What Is the Pregnancy Week Calculator?
This calculator tells you how many weeks and days pregnant you are based on the first day of your last menstrual period (LMP). Pregnancy is medically dated from the LMP rather than conception, which is why "gestational age" typically runs about two weeks ahead of the actual date of fertilization. The tool also identifies your current trimester and estimates how many days remain until your due date (40 weeks after LMP).
How to Use It
Enter the first day of your last menstrual period, the date you want to check (usually today), and your average menstrual cycle length. A standard cycle is 28 days; if yours is longer or shorter, the calculator adjusts the gestational age accordingly so your due-date estimate stays accurate.
The Formula Explained
The number of days pregnant equals the days between your LMP and today, minus an adjustment for cycle length:
$$\text{Days} = (\text{Today} - \text{LMP}) - (\text{Cycle} - 28)$$Dividing by 7 gives whole weeks, and the remainder gives extra days:
$$\text{Weeks} = \left\lfloor \frac{(\text{Today} - \text{LMP}) - (\text{Cycle} - 28)}{7} \right\rfloor$$Weeks 0–12 are the first trimester, 13–26 the second, and 27 onward the third.
$$T = \begin{cases} 1 & w < 13 \\ 2 & 13 \le w < 27 \\ 3 & w \ge 27 \end{cases}$$
Worked Example
Suppose your LMP was January 1, 2024 and today is March 11, 2024, with a 28-day cycle. That is 70 days between the dates, with no cycle adjustment. \(70 \div 7 = 10\) weeks exactly, so you are 10 weeks 0 days pregnant — firmly in the first trimester. Your estimated due date is October 7, 2024 (\(\text{LMP} + 280\) days), leaving 210 days to go.
FAQ
Is this a substitute for an ultrasound? No. LMP dating is an estimate; an early ultrasound is the most accurate way to date a pregnancy. Always confirm with your healthcare provider.
Why count from the LMP if I wasn't pregnant yet? It's the medical convention because the exact conception date is rarely known, but the start of a period is easy to recall.
What if I have irregular cycles? Adjusting the cycle length helps, but irregular ovulation makes LMP dating less reliable — rely on a clinical scan for accuracy.
