M6 Model Calculator NHS
The M6 model is a specialist clinical risk-prediction model used after a pregnancy has been classified as a pregnancy of unknown location (PUL). It estimates probabilities of failed PUL, intrauterine pregnancy and ectopic pregnancy/persisting PUL from biochemical measurements. This independent page explains the pathway and provides a safe hCG-change calculator; it does not reproduce or approximate the validated M6 probability algorithm.
48-hour hCG change calculator
Enter laboratory hCG results taken approximately 48 hours apart. This arithmetic is useful context for PUL follow-up but is not an M6 risk score.
Do not use these numbers alone to rule in or rule out ectopic pregnancy. Symptoms and clinical assessment take priority.
What does the real M6 model use?
NHS clinical pathways using M6 describe a two-step approach for PUL. The validated model uses an initial β-hCG result and a repeat β-hCG around 48 hours later; the M6P version also incorporates progesterone. An NHS Worcestershire pathway states that the original model was based on PUL with initial hCG above 25 IU/L and predicts failed PUL, intrauterine pregnancy, or ectopic pregnancy including persisting PUL.
Why this page does not generate an M6 probability
M6 is a validated multivariable prediction model. Replacing its coefficients with an invented formula or a simple hCG threshold could give a misleading ectopic-pregnancy risk. The updated M6P and M6NP models have been externally evaluated, and their published performance depends on the validated model rather than a home-made approximation. Therefore this page deliberately does not label an hCG percentage as an M6 result.
How PUL is assessed in NHS care
A PUL means a pregnancy test is positive but an ultrasound has not located the pregnancy. Possible outcomes include a very early intrauterine pregnancy, miscarriage/failed PUL, or ectopic pregnancy. NHS information commonly describes repeat hCG blood testing about 48 hours apart as part of follow-up. Clinical symptoms remain crucial.
NICE hCG guidance is not the same as M6
NICE NG126 gives clinical follow-up recommendations based on 48-hour hCG change. For example, a decrease greater than 50% suggests the pregnancy is unlikely to continue, while a decrease below 50% or an increase below 63% requires clinical review in the early pregnancy assessment service. These are NICE management recommendations, not M6 probabilities.
Worked example
If initial hCG is 500 IU/L and the repeat value is 700 IU/L, the change is (700 − 500) ÷ 500 × 100 = 40%, and the hCG ratio is 1.400. The calculator reports only those arithmetic values. It does not classify the pregnancy or estimate ectopic risk.
When to seek urgent help
An ectopic pregnancy can be life-threatening. New or worsening abdominal/pelvic pain, shoulder-tip pain, dizziness, fainting, heavy bleeding or feeling very unwell requires urgent clinical assessment. Follow the instructions from your Early Pregnancy Unit; in an emergency call 999 or attend emergency care.
Authoritative Sources
- NICE NG126: ectopic pregnancy and miscarriage
- NHS: ectopic pregnancy diagnosis
- Cambridge University Hospitals NHS: Pregnancy of unknown location
Related Calculators
NHS Calculator · NHS Pregnancy Calculator · Ovulation Calculator NHS · NHS IVF Eligibility Calculator · Period Calculator NHS
FAQs
Is this the official M6 calculator?
No. This independent page explains M6 and calculates only the arithmetic change between two hCG measurements. It does not reproduce the validated M6 prediction algorithm.
What is a pregnancy of unknown location?
It means a pregnancy test is positive but the pregnancy has not been identified inside or outside the uterus on ultrasound.
Does a rising hCG level rule out ectopic pregnancy?
No. hCG results must be interpreted with symptoms, ultrasound findings and specialist follow-up.
Can I use the result instead of my Early Pregnancy Unit plan?
No. Follow the clinical plan given by your Early Pregnancy Unit or clinician, and seek urgent assessment if symptoms worsen.
Disclaimer: This page is educational and is not a diagnostic or triage service. It does not calculate a validated M6 probability and must not be used to delay urgent assessment or replace an Early Pregnancy Unit care plan.