What is the threshold for recommending aspirin?

Per NICE NG133 and ACOG guidelines, low-dose aspirin (75–150 mg/day) is recommended when one high-risk factor is present OR two or more moderate-risk factors are identified in the first trimester.

Which risk factors are considered high risk?

High-risk factors include: previous preeclampsia, chronic hypertension, pre-existing diabetes (type 1 or 2), chronic kidney disease, and autoimmune conditions such as antiphospholipid syndrome or SLE.

Which factors are moderate risk?

Moderate-risk factors include: first pregnancy (nulliparity), age 35 or older, BMI ≥ 30 kg/m², multifetal pregnancy (twins or more), and a family history of preeclampsia (mother or sister).

When should aspirin be started?

For maximum benefit, low-dose aspirin should be started before 16 weeks of gestation. Starting after 16 weeks significantly reduces its protective effect.

Does this tool replace clinical judgment?

No. This calculator is a clinical decision-support tool intended to supplement, not replace, assessment by a qualified healthcare professional. Always consider the full clinical picture.

Is a low-risk result a guarantee?

No. A low-risk classification means no recognized major risk factors were identified, but preeclampsia can still occur. All pregnant women should be monitored at every antenatal visit.

Preeclampsia Risk Calculator

Enter maternal risk factors to estimate first-trimester preeclampsia risk and receive aspirin prophylaxis guidance per NICE/ACOG criteria.

Maternal Risk Factors

Select all that apply at the first antenatal visit.

High-Risk Factors (any one = high risk)

Previous preeclampsia in any prior pregnancy Chronic hypertension (pre-existing) Pre-existing diabetes (type 1 or type 2) Chronic kidney disease (any stage) Autoimmune condition (antiphospholipid syndrome, SLE, etc.)

Moderate-Risk Factors (two or more = high risk)

First pregnancy (nulliparous) Multifetal pregnancy (twins, triplets, etc.) Family history of preeclampsia (mother or sister)

Maternal Age (years)

Age ≥ 35 = moderate risk

Pre-Pregnancy BMI

Weight

Height

Complete the form and click Calculate Risk to see results.

Summary

This calculator applies NICE NG133 and ACOG preeclampsia screening criteria to stratify first-trimester risk based on clinical history, demographics, and comorbidities. High-risk classification (one high-risk factor or two or more moderate-risk factors) triggers a recommendation to initiate low-dose aspirin before 16 weeks of gestation. Use it at the booking visit alongside blood pressure measurement and uterine artery Doppler when available.

How it works

Select all clinical risk factors that apply to the patient.
Enter maternal age and pre-pregnancy BMI.
The calculator applies NICE/ACOG risk stratification criteria.
A single high-risk factor or multiple moderate-risk factors triggers a high-risk classification.
The result shows risk level and whether low-dose aspirin (150 mg/day before 16 weeks) is recommended.

Use cases

Screen first-trimester patients for preeclampsia risk at an antenatal visit.
Decide whether to initiate low-dose aspirin prophylaxis before 16 weeks gestation.
Counsel patients on modifiable and non-modifiable risk factors.
Document clinical risk stratification for obstetric referral letters.
Educate medical students and nurses on NICE preeclampsia screening criteria.
Quick bedside reference during obstetric triage.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu