Pregnancy due date calculator
Estimate a due date, gestational age and trimester dates from a last period or conception date.
A clear answer. Here’s how.
Naegele’s rule sets the due date 280 days (40 weeks) after the first day of the last menstrual period, moved by cycle length minus 28 days. From conception the due date is 266 days later. Gestational age counts days from the same adjusted start, and trimesters use 14 and 28 weeks; term categories follow ACOG (early term 37–38w6d, full term 39–40w6d, late term 41w, post-term 42w). Only an estimate: early ultrasound dating can override it.
How this is checked
3 reference checks passed These 3 cases were run when this page was built, on 20 October 2026, and all reproduce within the stated tolerance. The same cases are part of the automated tests that run on every change to Toolmoz. Engine version 1.
See the checks and where each expected answer comes from
Case 1
- Inputs
- Count from lmpDate 2026-01-01Usual cycle length (days) 28Show progress on 2026-04-01
- Expected, and what Toolmoz gave
- Estimated due date expected 2026-10-08 (Thursday), Toolmoz 2026-10-08 (Thursday)Gestational age on 2026-04-01 expected 12 weeks 6 days, Toolmoz 12 weeks 6 daysTrimester on that date expected First, Toolmoz First
- Where the expected answer comes from
- LMP + 280 days: 1 Jan is day 1, so day 281 is 8 Oct (273 days end 30 Sep); 280 = 40 weeks, so the weekday is unchanged (Thursday). 1 Jan to 1 Apr is 31 + 28 + 31 = 90 days = 12 weeks 6 days, before week 14
- Allowed difference
- exact
Case 2
- Inputs
- Count from conceptionDate 2026-01-01Usual cycle length (days) 28Show progress on 2026-04-01
- Expected, and what Toolmoz gave
- Estimated due date expected 2026-09-24 (Thursday), Toolmoz 2026-09-24 (Thursday)
- Where the expected answer comes from
- Conception + 266 days: day 1 + 266 = day 267; 1 Sep is day 244, so day 267 is 24 Sep; 266 = 38 weeks, so Thursday again
- Allowed difference
- exact
Case 3
- Inputs
- Count from lmpDate 2026-01-01Usual cycle length (days) 32Show progress on 2026-04-01
- Expected, and what Toolmoz gave
- Estimated due date expected 2026-10-12 (Monday), Toolmoz 2026-10-12 (Monday)
- Where the expected answer comes from
- A 32-day cycle moves the date by 32 − 28 = 4 days: 8 Oct + 4 = 12 Oct, a Monday (Thursday + 4)
- Allowed difference
- exact
What this shows: the engine reproduces independently worked values for these cases. It does not prove every possible input is right. To test an answer you were given, see how to check a number; to see what is and is not checked across Toolmoz, see the accuracy ledger.
See a worked example
Worked example
A fixed demonstration, separate from the values you enter above.
- Count from
- First day of last period
- Date
- 2026-01-01
- Usual cycle length (days)
- 28
- Show progress on
- 2026-04-01
Example result
- Estimated due date
- 2026-10-08 (Thursday)
- Estimated start (day 0)
- 2026-01-01 (Thursday)
- Gestational age on 2026-04-01
- 12 weeks 6 days
- Trimester on that date
- First
- Second trimester begins (14w)
- 2026-04-09 (Thursday)
- Third trimester begins (28w)
- 2026-07-16 (Thursday)
- Early term begins (37w)
- 2026-09-17 (Thursday)
- Full term begins (39w)
- 2026-10-01 (Thursday)
- Late term begins (41w)
- 2026-10-15 (Thursday)
- Post-term begins (42w)
- 2026-10-22 (Thursday)
Naegele’s rule: due date = first day of last period + 280 days, adjusted by cycle length minus 28 days; from conception, 266 days. Only an estimate: ultrasound dating early in pregnancy is more accurate than menstrual dating when they differ, and most babies are born before or after this date. This is not medical advice.