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Pregnancy Due Date

Estimate your baby's due date from the first day of your last period, and see how far along you are on a date you choose.

What this tool does

Estimate your baby's due date from the first day of your last period, and see how far along you are on a date you choose. You fill in these fields: First day of last menstrual period and Measure progress on. It shows estimated due date, weeks pregnant, extra days, days until due date, and trimester. Method: Days since the first day of the last period; due date by Naegele's rule (280 days).

Inputs

  • First day of last menstrual period: A date. Use the first day of your last period, not the last day.
  • Measure progress on: A date. Starts as today's date. Defaults to today. Change it to see how far along you will be on another date.

How to use

  1. Pick a date for "First day of last menstrual period".
  2. Pick a date for "Measure progress on".
  3. Read the result: estimated due date, weeks pregnant, extra days, days until due date, and trimester. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Measure progress on: February 12, 2026

Results:

  • Estimated due date: October 8, 2026 (280 days after the first day of your last period (Naegele's rule))
  • Weeks pregnant: 6 (completed weeks)
  • Extra days: 0 (beyond the completed weeks)
  • Days until due date: 238 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)

How this is worked out: Naegele's rule, 280 days after the first day of the last period. 1 Jan 2026 is day 1 of a 365-day year, 1 + 280 = day 281, and day 273 is 30 Sep (cumulative month lengths 31, 59, 90, 120, 151, 181, 212, 243, 273), so day 281 is 8 Oct. Elapsed days 1 Jan to 12 Feb = 30 + 12 = 42 = 6 weeks 0 days. Days until due = 280 - 42 = 238.

Example 2. Inputs:

  • First day of last menstrual period: March 15, 2026
  • Measure progress on: March 15, 2026

Results:

  • Estimated due date: December 20, 2026 (280 days after the first day of your last period (Naegele's rule))
  • Weeks pregnant: 0 (completed weeks)
  • Extra days: 0 (beyond the completed weeks)
  • Days until due date: 280 (negative once past the estimate)
  • Trimester: First (first to 13w6d, second to 27w6d, third from 28w0d)

How this is worked out: 15 Mar 2026 is day 31 + 28 + 15 = 74. 74 + 280 = day 354. Day 334 is 30 Nov (cumulative 304 at 31 Oct, plus 30), so day 354 is 20 Dec. Zero elapsed days gives 0 weeks 0 days and 280 days to go.

Example 3. Inputs:

  • First day of last menstrual period: June 10, 2023
  • Measure progress on: March 17, 2024

Results:

  • Estimated due date: March 16, 2024 (280 days after the first day of your last period (Naegele's rule))
  • Weeks pregnant: 40 (completed weeks)
  • Extra days: 1 (beyond the completed weeks)
  • Days until due date: -1 (negative once past the estimate)
  • Trimester: Third (first to 13w6d, second to 27w6d, third from 28w0d)
  • Note: This date is past the estimated due date.

How this is worked out: Crosses a leap year. 10 Jun 2023 is day 31 + 28 + 31 + 30 + 31 + 10 = 161 of 365. 161 + 280 = 441; 441 - 365 = day 76 of 2024, a leap year (31 + 29 = 60 at 29 Feb, 76 - 60 = 16), so 16 Mar 2024. 17 Mar 2024 is day 77, so elapsed = (365 - 161) + 77 = 281 days = 40 weeks 1 day, one day past the estimate.

Example 4. Inputs:

  • First day of last menstrual period: January 1, 2026
  • Measure progress on: April 9, 2026

Results:

  • Estimated due date: October 8, 2026 (280 days after the first day of your last period (Naegele's rule))
  • Weeks pregnant: 14 (completed weeks)
  • Extra days: 0 (beyond the completed weeks)
  • Days until due date: 182 (negative once past the estimate)
  • Trimester: Second (first to 13w6d, second to 27w6d, third from 28w0d)

How this is worked out: 9 Apr 2026 is day 31 + 28 + 31 + 9 = 99, so elapsed from day 1 is 98 days = 14 weeks 0 days (exactly 98 / 7). 280 - 98 = 182.

Limitations

  • The date is an estimate. Few babies arrive exactly on it.
  • It assumes a regular 28-day cycle and a known first day of the last period.
  • It does not replace dating by ultrasound or advice from a health professional.
  • Where a result is marked approximate, treat it as a typical value, not an exact one.

FAQs

Q: How is the due date worked out?
A: It is 280 days (40 weeks) after the first day of your last menstrual period. This is the convention stated in ACOG Committee Opinion 700 and is the day-count form of Naegele's rule. The calendar form of the rule (add a year, subtract three months, add seven days) can land up to a few days away.
Q: Why does the progress change from day to day?
A: Progress is measured on the date in the second field, which starts as today. Set it to any date on or after your last period to see how far along you will be then.
Q: Which weeks count as each trimester?
A: This tool treats weeks 0 to 13 as the first trimester, 14 to 27 as the second and 28 onward as the third, counting completed weeks from the first day of the last period. That is the split used by ACOG and the ICD-10-CM coding guidelines. Some sources start the second trimester at week 13.
Q: Does the tool allow for a cycle that is not 28 days?
A: No. This version assumes a 28-day cycle. The Pregnancy Due Date Calculator adjusts the estimate for your cycle length.

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