What this calculator does
This tool estimates how many weeks and days pregnant you are based on the first day of your last menstrual period (LMP). In obstetrics, pregnancy is dated by gestational age, which is counted from the LMP rather than from conception — typically about two weeks before ovulation. This is the same convention used by doctors and midwives, so the number you get here matches what you'll usually hear at appointments.
How to use it
Enter the first day of your last menstrual period in the LMP field. Then choose the "as of" date — leave it on today to find your current progress, or set it to a future appointment date to see how far along you'll be then. The result shows your gestational age in the standard weeks + days format (for example, "12 weeks 3 days"), plus the raw day count and a decimal-week figure.
The formula explained
First we compute the total number of whole days between LMP and the chosen date. Then:
$$\text{weeks} = \left\lfloor \frac{\text{daysSinceLMP}}{7} \right\rfloor$$ gives the completed weeks, and $$\text{days} = \text{daysSinceLMP} \bmod 7$$ gives the leftover days. So 87 days since LMP is \(\lfloor 87/7 \rfloor = 12\) weeks with \(87 - 84 = 3\) days remaining.
Worked example
Suppose your LMP was 2024-01-01 and today is 2024-03-28. That is 87 days. \(\lfloor 87 \div 7 \rfloor = 12\) weeks, and \(87 - (12 \times 7) = 3\) days. You are 12 weeks and 3 days pregnant.
FAQ
Is gestational age the same as how long since I conceived? No. Gestational age is counted from LMP, which is roughly 2 weeks before conception, so it's about 2 weeks more than the time since conception.
Why use LMP instead of the conception date? Most people know their last period date but not the exact day of conception, so LMP is the standard reference for dating a pregnancy.
Is this medically accurate? It's a useful estimate based on a regular 28-day cycle. An early ultrasound is the most accurate way to date a pregnancy — always confirm with your healthcare provider.
