Predict II – first-trimester prenatal screening (free β-hCG + PAPP-A), 11–13+6 weeks of gestation
2 days Predict II – first-trimester prenatal screening is a non-invasive prenatal screening investigation based on a maternal blood sample and first-trimester ultrasound data, performed between 11 – 13+6 weeks of gestation. The investigation is intended to estimate the individual pregnancy risk for common chromosomal aneuploidies, especially:
- trisomy 21 – Down syndrome;
- trisomy 18 – Edwards syndrome;
- trisomy 13 – Patau syndrome.
First-trimester screening allows early identification of pregnancies at increased risk for aneuploidies by combining maternal biochemical markers PAPP-A and free β-hCG with maternal age and first-trimester ultrasound parameters, in particular CRL / LCC — crown-rump length, NT – nuchal translucency, and nasal bone status: present, absent, or not assessed.
Within the 11 – 13+6 gestational weeks interval, trisomies 21, 18, and 13 are associated with an increased risk with advancing maternal age, increased NT, and reduced PAPP-A values. The biochemical profile differs among aneuploidies: in trisomy 21, free β-hCG is frequently elevated; in trisomy 18, free β-hCG is frequently reduced; in trisomy 13, the biochemical profile may vary depending on the algorithm used, however reduced PAPP-A and increased NT are frequently used elements in risk assessment.
Biochemical marker results are expressed in MoM — multiples of the median and are integrated into the risk calculation algorithm together with relevant maternal and ultrasound data.
The investigation is a screening test, not a diagnostic one. An increased-risk result does not confirm the presence of a chromosomal abnormality, but indicates the need for obstetric/genetic counseling and, if necessary, further investigations: NIPT as an additional screening test, chorionic villus sampling or amniocentesis, according to the recommendation of the obstetrician-gynecologist or maternal-fetal medicine specialist.
The calculated risk depends directly on the accuracy of the entered data, especially the precision of ultrasound measurements.
Test components
Component | Description |
PAPP-A — Pregnancy-Associated Plasma Protein A | Placental protein involved in placental and pregnancy development. Reduced values in the first trimester may be associated with an increased risk of trisomies 21, 18, and 13. In certain clinical contexts, low PAPP-A may also be associated with a risk of placental complications, but it should not be interpreted in isolation. |
| free β-hCG — the free beta subunit of hCG | Biochemical marker produced by trophoblastic/placental tissue. In trisomy 21, free β-hCG is frequently elevated; in trisomy 18, free β-hCG is frequently reduced; in trisomy 13, the biochemical profile may vary depending on the algorithm used. Interpretation is performed as part of integrated risk calculation. |
| CRL — crown-rump length | Ultrasound parameter used to confirm gestational age in the first trimester and to validate screening eligibility. For accurate risk calculation, the usual accepted range is CRL 45–84 mm. |
| NT — nuchal translucency | Ultrasound marker measured in the first trimester. Increased NT may be associated with aneuploidies, cardiac malformations, and other fetal anomalies. It should not be interpreted in isolation, but in the context of gestational age, CRL, biochemical markers, and maternal data. NT measurement must be performed according to standardized criteria by trained personnel, as measurement errors can significantly alter the calculated risk. |
| Nasal bone — NB | Additional ultrasound marker. Absence or hypoplasia of the nasal bone may increase the likelihood of trisomy 21, depending on gestational age, CRL / LCC, NT, and biochemical markers. |
These changes do not establish a diagnosis. They contribute to probability calculation and must be interpreted in a clinical, biochemical, and ultrasound context.
The Predict II report presents the measured values of the maternal biochemical markers PAPP-A and free β-hCG, expressed in concentration units and as MoM — multiples of the median. The report includes the calculation of individual risk for the evaluated aneuploidies, namely:
- age-related risk for trisomy 21;
- biochemical risk for trisomy 21, calculated based on serum markers;
- combined risk for trisomy 21, integrating first-trimester ultrasound data;
- combined calculated risk for trisomies 13 and 18;
- LR — likelihood ratio for T13/T18 risk;
- interpretation cut-offs for T21 and T13/18;
- graphical representation of T21 risk;
- interpretation of the result as low, intermediate/borderline, or high risk, according to report thresholds.
Role of the investigation
Predict II aims to early identify pregnancies with an increased probability of common chromosomal aneuploidies. The result allows risk stratification and appropriate clinical guidance:
- routine obstetrical follow-up;
- obstetric/genetic counseling;
- NIPT as an additional screening test;
- invasive prenatal diagnosis, if indicated.
