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Pregnancy Calculator: Due Date, Weeks and Trimester
Enter the first day of your last period and your usual cycle length to estimate your due date with Naegele's rule, see how many weeks and days pregnant you are on a chosen date, and which trimester it falls in.
How the due date is worked out
Naegele's rule adds one year, subtracts three months and adds seven days to the first day of the last menstrual period (LMP). In practice that equals 280 days, or 40 weeks, so this calculator adds exactly 280 days. The calendar version can differ by a day or two because months have different lengths.
The rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is longer or shorter, ovulation moves with it, so the calculator shifts the date by cycle length − 28 days. A 32-day cycle moves the due date four days later; a 25-day cycle moves it three days earlier.
Gestational age and trimesters
Gestational age counts from the start of the LMP, not from conception, which is why a pregnancy is "4 weeks" at about the time a period is first missed. The calculator subtracts the adjusted dating start from the date you choose and shows whole weeks and extra days.
One common convention puts the first trimester at 0 to 13 weeks 6 days, the second at 14 to 27 weeks 6 days and the third from 28 weeks. Some organisations draw the lines a week earlier or later, so do not worry if your care team uses slightly different dates. Term is described as early (37 to 38 weeks 6 days), full (39 to 40 weeks 6 days), late (41 to 41 weeks 6 days) and post-term from 42 weeks.
When the LMP date is unreliable
An LMP-based date is only as good as the memory and regularity behind it. It is less reliable if periods are irregular, if you conceived soon after stopping hormonal contraception, or if you were breastfeeding. Bleeding in early pregnancy can also be mistaken for a period.
Midwives and doctors often compare the LMP date with an early ultrasound measurement and may change the due date if the two differ by more than a set margin, which depends on the stage of pregnancy. The due date calculator includes conception, IVF transfer and ultrasound methods, and the conception calculator works backwards from a date.
Using the result
Treat the date as the centre of a window rather than a deadline. Booking appointments, screening tests and scans are scheduled by your maternity care team using their own dating. If you have bleeding, severe pain, reduced baby movements later in pregnancy or any other worry, contact your clinician or local emergency service without waiting.
This calculator is educational only. It is not medical advice or a diagnosis and cannot confirm that you are pregnant.
Worked example
A woman whose last period began on 1 January 2026 and who has a 28-day cycle checks her dates on 12 March 2026.
Inputs
| First day of last menstrual period | 2026-01-01 |
|---|---|
| Average cycle length | 28 days |
| Calculate for this date | 2026-03-12 |
Result
Estimated due date: 8 October 2026.
| Estimated due date | Thursday 8 October 2026 |
|---|---|
| Gestational age on 12 March 2026 | 10 weeks 0 days |
| Trimester | First trimester (up to 13 weeks 6 days) |
| Days until due date | 210 |
| Dating start used (LMP adjusted for cycle) | 1 January 2026 |
Pregnancy milestones from your dates
| Milestone | Date |
|---|---|
| Start of week 13 | 26 March 2026 |
| Second trimester begins (14 weeks) | 9 April 2026 |
| Halfway (20 weeks) | 21 May 2026 |
| Third trimester begins (28 weeks) | 16 July 2026 |
| Early term begins (37 weeks) | 17 September 2026 |
| Full term begins (39 weeks) | 1 October 2026 |
| Estimated due date (40 weeks) | 8 October 2026 |
| Late term begins (41 weeks) | 15 October 2026 |
- Only a minority of babies arrive on the estimated date; birth any time from about 37 to 42 weeks is common.
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How many weeks pregnant am I?
Count the days from the first day of your last period, adjusted for cycle length, and divide by seven. Enter the date and the tool shows whole weeks plus extra days.
How is the due date calculated from the last period?
Add 280 days, which is 40 weeks, to the first day of your last menstrual period, and adjust by the difference between your cycle length and 28 days. This is Naegele's rule with a cycle correction.
Why does my midwife give a different due date?
Dating can be revised using an early ultrasound scan, especially when periods are irregular or the date of the last period is uncertain. The clinical date is the one to follow.
Does the cycle length really matter?
Yes. Ovulation occurs about two weeks before the next period, so a longer cycle means a later ovulation and a later due date. The tool shifts the date day for day from a 28-day cycle.
Can this tell me if I am pregnant?
No. Only a pregnancy test and a clinician can confirm pregnancy. The calculator assumes you already know you are pregnant and want an estimated timeline.
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References and sources
- American College of Obstetricians and Gynecologists (2017). Committee Opinion No. 700: Methods for Estimating the Due Date. Link
- Naegele FK. Rule for estimating the date of confinement from the last menstrual period (the 280-day rule).
- World Health Organization: definitions of gestational age and preterm birth (before 37 completed weeks). Link