Enter your last period or conception date to estimate your baby's due date and current gestational age.

How to Use the Due Date Calculator

Switch between "From Last Period" and "From Conception Date" depending on which you know, then enter that single date — the calculator recalculates the due date and, if the date has already passed, your current gestational age in weeks and days.

A due date is the anchor everything else on this site works from — conception estimates, week-by-week weight gain targets, the whole pregnancy timeline gets measured against it. It's also one of the most misunderstood numbers in obstetrics: a single calendar date presented with a precision the underlying biology doesn't actually have.

Naegele's Rule

The math behind the "From Last Period" mode comes from Franz Karl Naegele, a German obstetrician working in the early 1800s, who proposed adding 280 days — 40 weeks — to the first day of the last menstrual period:

Due date = LMP + 280 days
Due date = Conception date + 266 days

The conception-date mode simply starts 14 days later in the cycle and subtracts the same 14 days from the total, landing on the same target. Both routes assume a 28-day cycle with ovulation smack in the middle, on day 14 — an average, not a universal constant. Someone with a 35-day cycle ovulates later than that, which nudges their real due date later than Naegele's rule alone would suggest. That precision matters most for people who know their conception date exactly — after IVF or a monitored ovulation cycle, for instance — which is exactly the case the second mode is built for; everyone else is stuck estimating from a period date instead, with the built-in cycle-length uncertainty above.

Why Only About 5% of Babies Arrive On Their Due Date

That statistic surprises a lot of expectant parents, but it's well documented: full-term birth spans a five-week window, not a single day. Weeks 37 through 42 all count as full term, and a healthy pregnancy can end anywhere in that range without anything having gone wrong. Naegele's rule gives you the midpoint of a distribution, not a forecast — the actual delivery date is closer to a weather forecast five weeks out than a scheduled appointment.

First babies also tend to run a few days past the calculated date more often than not, and cycle length variation stacks on top of that. None of this means the number is useless — it's still the standard reference clinicians schedule screenings and monitoring against — it just means "your due date" is shorthand for "the center of a normal range," not a prediction anyone should bank on.

This is an estimate, not a medical prediction. Naegele's rule gives a statistically reasonable target date based on averages — it is not a diagnosis, and it does not account for factors a clinician tracks through ultrasound dating and prenatal visits. Confirm and refine your due date with a doctor or midwife, and treat any date from this calculator as a planning reference, not a countdown to expect precisely.