Due Date Calculator
Last period date in — due date, current week, and trimester out.
How this is calculated
due date = LMP + 280 days (Naegele's rule, 40 weeks)
Example: LMP 1 January → due 8 October. Pregnancy weeks count from the LMP too, which is why "6 weeks pregnant" is possible four weeks after conception. Only ~5% of babies arrive on the date itself; the two weeks either side are all normal. A first-trimester scan may refine the estimate — when scan and dates disagree, clinicians go with the scan. Informational only — your antenatal care sets the clinical dates.
Frequently asked questions
How is a due date calculated?
By Naegele’s rule, the convention obstetrics has used for two centuries: 280 days (40 weeks) from the first day of the last menstrual period. Conception typically happens ~two weeks in, so "40 weeks pregnant" counts from the period, not conception.
How accurate is the due date?
It is a midpoint, not an appointment: only about 1 in 20 babies arrives on the exact date, while most arrive within two weeks either side. First-trimester ultrasound can shift the estimate if cycle dates are uncertain — the scan date wins when they disagree.
What if my cycles are longer or shorter than 28 days?
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Longer cycles push the real due date later (roughly a day per cycle-day over 28). Your care provider adjusts for this — mention your cycle length at the first visit.
What are trimester dates?
Counted from the same start: the first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days, and the third to birth. The result panel shows where the current week lands. This calculator is informational — antenatal care decides the clinical dates.