Pregnancy Calculator
Estimate gestational age, trimester, conception date, and due date from the first day of the last menstrual period (LMP). Dating uses Naegele’s rule: due date = LMP + 280 days.
Pregnancy Calculator tool
About the Pregnancy Calculator
Obstetric “weeks pregnant” count from the LMP, not from conception. That is why a typical pregnancy is called 40 weeks even though fertilization is about two weeks later. Conception on this page is LMP + 14 days.
This is a calendar estimate, not a diagnosis or an ultrasound report. Irregular cycles, late ovulation, and clinical dating can move the due date by days or weeks. Talk with a clinician for care decisions.
How the Pregnancy Calculator works
All outputs are UTC calendar dates. Gestational age is the whole number of days from LMP to the “as of” date, shown as weeks plus leftover days.
Due date = LMP + 280 days
Conception ≈ LMP + 14 days
Gestational days = as-of date − LMP
Example
LMP 1 January 2026 → conception 15 January 2026, due date 8 October 2026. On 16 April 2026 that is 15 weeks 0 days, second trimester.
How to Use the Pregnancy Calculator
- Enter the first day of the last menstrual period.
- Set the as-of date (today by default after load).
- Read gestational age and trimester.
- Note the estimated due date and conception date.
- If you have an ultrasound age, prefer the due date calculator’s ultrasound mode.
Pregnancy Calculator Examples
Textbook 40 weeks
LMP plus 280 days is the 40-week mark (40 × 7). Many births occur in the two weeks around that date.
First trimester cutoff
Through day 97 (13 weeks 6 days) this tool labels the first trimester; day 98 starts the second.
Before LMP
An as-of date before the LMP produces a negative gestational age and the label “Not yet pregnant.”
How to Calculate a Due Date from LMP
Add 280 days to the first day of the last period. On paper, some people add one year, subtract three months, and add seven days — that is the same rule on most dates.
Trimester cutoffs used here
These day counts are from the LMP.
| Trimester | Days from LMP | Weeks |
|---|---|---|
| First | 1–97 | through 13w6d |
| Second | 98–195 | 14w0d–27w6d |
| Third | 196–294 | 28w0d–42w0d |
| Past typical term | 295+ | after 42 weeks |
When LMP dating is a poor fit
Irregular periods, recent hormonal contraception, or a clear ovulation test may disagree with LMP + 280. Clinicians often switch to ultrasound dating in the first trimester when the difference is large. This page still reports Naegele’s date so you can compare; it will not pick a winner. Gestational “weeks + days” count from the LMP, which is why 15 weeks 0 days on 16 April 2026 matches an LMP of 1 January 2026 in the worked example. For IVF or known ovulation, prefer the due-date calculator’s conception or ultrasound mode.
Frequently Asked Questions
Why 280 days?
It is Naegele’s obstetric convention: LMP + 9 months + 7 days on a 28-day cycle.
Is conception exactly LMP + 14?
Only if ovulation is on day 14. Real cycles vary. This is a midpoint estimate.
Do you use local time?
Dates are calendar dates in UTC, so a timezone does not shift the day.
Can I date from conception instead?
Use the due date calculator’s conception mode (266 days).
Is this medical advice?
No. Follow your clinician’s dating.