Due date from LMP, conception, or ultrasound dating — Naegele’s rule with cycle-length adjustment, plus today’s gestational age, trimester ends, and days to go.
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FORMULA USEDPregnancy Due Date Calculator
Naegele’s rule
The 200-year-old standard counts 280 days (40 weeks) from the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14; if your cycles run longer or shorter, the due date shifts by the difference — a 34-day cycle adds six days. Gestational age already includes the two weeks before conception technically existed, which is why a due date is always counted from LMP, not from the sperm.
Three ways to date a pregnancy
LMP + 280 days is the calendar default. Conception date + 266 days works when you know the exact day (IVF, ovulation kits, careful timing). Ultrasound dating runs the tape backwards: EDD = scan date + (280 − gestation at scan). A first-trimester crown-rump length measurement is the most accurate method of the three, good to within about 5–7 days.
Trimesters, term, and milestones
The first trimester ends at 13 weeks 6 days (day 97), the second at 27 weeks 6 days (day 195). Delivery windows have their own vocabulary: early term 37–38w, full term 39–40w, late and post-term from 41w — babies arriving full-term do best on average, which is why scheduled deliveries wait for 39 weeks. This tool shows the trimester end dates and the current stage alongside the due date.
Worked example
LMP was Tuesday, March 10, 2026, with a textbook 28-day cycle.
280 days later → estimated due date Tuesday, December 15, 2026.
Cycle runs 32 days? Add 4 → December 19, 2026.
Estimated conception: LMP + 14 ≈ March 24, 2026.
How to use it
Choose the dating method you have: LMP, conception date, or ultrasound.
For LMP, enter the first day of the last period and your usual cycle length.
For ultrasound, enter the scan date and the gestation printed on the report.
Read the due date, today’s gestational age, trimester ends, and days remaining.
Frequently asked questions
How accurate is an estimated due date?
Only around 4–5% of babies are born on their exact due date, and roughly 80% arrive between 37 and 41 weeks. Treat the date as the middle of a delivery window, not an appointment.
What if my cycle is not 28 days?
Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. Longer cycles usually ovulate later, so the due date moves back by about one day per cycle day over 28 — this tool applies that adjustment automatically.
Why did my ultrasound give a different due date?
First-trimester scans measure the baby directly and are accurate to within a few days, so clinicians let them override the calendar date. If the two disagree by more than about a week in the first trimester, the scan wins.
What do early term, full term, and post-term mean?
Early term is 37–38 weeks, full term 39–40, late term 41, and post-term 42 and beyond. Outcomes are best in the full-term window, which is why elective deliveries are usually scheduled at or after 39 weeks.
Can I calculate a due date from conception instead of LMP?
Yes — add 266 days to the conception date, which is simply the 280-day rule minus the two pre-conception weeks. It suits IVF and ovulation-kit conceptions where the date is known precisely.
Should I plan my delivery around this date?
No — induction and C-section scheduling are clinical decisions made with your provider, usually based on 39-plus weeks and your health history. Use this estimate for milestones like announcing, leave planning, and the hospital bag.