What this calculator does
Pregnancy is dated from the first day of the last menstrual period. Naegele’s rule adds 280 days and adjusts for cycles that differ from 28 days.
This calculator shows the current week and day, trimester, estimated due date, days remaining and estimated conception. Ultrasound remains the most accurate dating method.
Pregnancy dating
Due date = last period + 280 days + (cycle length − 28) Gestational age is the number of days since the last period divided by seven. The first trimester runs to week 13, the second to week 27, and the third follows.
How this calculator works
- 01
Enter your last period
Choose the first day of your last menstrual period.
- 02
Add your cycle length
If your cycle is longer or shorter than 28 days, the due date shifts accordingly.
- 03
Review week and dates
See the pregnancy week, trimester, due date and days remaining.
Practical tips
- Use the date in your medical records as the authoritative pregnancy timeline.
- Seek urgent care for pain, bleeding or unusual symptoms.
Frequently asked questions
Why date pregnancy from the last period?
The exact conception date is often unknown, so clinicians use the first day of the last period as a common reference. Pregnancy is therefore counted as starting about two weeks before conception.
Is the due date exact?
No. Naegele’s rule is an estimate and only a small share of babies arrive exactly on the due date. Ultrasound provides a more accurate estimate.
What does 12+3 mean?
It means 12 completed weeks and 3 days of pregnancy. This notation is common in medical records.
Data basis: 2026 · Annual constants live in structured JSON files and are updated once a year.
Results are unverified model calculations and do not replace tax, legal or financial advice.