Pregnancy Weeks Calculator
Find out how many weeks pregnant you are and your estimated due date. Enter the first day of your last period and your usual cycle length. The result is an approximate count, not a replacement for your prenatal care.
An approximate count
How far along you are
10 weeks and 0 days
Check the highlighted fields to see your result.
- Estimated due date
- June 12, 2027
- Trimester
- First
- Days to the due date
- 210
- Approximate conception
- September 19, 2026
Results are estimates for information only and do not constitute financial, medical, or legal advice.
How it works
A pregnancy lasts about 40 weeks (280 days), counted from the first day of the last period. The calculator counts the days from that date to today and adds 280 days to find the estimated due date. If your cycles are regularly longer or shorter than 28 days, it shifts the dates by the difference.
The formulas
Due date = first day of last period + 280 days (+ cycle length − 28)
Weeks pregnant = days since the last period ÷ 7
First trimester: up to 13 weeks and 6 days · Second: 14 to 27 weeks · Third: from 28 weeks.
Worked example
Your last period started on January 1, 2026, your cycles are 28 days long, and today is March 12, 2026.
- Days since January 1: 70, which is 10 weeks and 0 days (first trimester).
- Due date: January 1 + 280 days = October 8, 2026.
Sources
- NHS.Pregnancy due date calculator(due date counted from the first day of the last period).
Frequently asked questions
Why are pregnancy weeks counted from my last period?
The exact day of conception is usually unknown, but the first day of the last period is easy to remember. So pregnancy is counted from that day, about two weeks before conception. That's why a pregnancy lasts about 40 weeks (280 days) on this count.
How accurate is the due date?
It is an estimate. Most babies are born within a week or two of the due date, and only a small share arrive on the exact day. An early ultrasound often adjusts the date, and your doctor or midwife will tell you which date to follow.
What if I don't remember the date of my last period?
Then this calculator can’t give a useful estimate. A health professional can date the pregnancy with an ultrasound.