 3:

Maternal monitoring:

• Enhanced patient education on signs/symptoms of preeclampsia (headache, visual changes, right upper quadrant pain, sudden swelling)
• Increased frequency of antenatal visits starting in second trimester
• Blood pressure monitoring at each visit
• Consider uterine artery Doppler at 22-24 weeks (abnormal Doppler with low PAPP-A identifies highest risk group) 3

Fetal monitoring:

• Serial growth ultrasounds (recommend every 3-4 weeks starting at 24-28 weeks)
• Amniotic fluid assessment
• Umbilical artery Doppler if growth restriction suspected
• Consider third trimester fetal surveillance (biophysical profile, non-stress testing) starting at 32-34 weeks 3

Cervical assessment:

• Consider cervical length measurement at 18-24 weeks given increased preterm birth risk 3

Step 4: Consider Preventive Therapy

Low-dose aspirin (60-81 mg daily):

• Limited evidence suggests benefit in women with elevated second trimester hCG and/or abnormal uterine artery Doppler at 22-24 weeks 3
• May consider initiating aspirin therapy in women with low PAPP-A (<0.4 MoM), particularly if combined with other risk factors (prior preeclampsia, chronic hypertension, diabetes, renal disease) 3
• Optimal timing is before 16 weeks gestation for preeclampsia prevention

Step 5: Offer Second Trimester Screening

All women with first trimester screening should be offered MSAFP screening at 16-18 weeks for neural tube defect detection 1. This is independent of the first trimester aneuploidy screen result.

Important Clinical Caveats

Gestational age accuracy is critical: PAPP-A measurements are most discriminatory at 9 weeks gestation and timing significantly affects interpretation 10. Always confirm dating with first trimester ultrasound.

Do not repeat PAPP-A testing: Obtaining a second specimen for repeat testing is discouraged except for suspected sample mix-up 10. Serial PAPP-A measurements do not improve prediction.

Twin pregnancies: First trimester screening is less effective in twins. PAPP-A levels reflect the overall pregnancy rather than individual fetuses, though NT measurements remain fetus-specific 10.

Isolated low PAPP-A has limited positive predictive value: While relative risks are elevated, the absolute risk remains modest. In one large low-risk cohort, PAPP-A <0.4 MoM had only 6.3% positive predictive value for preeclampsia and 4.4% for SGA 11. Therefore, low PAPP-A should not be used as sole indication for aspirin therapy or trigger invasive interventions, but rather guide intensified surveillance 11.

No specific treatment protocol exists: At present, no specific therapeutic protocol has been proven to clearly improve outcomes in pregnancies with isolated low PAPP-A 3. Management focuses on early detection of complications through surveillance.

Pathophysiology: Low PAPP-A likely reflects early placental dysfunction, with histopathologic correlates including distal villous hypoplasia, accelerated villous maturation, and avascular villi 7. This explains the association with both early-onset preeclampsia and IUGR.

What NOT to Do

• Do not offer diagnostic testing (CVS/amniocentesis) solely for isolated low PAPP-A if the combined first trimester screen is negative 9
• Do not use PAPP-A as a population-based screening method for adverse pregnancy outcomes outside research protocols, as sensitivity is low and false positive rates are high 3
• Do not rely on predetermined age-based cutoffs (e.g., age 35); instead, counsel women based on their individualized adjusted risk 1