Due Date Calculator
Calculate your estimated due date from your last menstrual period (with cycle-length adjustment) or conception date, plus trimester dates and key milestones.
How the due date is calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception.
The classic estimate, Naegele’s rule, adds 40 weeks:
Due date = LMP + 280 days + (cycle length − 28)
The adjustment matters because the 280-day rule assumes ovulation on day 14 of a 28-day cycle. Ovulation typically
occurs about 14 days before the next period, so a longer cycle means later ovulation, later conception,
and a later due date — one day for each day of cycle length beyond 28.
If you know the conception date directly (from IVF, an ovulation test or tracking), the calculation skips the
cycle assumption entirely:
Due date = conception date + 266 days (38 weeks)
Worked example
LMP on 1 January 2026 with a 28-day cycle: 1 January + 280 days = 8 October 2026.
With a 31-day cycle the date shifts 3 days later, to 11 October. Conception is estimated around
1 January + (31 − 14) = 18 January, and the first trimester ends 13 weeks after the LMP, on 2 April.
What the milestones mean
- Estimated conception — around LMP + (cycle − 14) days, the likely ovulation day.
- End of 1st trimester — 13 weeks from LMP; miscarriage risk drops markedly after this point.
- End of 2nd trimester — 27 weeks from LMP; the third trimester begins at week 28.
- Weeks pregnant today — your current gestational age, cycle-adjusted to match the due date.
Remember that a due date is a midpoint, not a deadline: full term spans 37 to 42 weeks, and only around 5% of
babies arrive exactly on the estimated day.
Frequently asked questions
How accurate is a due date?
Only about 1 in 20 babies arrives on the exact due date — most are born within two weeks either side. The due date is a planning estimate: a first-trimester ultrasound is the most accurate way to date a pregnancy, and your provider may adjust the date after a scan.
How is the due date calculated from my last period?
The standard rule (Naegele’s rule) adds 280 days — 40 weeks — to the first day of your last menstrual period. Because that rule assumes a 28-day cycle with ovulation on day 14, this calculator also shifts the date by one day for every day your cycle is longer or shorter than 28 days.
Why does cycle length change the due date?
Ovulation usually happens about 14 days before the next period, so with a 32-day cycle you likely ovulate — and conceive — around day 18, four days later than the 28-day assumption. The due date moves later by those same four days.
Which is more accurate: LMP or conception date?
If you know the actual conception date (for example from IVF or ovulation tracking), it is more direct: due date = conception + 266 days. The LMP method estimates ovulation from your cycle length, which adds some uncertainty.
What are the trimester cut-offs?
The common convention used here is: first trimester through the end of week 13, second trimester through the end of week 27, and third trimester from week 28 until birth.
This tool provides general health information, not medical advice. Consult a healthcare professional before making decisions based on it. Dates are estimates — only an ultrasound can date a pregnancy accurately; always confirm with your healthcare provider.