Pregnancy is counted in weeks, not months — and once you know your LMP (first day of your last menstrual period), the question "what week am I?" has a precise answer. This pregnancy week calculator takes your last period start date and the date you want to measure against, then tells you your exact completed week, the days into that week, which trimester you are in, how far through pregnancy you are as a percentage, and your estimated due date.
Because doctors date pregnancy from the first day of your last period rather than from conception, you are technically about two weeks pregnant on the day you actually conceive. That is why a full-term pregnancy is described as 40 weeks (280 days) long even though conception-to-birth is only about 38 weeks. Enter your dates below and every figure updates instantly.
How to Use
- Enter the First Day of Last Period (LMP) — the date your most recent period started before you became pregnant. This is the anchor obstetricians use.
- Set your Average Cycle Length. The textbook value is 28 days. If your cycles are regularly longer (for example 32 or 35 days), enter that number; it nudges your due date later by the extra days beyond 28.
- Leave Measure As Of set to today, or pick any other date to see what week you were (or will be) on that day.
- Read the result card: Current Pregnancy Week is the completed number of weeks, Days Into This Week rounds out your exact age (for example 18 weeks and 4 days), and Trimester tells you whether you are in the first, second, or third trimester.
- Compare Progress Through Pregnancy (a percentage) and Days Until Due Date to see how much remains.
The engine works on simple date arithmetic: days pregnant = (measure date − LMP), completed weeks = floor(days ÷ 7), and your estimated due date = LMP + 280 days, adjusted by (cycle length − 28). Trimester cutoffs are weeks 13 and 28.
How Gestational Age Is Counted
Obstetric gestational age deliberately starts two weeks before conception. On the day of ovulation you are already "4 weeks 0 days" pregnant in clinical terms, even though fertilization has just happened. This convention exists because the LMP is a date most people can remember, whereas the exact conception date is usually uncertain.
The trimesters divide that 40-week window roughly into thirds, though the cutoffs are not exactly even:
- First trimester (weeks 0–12) — from LMP to the end of week 12. This is when organ formation happens and when most early screening is scheduled.
- Second trimester (weeks 13–27) — often called the comfortable middle, when the anatomy scan around week 20 takes place.
- Third trimester (weeks 28–40 and beyond) — fetal weight gain accelerates and weekly monitoring may begin near term.
Only about 5% of babies are born exactly on their estimated due date; anything from 38 to 42 weeks is considered full or late term, so treat the due date as an estimate rather than a deadline.
Examples
Example 1 — The anchor case. Someone whose last period started on January 1, 2026, measured on October 24, 2026, has 296 days pregnant, which is 42 weeks and 2 days. That places her in the third trimester, about 105.7% of the way through a 280-day pregnancy, with an estimated due date of October 8, 2026 — meaning the pregnancy has run roughly 16 days past the estimated date and is in late/post-term territory where a clinician would usually reassess.
Example 2 — A routine 28-week checkup. With an LMP of April 11, 2026, measured on October 24, 2026, the count is 196 days = 28 weeks and 0 days, squarely at the start of the third trimester, 70.0% complete, with a due date of January 16, 2027 and 84 days remaining.
Example 3 — Longer cycles adjust the due date. If your cycles average 35 days instead of 28, the engine adds 7 days to your estimated due date and pushes the estimated conception date about 3–4 days later. This is why two people with the same LMP but different cycle lengths can land on different due dates by up to a week.
FAQ
How many weeks pregnant am I?
Subtract the first day of your last menstrual period from today, divide by 7, and round down — that is your completed pregnancy week. Add the leftover days for the exact "weeks + days" figure. This calculator does that math for you and also reports the trimester and due date.
Why does pregnancy start before I conceived?
Clinicians date pregnancy from the LMP because that date is reliably remembered, while conception usually happens around ovulation, roughly two weeks after the last period. So at conception you are already counted as about 4 weeks pregnant, and a full-term pregnancy is 40 weeks rather than 38.
What trimester am I in?
Weeks 0–12 are the first trimester, weeks 13–27 are the second, and week 28 onward is the third. The moment you complete week 27, you move into the third trimester. The result card shows this automatically.
How accurate is my estimated due date?
The Naegele estimate (LMP + 280 days) is accurate within about a week for people with regular 28-day cycles, but only around 5% of births happen on the exact due date. Ultrasound measurements in early pregnancy refine the date and often override the LMP estimate if they differ by more than a few days.
What if my cycles are irregular or I don't remember my LMP?
Use the date of your earliest positive pregnancy test, or better, the due date from a first-trimester ultrasound, and enter that as your LMP substitute. Irregular cycles make the LMP estimate less reliable, so confirm your true gestational age with your prenatal provider.
Can this calculator tell me when I conceived?
Approximately. Estimated conception is usually about half a cycle length after the LMP — around day 14 for a 28-day cycle. The result card shows this estimate, but it is a rough guide, not a paternity-grade date.
What does the progress percentage mean?
It is days pregnant divided by the full 280-day pregnancy length (adjusted for your cycle), shown as a percentage. A figure over 100% simply means you have passed your estimated due date and should check in with your care team.
Disclaimer
This pregnancy week calculator is provided for informational purposes only and is not medical advice. Gestational age and due-date estimates based on LMP are approximate; only a qualified healthcare professional, using ultrasound and your full history, can confirm your true due date and assess fetal well-being. If you are late, bleeding, in pain, or have any concern, contact your obstetric provider or maternity unit promptly rather than relying on this calculator.