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.

Inputs

The first day of your last menstrual period.

Date to calculate gestational age for (defaults to today).

Result

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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 days

Dating runs from the LMP; 40 weeks defines term, and ovulation/conception is assumed two weeks in.

Source: ACOG — How your fetus grows during pregnancy

Worked examples

Example 1
LMP 2026-01-01, today 2026-05-21
Inputs:
lmp=2026-01-01, today=2026-05-21
Output:
20w 0d; due 2026-10-08
Example 2
LMP 2026-03-01, today 2026-05-21
Inputs:
lmp=2026-03-01, today=2026-05-21
Output:
~11w 4d; due 2026-12-06
Example 3
Due date for LMP 2026-06-10
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.

Frequently asked

How is pregnancy gestational age calculated?+
It is counted from the first day of your last menstrual period (LMP). The number of days since the LMP, divided into weeks and days, is your gestational age. So "10 weeks pregnant" means about 70 days since the LMP — roughly 8 weeks since conception.
What is Naegele’s rule?+
Naegele’s rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. Equivalently, take the LMP, subtract three months, and add seven days and one year.
Why is pregnancy dated from the LMP and not conception?+
Because the LMP is a known, observable date, while the exact day of conception is usually uncertain. Conception typically occurs about two weeks after the LMP, so gestational age runs roughly two weeks ahead of the actual embryonic age.
How accurate is the due date?+
It is an estimate. Naegele’s rule assumes a regular 28-day cycle with ovulation on day 14; longer or shorter cycles shift the date. An ultrasound in the first trimester provides more accurate dating. Only about 5% of babies are born on the due date itself.
What if my cycle is not 28 days?+
The LMP method assumes a 28-day cycle. If yours is consistently longer or shorter, your actual ovulation and due date shift accordingly. Your provider may adjust the dating, especially using ultrasound measurements.
When do the trimesters start and end?+
Roughly: the first trimester is weeks 1–13, the second is weeks 14–27, and the third is weeks 28 to birth. This tool labels the trimester based on your current gestational age.

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