Pregnancy Due Date Calculator
The pregnancy calculator estimates your due date by adding 280 days to the first day of your last menstrual period, and shows your current gestational age and trimester.
verified_userReviewed by the Calculopedia editorial teamLast updated 2026-08-14
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quizExample
How this calculator works, with real numbers (no JavaScript needed):
Inputs
- First day of last menstrual period
- 2026-01-15
- As of date
Results
- Estimated due date
- 22 October 2026
- Gestational age
- 32
- Extra days
- 3
- Trimester
- Third trimester (weeks 28+)
- Estimated conception date
- 29 January 2026
- Weeks remaining
- 7.6
functionsThe formula
Pregnancy is dated from the first day of your last menstrual period (LMP), even though conception actually happens about two weeks later. This is standard medical practice. It can feel odd that a pregnancy counts as "two weeks along" before it even begins, but dating from a memorable date like the LMP is far more practical than trying to pin down the exact moment of conception.
Naegele's rule
Due date = first day of LMP + 280 days (40 weeks).
If your LMP began on 15 January 2026:
Due date = 15 January + 280 days = 22 October 2026
About 4% of babies arrive exactly on the due date — most are born within two weeks either side. Treat the date as a strong estimate, not a promise.
Why 40 weeks — and why it varies
The classic 40-week gestation is our best central estimate, but healthy pregnancies simply are not all the same length. A baby can be born late-term and perfectly healthy, or arrive early for a range of reasons. Because so much natural variation exists, doctors treat the due date as a guide for scheduling scans, monitoring and planning — not as a deadline the baby is expected to honour. This is also why the phrase "estimated due date" deliberately contains the word estimated.
Gestational age
Gestational age counts from the LMP. If today is 14 August 2026, that's 211 days after 15 January:
211 ÷ 7 ≈ 30 weeks and 1 day
The pregnancy calendar is split into three trimesters:
| Trimester | Weeks | Milestones |
|---|---|---|
| First | 1–13 | Organs form; nausea and fatigue common |
| Second | 14–27 | Movement felt; ultrasound can show the baby's sex |
| Third | 28–40 | Rapid growth; preparing for birth |
Conception date
Ovulation typically happens about 14 days after the LMP, so the estimated conception date is LMP + 14 days. (Sperm can survive a few days, so this is an estimate.) Interesting to note: this means the estimated conception date and your due date are simply 266 days apart — the 280-day rule is essentially "conception plus 38 weeks".
This tool is for general information and is not medical advice. Your doctor's ultrasound dating — especially the first-trimester scan — is the most accurate. Always follow your clinician's guidance.
helpFrequently asked questions
question_markHow is my due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period. This is known as Naegele's rule and is the standard way to estimate a due date.
question_markHow accurate is the due date?
Only about 4% of babies are born exactly on the due date. Most arrive within two weeks before or after. First-trimester ultrasound dating is more accurate than LMP-based dates.
question_markWhat is gestational age?
Gestational age is the age of the pregnancy, counted in weeks from the first day of your last menstrual period — not from conception.
question_markWhen does the third trimester start?
The third trimester starts at 28 weeks of pregnancy and continues until birth (around week 40).