Only about 5% of babies arrive on their due date, but that date still anchors everything: prenatal appointments, screening windows and maternity planning all count from it. This calculator uses Naegele's rule — the same method most clinicians start with — to estimate it from the first day of your last menstrual period.
You will also get an estimated conception date (roughly two weeks after your last period began, near ovulation) and a timeline showing which trimester you are in and what typically happens in each. If you know your conception date from fertility treatment, you can enter that instead for a more precise estimate.
Part of the Health Calculators collection on CountCrunch.
How the due date is figured
Example: last period began 1 January → subtract 3 months (1 October), add 7 days (8 October), add a year → 8 October. The method assumes a 28-day cycle with ovulation on day 14; the calculator adjusts if your cycle is longer or shorter.
Trimester guide
- First trimester (weeks 1–12): organ development; first ultrasound usually around weeks 8–12; dating scan can refine the due date.
- Second trimester (weeks 13–27): anatomy scan around week 20; many people feel movement around weeks 18–22.
- Third trimester (weeks 28–40): growth scans, birth planning; full term is 39–40 weeks.
How accurate is the estimate?
A first-trimester ultrasound dates a pregnancy more accurately than LMP math, especially with irregular cycles. Your clinician may adjust the due date after the dating scan — that is normal and expected.
Frequently asked questions
How is my due date calculated from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. This is Naegele's rule, the standard clinical starting point.
What if my cycles are irregular or longer than 28 days?
Enter your average cycle length and the calculator shifts the estimate — ovulation (and therefore conception) happens about 14 days before your next period, not 14 days after the last one.
Can I use my conception date instead?
Yes, and it is more precise. Add 266 days (38 weeks) to the conception date. This is the option to use after IVF or known ovulation.
How many babies are born on their due date?
Only about 1 in 20. Most arrive between 37 and 42 weeks; first babies often run a little late.
When should I see a doctor after a positive test?
Most guidelines suggest booking a first prenatal visit around 8–10 weeks. If you have pain, bleeding or a high-risk history, call sooner.