Pregnancy Calculator
Enter the first day of your last menstrual period and your average cycle length to estimate the due date, current gestational age and trimester.
Estimated due date
Trimester milestones
| Trimester | Weeks | Key milestones |
|---|---|---|
| First trimester | Weeks 0–13 | Organs form and the heartbeat becomes visible on ultrasound |
| Second trimester | Weeks 14–27 | Movements are felt, anatomy scan around weeks 18–22 |
| Third trimester | Weeks 28–40 | Rapid growth and lung maturation; full term begins at week 37 |
Method: estimated due date = first day of last period + 280 days + (average cycle length − 28) days — the classic rule adjusted for cycle length.
How to use
- Enter the first day of your most recent menstrual period.
- Adjust the average cycle length if it differs from the standard 28 days.
- Click Calculate to see the estimated due date, gestational age, trimester and days remaining.
Frequently asked questions
How accurate is a due date from the last period?
It is an estimate: most babies arrive within about ten days of the predicted date, and only a small minority on the exact day. Cycles longer or shorter than 28 days shift ovulation, and irregular cycles shift it more. An ultrasound scan in the first trimester is more accurate for dating than the last menstrual period, so follow the date your doctor confirms.
Is this tool medical advice?
No. The result is a population-level estimate based on a standard 280-day model, not a personal prediction or a medical assessment, and every pregnancy is different. Use the dates for orientation and planning, and consult your doctor or midwife for anything that affects your care — they will confirm the dating with an ultrasound where needed.
Why does cycle length change the due date?
The classic rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is longer, ovulation — and therefore conception — happened later relative to the first day of bleeding, so the estimate shifts by the difference. The adjustment is crude: it assumes every cycle is the same length, which is rarely true in practice.
What is gestational age and why count from the period?
Pregnancy age is traditionally counted from the first day of the last menstrual period, even though conception typically happens about two weeks later. Doctors worldwide use this convention so estimates are comparable, which is why week 40 does not mean 40 weeks since conception. The tool reports age in the same weeks-and-days format used in maternity notes.
What if my period date is in the future?
The calculator still shows the projected due date for that date, but it cannot compute a gestational age or trimester for a pregnancy that has not started, so those boxes show a dash. If you meant a past date, correct the entry and calculate again — the remaining results fill in immediately, with no extra steps needed.
Is my data stored or shared?
No. The calculation happens entirely in your browser — the dates you enter are never sent to any server, logged or stored. Closing or reloading the page clears everything, and the tool keeps working offline once loaded, so you can plan and track privately from any device.