Pregnancy Week Calculator
Find out how many weeks pregnant you are from your last period.
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How Pregnancy Weeks Are Counted
Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. This is the convention used by healthcare providers worldwide, so "12 weeks pregnant" means 12 weeks since the LMP, which is about 10 weeks since conception.
Weeks pregnant = (today - LMP) / 7Pregnancy is divided into 40 weeks across three trimesters, with full term reached at 37–42 weeks.
Worked Example
If your last period began 80 days ago, you are 80 / 7 = 11 weeks and 3 days pregnant, placing you in the first trimester with roughly 29 weeks until your due date.
Trimester Breakdown
| Trimester | Weeks |
|---|---|
| First | 1 – 13 |
| Second | 14 – 27 |
| Third | 28 – 40 |
Frequently Asked Questions
Why count from the last period and not conception?
The last period is a known, datable event, whereas the exact conception date is usually unknown. Dating from the LMP gives a consistent standard, which is why gestational age is about two weeks ahead of fetal age.
How many weeks is full term?
37 to 42 weeks is considered full term. 40 weeks is the standard due-date marker, but a healthy birth can occur across that range.
Is an ultrasound more accurate?
Yes. An early dating scan measures the embryo directly and is more accurate than LMP-based counting, especially with irregular cycles.
Note: Estimates are for informational purposes only and are not medical advice. Consult a qualified healthcare provider for accurate dating and pregnancy care.