Risk of Preeclampsia at 19–24 Weeks According to FMF: Routing
What Exactly the FMF 2025 Model Assesses
The work of the FMF group was published in 2025 in Ultrasound in Obstetrics & Gynecology and is available in open access: Assessment of risk for pre-eclampsia at mid-gestation to define subsequent care. This is a competing risks model for the 19–24 week visit, when second-trimester ultrasound data and uterine artery Doppler are already available. The result of the calculation is the individual probability of preeclampsia requiring delivery within specified gestational intervals.
The key difference from a simple 'normal or elevated PI' is that the model combines several independent blocks of information. Therefore, the same uterine artery PI carries different weight for patients with different histories, different estimated fetal weights, and different mean arterial pressures.
Input Data: What Needs to Be Collected at the Appointment
For correct calculation, four groups of parameters are used: maternal factors, estimated fetal weight, mean arterial pressure, and uterine artery pulsatility index. In practical work, this means that the ultrasound report should not be limited to the phrase 'blood flow in the uterine arteries is not impaired': the model requires a numerical PI, preferably converted to MoM in a validated FMF calculator.
| Data Block | What is Entered into the Model | Clinical Meaning |
|---|---|---|
| Maternal Factors | History and basic characteristics of the pregnant woman | Form the a priori risk of preeclampsia |
| EFW | Estimated fetal weight on ultrasound at 19–24 weeks | Reflects the contribution of early placentation and growth disturbances |
| Mean Arterial Pressure | Mean arterial pressure measured according to a standardized protocol | Increases the accuracy of the prognosis compared to history and ultrasound alone |
| Uterine Arteries | Uterine artery PI, preferably as MoM | Marker of uteroplacental blood flow resistance |
Gestational Risk Thresholds
The FMF calculation in the second trimester is not aimed at a binary answer 'will/will not have preeclampsia', but at the timing of the anticipated clinical event. It is the timing of delivery that determines the urgency of further monitoring. The risk of preeclampsia before 28 weeks carries different management weight than the risk of preeclampsia after 36 weeks.
| Model Output | How to Interpret | Routing Priority |
|---|---|---|
| Preeclampsia with delivery before 28 weeks | The earliest and clinically severe scenario | Maximum |
| Preeclampsia with delivery before 32 weeks | Early scenario with high significance for perinatal outcome | Very high |
| Preeclampsia with delivery before 36 weeks | Preterm preeclampsia | High |
| Preeclampsia after 36 weeks | Late scenario | Planned, but not zero |
How to Use the Result for Routing
The practical algorithm begins with determining the earliest interval in which the calculated risk is recognized as elevated according to the institution's accepted threshold. If the high risk pertains to an event before 28 weeks, the patient should not wait for the standard late third-trimester visit: early involvement of the obstetric team, monitoring of symptoms, blood pressure, fetal growth, and Doppler parameters is required. If the risk is shifted to the interval before 32 or 36 weeks, monitoring is also intensified, but the urgency is lower.
Important: the FMF 2025 article validates the stratification method but does not replace the local clinical protocol. The threshold at which a woman is transferred to a more intensive route depends on the availability of specialized consultations, the frequency of ultrasound monitoring, and the organization of perinatal care.
The Role of Uterine Artery Doppler
The uterine artery PI in the second trimester remains one of the central ultrasound markers of risk. However, in the FMF model, it is not interpreted in isolation. One cannot mechanically apply a universal 'normal PI' to all patients: the calculation is performed through the distribution of the indicator taking into account gestational age and maternal characteristics, usually in the MoM format.
For the ultrasound diagnostician, this shifts the focus of the protocol. Reproducible measurement technique, indication of the side or average value for the uterine arteries according to the accepted protocol, numerical PI, and absence of rounding to qualitative categories are needed. The more accurate the input data, the more reliable the individual risk.
Mean Arterial Pressure: Why It Is Included in the Ultrasound-Oriented Model
Mean arterial pressure is not an ultrasound indicator, but its inclusion is fundamental to the competing risks model. Preeclampsia develops as a clinical syndrome, and the hemodynamic component enhances the predictive value of the calculation. Therefore, the optimal scenario for 19–24 week screening is a single visit where ultrasound, Doppler, and standardized blood pressure measurement are performed together.
If the ultrasound office only provides EFW and uterine artery PI, and mean arterial pressure is entered later by the obstetrician, it is necessary to maintain the original numerical values without loss of accuracy. Repeated manual recalculation based on incomplete data reduces the meaning of the FMF approach.
EFW as Part of the Preeclampsia Prognosis
The inclusion of estimated fetal weight reflects the connection between early placental dysfunction, growth retardation, and preeclampsia. At 19–24 weeks, there may not yet be a pronounced growth lag, but EFW already adds information to maternal factors and Doppler. Therefore, biometry should be technically sound, with the correct gestational age and a consistent method of weight calculation.
What Not to Do
- Do not use the model as a diagnostic criterion for preeclampsia: it assesses future risk, not confirms the disease.
- Do not replace the numerical uterine artery PI with a textual assessment of 'normal' or 'elevated'.
- Do not interpret high risk of late preeclampsia the same as high risk of an event before 28 or 32 weeks.
- Do not apply unvalidated local PI thresholds if the calculation is performed according to FMF logic with MoM.
- Do not consider the result as an independent indication for delivery: the decision depends on the clinic, blood pressure, laboratory data, fetal condition, and gestational age.
Practical Conclusion Template
In the second-trimester ultrasound protocol, it is advisable to separately indicate: the study period of 19–24 weeks, EFW, uterine artery PI, method of obtaining the average value, presence or absence of technical limitations. In the obstetric conclusion according to the FMF model, mean arterial pressure and final individual risks by gestational intervals: before 28, before 32, before 36 weeks, and after 36 weeks are additionally recorded.
The final wording should be managerial: 'according to the FMF 2025 model, the risk belongs to the group of early/preterm/late event; routing is recommended according to the local protocol for monitoring high-risk pregnancies'. This format is understandable to the obstetrician, ultrasound diagnostician, and perinatal council.
Frequently asked questions
Can risk be assessed solely by uterine artery PI?
No. In the FMF 2025 model, uterine artery PI is one of the components alongside maternal factors, EFW, and mean arterial pressure. An isolated assessment of PI is not equivalent to calculating individual risk.
At what gestational age is the model applied?
The model is intended for risk assessment in mid-pregnancy, at the 19–24 week visit, when fetal biometry, EFW, uterine artery Doppler, and mean arterial pressure are available.
Is high risk an indication for delivery?
No. A high calculated risk determines the monitoring route and frequency of clinical control but is not a diagnosis and does not replace the assessment of symptoms, blood pressure, laboratory indicators, and fetal condition.