Pregnancy Due Date Calculator — Weeks, Trimester and Milestones
This pregnancy calculator estimates your due date, tells you exactly how many weeks and days pregnant you are, identifies your current trimester, and maps key milestones along the way. It uses Naegele's rule, the standard method in obstetrics: the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period (LMP). If you know your conception date instead, the estimate is 266 days from conception.
The due date is a statistical reference, not a promise — only a small share of babies arrive exactly on it, and delivery anywhere between 37 and 42 weeks is generally considered term. Ultrasound measurements early in pregnancy can refine the estimate, especially when cycles are irregular. Use this calculator for planning and curiosity, and always follow the dates and guidance confirmed by your obstetrician or prenatal care provider.
How it works
Naegele's rule: due date = first day of LMP + 280 days (equivalently LMP + 1 year − 3 months + 7 days). From conception: due date = conception + 266 days. Gestational age = days elapsed since LMP ÷ 7, expressed as weeks + days.
Use cases
- Estimating your due date from the last menstrual period
- Finding out how many weeks and days pregnant you are today
- Identifying your current trimester and when the next one starts
- Planning prenatal appointments, screening windows, and maternity leave
- Estimating the likely conception date from a known due date
- Sharing week-by-week progress with family or a birth plan team
Frequently asked questions
How is the due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. A practical shortcut gives the same result: take the LMP date, add one year, subtract three months, and add seven days. The rule assumes a 28-day cycle with ovulation on day 14, so an early ultrasound may adjust the date if your cycles differ.
How do I know how many weeks pregnant I am?
Gestational age is counted from the first day of your last menstrual period, not from conception. Divide the days elapsed since the LMP by seven: 10 weeks and 3 days means 73 days have passed. This convention means you are counted as roughly two weeks pregnant at the moment of conception, which surprises many people but is the standard used in all prenatal care.
When does each trimester start and end?
The most common division puts the first trimester from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Boundaries vary slightly between sources. Each trimester has its own typical exams and screening windows, which is why knowing your exact week matters for scheduling prenatal care.
How accurate is the estimated due date?
It is an estimate: only about 1 in 20 babies is born on the exact due date, and birth anywhere from 37 to 42 weeks is generally considered term. Accuracy is lower with irregular cycles, since Naegele's rule assumes ovulation on day 14. A first-trimester ultrasound measuring the embryo is the most accurate way to date a pregnancy, and your doctor may update the due date based on it.
What if I know my conception date instead of my last period?
Add 266 days to the conception date — that is the 280-day rule minus the roughly two weeks between the LMP and ovulation. Conception date is best known through IVF transfers or ovulation tracking. If both dates are available and disagree, your prenatal care provider will decide which reference to use, usually favoring early ultrasound measurements.