Pregnancy Due Date Calculator
The estimated due date anchors every prenatal appointment and test window. This calculator uses Naegele's rule, counting 280 days from the first day of your last period, and adjusts for cycle length. It is an estimate: only about 4% of births land exactly on it.
Inputs
Result
Select a date
How the pregnancy due date calculator works
Naegele's rule: due date = first day of last menstrual period + 280 days, or LMP + 1 year − 3 months + 7 days.
This assumes a 28-day cycle with ovulation on day 14. For other cycle lengths, adjust by (cycle length − 28) days.
Gestational age counts from LMP; conception occurs roughly two weeks later, which is why fetal age is about two weeks behind gestational age.
Worked example: LMP on 12 March, 31-day cycle
- Naegele's baseline: 12 March + 280 days = 17 December.
- Cycle adjustment: 31 − 28 = +3 days, giving 20 December.
- Estimated conception: around 28 March.
- First trimester ends around 12 June, second around 18 September.
- Full term spans 37–42 weeks, so a normal delivery window here is roughly 27 November to 1 January.
Common mistakes to avoid
Counting from conception instead of LMP
Pregnancy is dated from the last period, not conception. Using a conception date adds about two weeks and misaligns every appointment.
Ignoring an irregular cycle
Naegele's rule assumes 28 days with day-14 ovulation. With irregular cycles, an early ultrasound gives a much better date.
Treating the date as fixed
Only about 4% of births occur on the estimated date, and around 80% fall within ten days either side. It is a midpoint, not a deadline.
Frequently asked questions
How accurate is a due date from the last period?
It sets a reasonable midpoint, but a first-trimester ultrasound is more accurate — usually within five to seven days.
How many weeks pregnant am I?
Count from the first day of your last period to today, then divide by seven. That is gestational age, which is what clinicians use.
What counts as full term?
37 to 42 weeks. Before 37 weeks is preterm; after 42 is post-term, and induction is usually discussed around 41 weeks.
Does a long cycle change the due date?
Yes. Add the difference between your cycle length and 28 days, because ovulation happens later.
When are the trimesters?
First 0–13 weeks, second 14–27 weeks, third 28 weeks to delivery.
Related calculators
Ovulation Calculator
Fertile window and ovulation day from your cycle.
Child Height Predictor
Mid-parental height estimate for a child.
BMI Calculator
Body Mass Index from height and weight, with WHO category.
Water Intake Calculator
Recommended daily water intake for your body weight.
Healthy Weight Calculator
Healthy weight range for your height based on BMI 18.5–24.9.