How does this work?
A due date is a single day, but birth is a range. Your due date is worked out as 40 weeks from the start of your last menstrual period — a convention, not a prediction. Very few babies read the memo: only about one in 29 arrives on the estimated date, and while 59% land within a week of it, that still leaves a fortnight-wide window most pregnancies fall somewhere inside.
Rather than give you one more date to stare at, we give you the whole distribution: the chance your baby arrives on each remaining day, and the chance they’ve arrived by each one.
Where the data comes from
Our figures come from birth certificate data published by the CDC’s National Center for Health Statistics, covering 10.3 million U.S. live births between 2022–2024. These are records of births that actually happened, not a model of what should happen.
Gestational age in those records is the obstetric estimate — the clinical assessment of how far along a pregnancy was at delivery. It replaced an older measure based on the last menstrual period, which tended to overstate long pregnancies.
How we get to individual days
One honest caveat about the calendar. Birth records give gestational age in completed weeks, not days. So while the chance of arriving during any given week comes straight from the data, spreading that week across its seven days is our own work.
We do it with a smooth curve fitted through the weekly figures, chosen so that each week’s seven days add back up to exactly the number the records show. The weekly totals are never altered — the only assumption is that the odds change gradually from one day to the next rather than jumping at week boundaries. Treat a single day’s figure as an estimate within a well-grounded week.
What we do with your answers
The five questions correspond to characteristics that measurably shift when babies arrive: whether you’re expecting one baby or twins, the baby’s sex, whether this is a first birth, your age, your education, and your racial or ethnic background. We find the births in the data matching your answers and show you when those babies arrived.
Each time you load your results we drop the days that have already passed and spread the remaining probability across the days still ahead — which is why the numbers change as your pregnancy progresses, and why the bookmark is worth keeping.
What this can’t tell you
A prediction is a best guess from what happened to other people. It cannot know about your pregnancy in particular — your health, your baby, whether your care team is planning an induction or a caesarean. The averages are right on average, and nobody is average.
We can’t help with triplets or more: there simply aren’t enough births in the record to say anything trustworthy.
This is general information, not medical advice. Your midwife or doctor knows your pregnancy; we know a spreadsheet.
Questions? Email info@whentoexpect.com.