Due Date Calculator
Naegele’s rule estimates a due date as the first day of the last menstrual period plus 280 days — 40 weeks. Because the rule assumes a 28-day cycle with ovulation on day 14, this calculator shifts the whole dating window by the difference when your cycle is longer or shorter, then reports the gestational age, the trimester, and the dates the usual milestones fall on. It is an estimate: only about 4% of babies are born on the day it names.
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Estimated due date
29 May 2027
Trimester
First
Details
Updates as you typeFirst day of one period to the first day of the next. Naegele’s rule assumes 28 days.
The first day of bleeding, not the last. Must fall on or before the date below.
Defaults to today. Change it to read the gestational age on any other date.
Summary
First trimester — up to 13 weeks 6 daysEstimated due date
29 May 2027
40 weeks of gestation
280 days
- Days completed00%
- Days remaining280100%
- Gestational age
- 0w 0d
- Trimester
- First
- Days to the due date
- 280
- Weeks completed
- 0.0
- Estimated conception
- 2026-09-05
- Full term begins
- 2027-05-22
- Naegele’s rule adds 280 days — 40 weeks — to the first day of the last period, assuming a 28-day cycle with ovulation on day 14.
- This is an estimate, not an appointment. Only about 4% of babies arrive on their estimated due date, and most births fall within roughly two weeks either side of it.
- A dating ultrasound in the first trimester measures the pregnancy directly and is more accurate than the last period. Where the two disagree by more than about a week, the scan date is normally the one used.
How this is calculated
- First day of last period
- 2026-08-22
- Cycle length
- 28 days
- Cycle adjustment (length − 28)
- 0 days
- Adjusted start of dating
- 2026-08-22
- Estimated conception (day 14)
- 2026-09-05
- Second trimester begins (14 weeks)
- 2026-11-28
- Anatomy scan window opens (18 weeks)
- 2026-12-26
- Third trimester begins (28 weeks)
- 2027-03-06
- Early term begins (37 weeks)
- 2027-05-08
- Full term begins (39 weeks)
- 2027-05-22
- Estimated due date (40 weeks)
- 2027-05-29
- Late term begins (41 weeks)
- 2027-06-05
- Post-term begins (42 weeks)
- 2027-06-12
- Gestational age worked out on
- 2026-08-22
- Days since the adjusted start
- 0
- That age in weeks and days
- 0w 0d
Compare scenarios
See how one change moves the result
- CurrentYour inputs as they stand29 May 2027Current
- Average cycle lengthdays 355 June 2027
For informational purposes only. This is not medical advice — consult a healthcare professional about your health.
Frequently asked questions
How is a due date calculated from the last period?
By Naegele’s rule: add 280 days — 40 weeks — to the first day of the last menstrual period. The traditional shorthand is to subtract three months, add seven days, and add a year, which gives the same answer for almost every start date. The 280 days are counted from the period rather than from conception because the period is the event most people can date accurately, and the rule builds in a 14-day wait for ovulation.
What if my cycle is not 28 days long?
Naegele’s rule assumes ovulation on day 14 of a 28-day cycle. A longer cycle usually means a longer stretch before ovulation, not a longer second half, so this calculator adds the difference between your cycle length and 28 to the start of dating and moves every date with it. A 35-day cycle pushes the due date seven days later; a 21-day cycle pulls it seven days earlier.
How likely is a baby to arrive on the due date?
Around 4% of births fall on the estimated date itself. Roughly half arrive within a week of it and the large majority within two weeks either side, so the due date is best read as the middle of a range rather than as a deadline. Births are classed as early term from 37 weeks, full term from 39 weeks, late term from 41 weeks, and post-term from 42 weeks.
What does gestational age in weeks and days mean?
Gestational age is counted from the first day of the last period, not from conception, so at the moment of conception the pregnancy is already about two weeks old. It is written as completed weeks plus days — "12w 3d" means twelve full weeks and three days. Fetal age, or conceptional age, is roughly two weeks less, which is why the two numbers rarely match.
When do the trimesters start and end?
The first trimester runs to the end of week 13, the second from 14 weeks 0 days to the end of week 27, and the third from 28 weeks 0 days until birth. The boundaries are conventions rather than physiological events, and some sources place them a week either side of these, so a date near a boundary can be described differently elsewhere.
Is an ultrasound due date better than this one?
Usually, yes. A first-trimester scan measures the pregnancy itself and is accurate to within about five days, while last-period dating depends on remembering the date and on ovulation happening when the rule assumes. Where the two disagree by more than roughly a week, clinicians normally redate the pregnancy to the scan.