Compare labour probability calculators
What to look for, and how this one differs
Last updated
Most labour and due date calculators give one date, or a probability curve with no stated data or model. The useful questions are: what data is the curve fitted to, does it handle preterm births, does it condition on how far along you are, and which personal factors does it adjust for.
Feature comparison
| Feature | Is Baby Coming? | Typical calculators |
|---|---|---|
| Statistical model | Johnson SU distribution, truncated at 42 weeks | Simple curves, often undocumented |
| Data | NHS Maternity Statistics 2023-24 and CDC 2023 | Older or unstated |
| Preterm births | Calibrated to a 10% preterm share | Often ignored |
| Conditional probability | Yes: odds assume you are still pregnant today | Varies |
| Parity | First vs later pregnancies | Sometimes |
| Baby's sex | About 1 day later for boys | Rare |
| BMI | Five categories | Rare |
| Twins and triplets | Yes, 3 to 7.5 weeks earlier | Rare |
| Interpregnancy interval | Yes | Rare |
| Date planner | Odds before, during and after any dates | Rare |
| Sources shown | Every factor links its study | Rare |
What makes the difference
Recent official data
The model is fitted to NHS Maternity Statistics for 2023-24 and CDC natality data for 2023, so it reflects current birth patterns rather than studies from decades ago.
A model that handles the preterm tail
The Johnson SU distribution has a heavy left tail. It fits the roughly 10% of births that happen before 37 weeks as well as the peak at 40 to 41 weeks, which simple curves cannot do at the same time.
Conditional odds and personal factors
Odds are recalculated for where you are today, and the curve shifts for parity, BMI, the baby's sex, twins or triplets, and the gap since a previous birth. Each adjustment cites its study on the methodology page.
What no calculator can do
No calculator can predict your day. Individual prediction has limited accuracy in research, and inductions and planned births follow a schedule rather than the curve. Treat any calculator as a planning aid and talk to your midwife or doctor about your own care.