Enter the first day of your last period, your due date or your conception date. The calculator shows how many weeks pregnant you are, your trimester, your due date and the likely scan windows.
Results are estimates for planning. Check the figures before you make a financial or health decision.
How the Pregnancy week calculator works
Pregnancy weeks are counted from the first day of your last period, not from conception. Whatever you enter, the calculator works back to that date:
- Last period: used as it is.
- Due date: the start is 280 days (40 weeks) earlier.
- Conception date: the start is 266 days before the due date, which is 14 days after the first day of your last period.
It then counts the days from that start to the date you choose, shown as weeks and days. The due date is 280 days after the start, which is Naegele’s rule and assumes a 28-day cycle. Only a small minority of babies arrive on their due date.
Trimesters are shown as first (to the end of week 13), second (weeks 14 to 27) and third (week 28 onwards). Scan windows shown are 10 to 14 weeks for the dating scan and 18 to 20 weeks for the anomaly scan, and full term is from 37 weeks. If the dates are before the start or more than 42 weeks, you get an error to check them.
Tips and common mistakes
If your cycles are not about 28 days, or you were unsure of your last period, your due date from this tool may be off by days or weeks. Your midwife will confirm your dates at your first scan, and that date is the one to follow.
Use the “show me the week on” box to look ahead, for example to plan around a scan or time off work. For ovulation timing or fertility tracking, try the Ovulation and due date calculator. This is general information, not medical advice. Contact your midwife or GP with any concerns.
Frequently asked questions
Why are weeks counted from my last period?
It is the date most people can identify. This means you are counted as pregnant for about two weeks before conception, so a 40-week pregnancy lasts about 38 weeks from conception.
How accurate is the due date?
It is an estimate. Only a small minority of babies are born on the due date, and your scan may change it. Follow the date your midwife gives you.
When are my scans?
The tool shows the 10 to 14 week dating scan window and the 18 to 20 week anomaly scan window. Your hospital will send your actual appointment dates.