Pregnancy Due Date Calculator — LMP, Conception or Ultrasound

Pregnancy is dated from your last period, not conception

When you conceive, you are already counted as roughly two weeks pregnant. This confuses almost everyone the first time. Here's why: clinicians date pregnancy from the first day of your last menstrual period (LMP), not from conception. A "40-week" pregnancy contains about 38 weeks of actual gestation — the two-week gap accounts for the time from your period to ovulation to fertilization.

The standard formula is Naegele's rule: add 280 days to the first day of your last period. That is exactly 40 weeks, or 10 lunar months of 28 days each. So if the first day of your last period was 1 January, the estimated due date is 8 October — a day earlier in a leap year, since February gains a day along the way.

Adjust for your actual cycle length — this calculator does

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, your ovulation day shifts, and so does your true due date. Someone with a 35-day cycle typically ovulates around day 21, about seven days later than the formula assumes — so their due date should be about seven days later too.

This calculator lets you input your average cycle length (from 21 to 45 days). If you enter 35, the due date calculation automatically adds those extra days. Most online calculators do not offer this adjustment, so it's a real differentiator if your cycles are irregular. Track your cycle separately if you want to nail down your pattern first.

A due date is the middle of a range, not an appointment

Only about 5% of babies arrive on their calculated due date. Full-term births span 37 to 42 weeks — a five-week window. Your due date is the midpoint of that range, useful for planning but not predictive for any single person.

An early-pregnancy ultrasound (usually around 11–14 weeks) is more accurate than any date-based formula. If a scan shows your baby is ahead or behind the due date the calendar gave you, clinicians will revise it. This revision is normal and not a sign anything is wrong — it just means the measurement is more precise than the math.

Confirm with your healthcare provider

This calculator is an estimate for planning. Your midwife or doctor will date your pregnancy at your first antenatal appointment and refine it as scans provide more detail. If you have questions about your due date or any milestones in your pregnancy, ask your clinician — they have your full medical history and can give you personalized advice.