Pregnancy week calculator
My last period started on
My cycles usually last .
Pick the first day of your last period to see how many weeks pregnant you are.
General information. Not a substitute for your doctor.
The gestational age written on the scan report.
If you are not sure, leave it at 28 — the estimate already allows for that.
Gestational age counts the weeks from the first day of the last period, not from conception: 59 days after it are 8 weeks and 3 days. On a 28-day cycle, the due date falls 280 days, or 40 weeks, after that day (ACOG, 2017).
An ultrasound in the first trimester is the most accurate way to date a pregnancy: in one study, 40% of the women who had one saw their due date moved by more than 5 days.
Source: ACOG, 2017.
How the calculator counts the weeks
Gestational age counts weeks and days from the first day of the last period, even when it comes from an ultrasound.
From your last period
- 1
The first day of your last period is day zero.
- 2
The days since then, in weeks and days: 59 days are 8 weeks and 3 days.
- 3
Your cycle: each day it runs longer than 28 takes a day off the count, and each day shorter adds one.
From an ultrasound
- 1
The gestational age the scan report gives, on the day of the exam.
- 2
Plus the days since the exam: a scan of 8 weeks and 3 days, done 14 days ago, makes 10 weeks and 3 days today.
Questions about pregnancy weeks
How do I work out how many weeks pregnant I am?
Count the days since the first day of your last period and divide by 7: the whole number is the weeks, and the remainder the days. 59 days, for example, are 8 weeks and 3 days.
Why does the count start at the last period, not at conception?
Because the date of the last period is usually known, and the date of conception is not. The convention assumes ovulation 14 days after it (ACOG, 2017), so at conception a pregnancy already counts about two weeks.
My ultrasound and my last period give different ages. Which one counts?
In the first trimester, up to 13 weeks and 6 days, an ultrasound is the most accurate way to date a pregnancy (ACOG, 2017). When the two disagree, your doctor decides which date to use.
How many weeks does a pregnancy last?
About 40 weeks, or 280 days, from the first day of the last period to the due date (ACOG, 2017). Only about 1 in 20 babies arrives on that date (Khambalia et al., 2013).
Can I use it for an IVF pregnancy?
No. In a pregnancy from assisted reproduction, the gestational age comes from the treatment's own dates (ACOG, 2017), and this calculator doesn't do that calculation.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017;129(5):e150–4.
- Khambalia AZ, Roberts CL, Nguyen M, et al. Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics, 2013;123(2):105–9.
General information. Not a substitute for your doctor. Updated September 27, 2026.