About

Early-onset preeclampsia (before 34 weeks) represents a major cause of maternal and perinatal morbidity. Traditional screening based solely on maternal history detects only about 30% of cases. The combined screening approach, utilizing the Fetal Medicine Foundation (FMF) competing risks model, integrates maternal factors with biophysical and biochemical markers to improve detection rates to approximately 90%.

This tool processes inputs for Mean Arterial Pressure (MAP), Uterine Artery Pulsatility Index (UTPI), and serum markers PLGF and PAPP-A. It adjusts these raw values into Multiples of the Median (MoM) based on Crown-Rump Length (CRL) or Gestational Age standards. The calculated risk probability aids obstetricians in prescribing prophylactic aspirin therapy (150 mg/day) before 16 weeks, which has been shown to significantly reduce the prevalence of the severe form of the disease.

Formulas

The screening logic employs a Bayes' theorem approach. The posterior risk is calculated by multiplying the prior risk (from maternal history) by the Likelihood Ratios (LR) of the biomarkers. The MoM (Multiple of Median) calculation for any marker M is:

MoM = Measured_ValueExpected_Median(CRL)

The simplified probability model for early preeclampsia risk (P) follows a logistic regression structure:

Odds = P1 − P = PriorOdds × LR_MAP × LR_UTPI × LR_PLGF

Likelihood ratios are derived from Gaussian distributions of log-transformed MoMs. For example, in Preeclampsia cases, MAP is typically shifted right (higher), while PLGF is shifted left (lower).

Reference Data

Gestational Age (CRL)	Median MAP (mmHg)	Median UTPI	Median PLGF (pg/mL)	Median PAPP-A (IU/L)
45 mm (11w)	83.2	2.35	36.1	1.89
50 mm (11w+4d)	83.0	2.20	41.5	2.35
55 mm (12w+1d)	82.9	2.10	46.8	2.90
60 mm (12w+5d)	82.8	1.98	52.5	3.55
65 mm (13w+1d)	82.7	1.85	58.9	4.20
70 mm (13w+3d)	82.6	1.75	66.0	4.95
75 mm (13w+5d)	82.5	1.68	73.2	5.80
80 mm (14w+0d)	82.4	1.60	81.5	6.75
84 mm (14w+2d)	82.3	1.55	88.0	7.50

Frequently Asked Questions

This period is optimal for two reasons. First, biomarkers like PAPP-A and PLGF show the most significant deviation in affected pregnancies during the first trimester. Second, prophylactic aspirin is most effective when initiated before 16 weeks of gestation to facilitate proper placentation.

A risk cutoff of 1 in 100 is commonly used in FMF protocols. Patients with a risk greater than this threshold (e.g., 1 in 50) are typically recommended for aspirin therapy and increased surveillance.

Yes, but sensitivity decreases. The algorithm detects approximately 40% of cases with maternal history alone, 70% with MAP and UTPI added, and 90% when PLGF is included. Biochemical markers provide the highest specificity and sensitivity.

Mean Arterial Pressure should be measured twice in each arm while the patient is seated and relaxed. The final MAP used for calculation is the average of all four measurements.