First-trimester screening contributes to the integrated assessment of pregnancy at a stage when ultrasound allows confirmation of fetal viability, determination of gestational age, measurement of CRL and NT, evaluation of the nasal bone, and early detection of major structural anomalies.
Indications
The investigation is indicated for pregnant women between 11 – 13+6 weeks of gestation, as part of first-trimester prenatal screening.
It may be particularly recommended in the following situations:
- routine prenatal screening for trisomies 21, 18, and 13;
- advanced maternal age, especially ≥35 years at the estimated date of delivery;
- previous pregnancy with fetal aneuploidy;
- relevant family history of chromosomal abnormalities;
- suggestive ultrasound markers or increased NT at first-trimester scan;
- assisted reproductive technology pregnancy;
- patient request, after counseling regarding the purpose and limitations of screening.
Particularities in twin pregnancy
In twin pregnancies, interpretation of first-trimester screening is more complex than in singleton pregnancy.
Maternal biochemical markers PAPP-A and free β-hCG reflect the placental/feto-placental contribution of the twin pregnancy, and their values require interpretation using a validated algorithm for multiple pregnancies. For risk calculation, first-trimester ultrasound data are essential, including CRL, NT for each fetus, number of fetuses, and chorionicity.
In twin pregnancy, calculated risk may be influenced by the type of twin gestation. In monochorionic pregnancies, risk is generally interpreted at pregnancy level, whereas in dichorionic pregnancies, risk may be estimated separately for each fetus, depending on the algorithm used.
The result must be interpreted by an obstetrician-gynecologist or maternal-fetal medicine specialist, in correlation with first-trimester ultrasound, chorionicity, and maternal clinical data.
Procedure
Patient identity, sampling date, gestational age, mandatory maternal data, and first-trimester ultrasound report are verified.
Blood is collected from a vein via standard venipuncture. The sample is analyzed in the laboratory to determine PAPP-A and free β-hCG concentrations. The values are then expressed in MoM and integrated into the risk calculation algorithm.
The sampling procedure takes a few minutes and carries no significant risks for mother or fetus.
Interpretation of results
The result is not diagnostic and must be interpreted by an obstetrician-gynecologist or maternal-fetal medicine specialist.
A low-risk result reduces the probability of the evaluated aneuploidies but does not completely exclude a chromosomal abnormality.
A high-risk result for trisomy 21, 18, or 13 does not establish a diagnosis. It indicates the need for medical counseling and discussion of further options, including NIPT as an additional screening test or invasive diagnostic testing, according to clinical management.
Sources:
https://pmc.ncbi.nlm.nih.gov/articles/PMC3547446/
https://www.isuog.org/static/f465db45-655c-42eb-96a196bcd2d34547/ISUOG-Practice-Guidelines-Updated-performance-of-11-14-week-ultrasound-scan.pdf
https://pmc.ncbi.nlm.nih.gov/articles/PMC6596981/
https://tjodistanbul.org/uploads/screening-for-fetal-chromosomal-abnormalities-number-226.pdf
Preparation:
No special preparation is required.
It is recommended to:
- have the blood sample collected approximately 2–3 hours after the last meal;
- provide the first-trimester ultrasound report, preferably performed on the day of blood collection or one day before the test;
- provide all information required for the individual risk calculation.
The following information is required for risk calculation:
◦ patient's date of birth (maternal age);
◦ gestational age at the time of blood collection;
◦ maternal body weight;
◦ ethnicity;
◦ smoking status (smoker / non-smoker);
◦ presence of maternal diabetes mellitus (if applicable);
◦ spontaneous pregnancy or pregnancy achieved through assisted reproductive technologies (ART);
◦ oocyte donation (if applicable);
◦ previous pregnancy affected by trisomy 21, 18 or 13 (if applicable);
◦ number of fetuses;
◦ chorionicity in twin pregnancy (if known);
◦ first-trimester ultrasound report including CRL (Crown-Rump Length) and NT (Nuchal Translucency) measurements;
◦ nasal bone status: present / absent / not assessed. If the required maternal or ultrasound data are missing, the risk calculation may be incomplete, inaccurate, or impossible.
The final result depends on the accuracy of the clinical and ultrasound information entered into the risk assessment algorithm.
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