Pregnancy Due Date Calculator
Calculate your estimated due date, current pregnancy week and trimester from the first day of your last period.
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How Is the Due Date Calculated?
The estimated due date follows Naegele's rule, published in the 19th century and still the standard starting point in obstetrics: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Pregnancy weeks are counted from that same day — not from conception — which is why "week 1" of pregnancy technically begins about two weeks before the egg is even fertilised.
Worked example: if your last period started on 1 January, adding 280 days lands on 8 October — your estimated due date. Today's date is then compared against the LMP to show how many weeks and days pregnant you are, which trimester you are in, and how many days remain until the due date.
Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14. If your average cycle is longer or shorter than 28 days, the true due date shifts accordingly — a 35-day cycle, for example, tends to mean ovulation and conception happened later than the rule assumes, pushing the real due date several days later than the simple calculation suggests.
What You Should Know
A due date is a statistical estimate, not a deadline: only about 4–5% of babies are born on their exact calculated date, and a birth anytime between week 37 and week 42 is considered full term and normal. First-trimester ultrasound — typically between weeks 8 and 13 — measures the embryo or fetus directly and is considered more accurate than LMP-based dating alone, especially for people with irregular cycles, who conceived via fertility treatment, or who are unsure of their last period's exact start date.
If you conceived through IVF or know your exact ovulation date from fertility tracking, your due date is usually calculated from that known date rather than the LMP, since it removes the guesswork around cycle length entirely. The trimesters break down as: first trimester weeks 1–13, second trimester weeks 14–27, third trimester week 28 until birth. Each trimester carries different typical screening tests and milestones that your prenatal care provider will guide you through. This calculator is a starting-point estimate for planning purposes only; it does not replace ultrasound dating or the ongoing prenatal care and monitoring that only a qualified doctor or midwife can provide.
Frequently Asked Questions
How accurate is the due date?
It is an estimate: only about 4-5% of births happen on the exact date, and most occur within two weeks either side. First-trimester ultrasound gives the most reliable dating, especially when your cycle length is not a standard 28 days.
Why do pregnancy weeks start before conception?
By convention, gestational age counts from the first day of the last period — roughly two weeks before ovulation and conception. So at "6 weeks pregnant" the embryo is about 4 weeks old.
What are the trimesters?
The first trimester covers weeks 1-13, the second weeks 14-27, and the third week 28 to birth. The tool shows which one you are in and how many weeks remain.
What if my cycle is not 28 days long?
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycles typically run longer, ovulation likely happened later than day 14, so your real due date is probably somewhat later than the simple LMP-based calculation; with shorter cycles, the reverse applies. This is one reason early ultrasound dating is preferred when cycle length is known to be irregular.
Does the due date change after an ultrasound?
Often, yes, especially if it is done in the first trimester. Measuring the crown-rump length between roughly weeks 8 and 13 is considered more accurate than LMP-based dating alone, and if the two estimates differ by more than about 5–7 days, most care providers will adjust the official due date to match the ultrasound.
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