due date
calculator.
a quiet, evidence-based estimate.
Most due-date calculators online ask for a date, then bury the answer in a pop-up. This one is open, instant, and based on Naegele's rule (the same formula your obstetrician uses). No signup, no email, no ads.
estimated due date
Sunday, April 11, 2027
how the formula works
Naegele's rule estimates the due date as 280 days (40 weeks) from the first day of your last menstrual period — or 266 days (38 weeks) from the date of conception. It assumes a regular 28-day cycle. If your cycle is longer or shorter your obstetrician may adjust the estimate after the first ultrasound.
This calculator is for general guidance only. It is not a replacement for medical assessment.
how accurate is a due date?
Honestly — it's an estimate, not an appointment. Only about 4 in 100 babies arrive on their exact due date; most are born within a two-week window either side. A full-term pregnancy is anything from 37 to 42 weeks, which is why midwives talk about a “due month” more than a due day.
The most reliable dating comes from the first-trimester ultrasound (usually between 8 and 14 weeks), when babies grow at a very predictable rate. If the scan estimate differs from your period-based date by more than a few days, your provider will normally switch to the scan date — and that becomes the date everything else is planned around.
IVF and irregular cycles
After IVF the due date is calculated from the transfer date rather than a period: 266 days minus the embryo's age at transfer (usually 3 or 5 days). Because the conception day is known exactly, IVF due dates are actually more precise than natural ones. If your cycles are irregular or longer than 28 days, the period-based method drifts — count on the ultrasound to settle it.
common questions
Can my due date change?
Yes — most often after the first ultrasound, if the scan measurement disagrees with your period-based estimate by more than about five to seven days. After that first-trimester dating is set, providers rarely change it again.
How many weeks pregnant am I right now?
Enter your last period date above — the calculator shows your current week alongside the due date. Pregnancy is counted from the first day of the last period, so you're usually about two weeks “further along” than the actual conception.
What if I don't remember my last period?
Use the conception date if you know it, or wait for the dating ultrasound — it estimates gestational age from the baby's measurements and works even with no period information at all.
Is 40 weeks the same as 9 months?
Slightly more — 40 weeks is about 9 months and one week, counted from the last period. That's why trimester math never seems to add up neatly: months and weeks just don't divide evenly.