Pregnancy Week Estimator (LMP-based)
Estimate gestational age and due date from the first day of your last menstrual period (LMP), using Naegele’s rule.
Result
How to use this calculator
- Enter the first day of your last menstrual period.
- Leave the current date as today, or set a specific date.
- Read your gestational age in weeks and days and your estimated due date.
- Confirm with your healthcare provider and an early ultrasound.
About this calculator
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception, because the LMP is a date most people can identify while ovulation is harder to pin down. By convention a full-term pregnancy lasts 40 weeks (280 days) from the LMP, which is the basis of Naegele’s rule for the estimated due date. This calculator counts the days from your LMP to today to give your current gestational age in weeks and days, identifies the trimester, and projects the due date by adding 280 days. It also estimates the approximate conception date as two weeks after the LMP, assuming a typical 28-day cycle. These are estimates: cycle length varies, and an early ultrasound gives the most accurate dating. Only a small fraction of babies actually arrive on the predicted due date.
How it works — the formula
Gestational age = days(today − LMP), shown as weeks + days
Due date = LMP + 280 days (Naegele’s rule)
Conception ≈ LMP + 14 daysDating runs from the LMP; 40 weeks defines term, and ovulation/conception is assumed two weeks in.
Worked examples
- Inputs:
- lmp=2026-01-01, today=2026-05-21
- Output:
- 20w 0d; due 2026-10-08
- Inputs:
- lmp=2026-03-01, today=2026-05-21
- Output:
- ~11w 4d; due 2026-12-06
- Inputs:
- lmp=2026-06-10, today=2026-06-10
- Output:
- 0w 0d; due 2027-03-17
Limitations
- Assumes a 28-day cycle with day-14 ovulation.
- Ultrasound dating is more accurate, especially with irregular cycles.
- An estimate — not a medical determination of due date.
Educational estimate, not medical advice; confirm with your healthcare provider.