What Is the Weeks Pregnant Calculator?
This calculator tells you how far along a pregnancy is on any given date, expressed in the standard obstetric format of weeks and days (for example, "12w 3d"). Gestational age is conventionally measured from the first day of the last menstrual period (LMP), not from the date of conception, because the LMP is a date most people can reliably remember.
How to Use It
Enter the first day of your last menstrual period and the date you want to calculate for (today by default). The tool counts the number of days between the two dates, divides by 7 to get completed weeks, and shows the leftover days. A full-term pregnancy is about 40 weeks (280 days) from the LMP.
The Formula Explained
Let the elapsed days be \(D = \text{today} - \text{LMP}\). Then:
$$\text{weeks} = \left\lfloor D \div 7 \right\rfloor \quad\text{and}\quad \text{days} = D \bmod 7.$$
For example, if 87 days have passed, \(87 \div 7 = 12\) remainder \(3\), so you are 12 weeks and 3 days pregnant.
Worked Example
Suppose your LMP was January 1, 2024 and today is March 28, 2024. That is 87 days. \(87 \div 7 = 12\) with remainder \(3\), so the result is 12 weeks and 3 days — squarely in the first part of the second trimester.
FAQ
Is gestational age the same as the age of the fetus? No. Gestational age is counted from the LMP, roughly two weeks before conception, so the actual embryonic age is about two weeks less.
What if I don't know my LMP? A healthcare provider can estimate gestational age using an early ultrasound, which is often more accurate, especially with irregular cycles.
How many weeks is full term? Term is generally 37–42 weeks, with the due date set at 40 weeks (280 days) from the LMP. This tool is informational and not a substitute for medical advice.
