🤰 Pregnancy Week Calculator
Pregnancy is dated from the first day of your last period, not conception — so "how far along" you are is simply the number of weeks and days since that date, and the due date is 280 days (40 weeks) after it. Enter your values below — results update instantly, entirely on your device.
Fill in the fields above — results appear instantly.
How the pregnancy week calculator works
Gestational age = today − first day of last menstrual period (LMP), expressed in completed weeks and days. Trimesters: weeks 0–12 (first), 13–26 (second), 27+ (third). Estimated due date = LMP + 280 days (Naegele's rule).
Example: If your last period began 10 weeks and 3 days ago, you are 10 weeks 3 days pregnant — in the first trimester, with a due date about 40 weeks after that period.
Dating from the last period means the first ~2 weeks of "pregnancy" are before conception even occurs — this is the standard clinical convention. It is an estimate: an early ultrasound is more accurate, especially with irregular cycles, and this tool is not a substitute for prenatal care or medical advice. Your date stays on your device and is never uploaded.
Frequently asked questions
How many weeks pregnant am I?
Count from the first day of your last menstrual period to today. Enter that date and the calculator shows completed weeks and days, your trimester, and an estimated due date.
Why is pregnancy dated from the last period, not conception?
Because the last period is a date most people know precisely, while the exact conception day usually isn't. By convention, gestational age starts from the last period — which is why you're considered ~2 weeks "pregnant" at conception.
How is the due date calculated?
By Naegele's rule: last period plus 280 days (40 weeks). It assumes a regular 28-day cycle, so it's an estimate — only about 1 in 20 babies arrive exactly on the due date.
Is this a substitute for a doctor?
No. It's a convenience estimate. An early ultrasound gives more accurate dating, and prenatal care should come from your healthcare provider.
Is this pregnancy week calculator accurate and private?
Yes. It uses the standard published formula, shows its working under every result, and computes locally in your browser — your inputs are never sent to a server, and the page works offline.