Pregnancy Due Date Calculator
Estimate your baby's due date from your last period, plus how many weeks pregnant you are and your trimester.
Last reviewed July 1, 2026
Based on Naegele's rule (LMP + 280 days), adjusted for your cycle length.
An estimate — only about 5% of babies arrive on the exact due date. Your doctor's dating scan is the most accurate. Not medical advice.
Finding out when your baby is due is one of the first things every expectant parent wants to know. This calculator estimates your due date from the first day of your last menstrual period (LMP), and tells you how far along you are today and which trimester you're in.
It uses Naegele's rule — the standard method — which adds 280 days (40 weeks) to your last period, adjusted for your cycle length if it isn't the typical 28 days.
The Pregnancy Due Date formula
Estimated due date = First day of last period + 280 days (± cycle-length adjustment)First day of last period (LMP) = the start of your most recent period · 280 days = 40 weeks. If your cycle isn't 28 days, add or subtract the difference from 28.
Worked example
If your last period began on 1 January and you have a typical 28-day cycle, the estimated due date is 1 January + 280 days ≈ 8 October. A longer 32-day cycle would push it about 4 days later.
How the due date is estimated
Pregnancy is counted from the first day of your last period, not from conception — which is why 40 weeks is 'full term' even though conception happens about two weeks in. The calculator adds 280 days to your LMP, then adjusts for a longer or shorter cycle.
Only about 1 in 20 babies actually arrives on the exact due date; most come within a week or two either side. Your doctor's early dating ultrasound gives the most accurate estimate.
Frequently asked questions
▶How is my due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length. This calculator does it instantly.
▶How accurate is a due date?
It's an estimate — only about 5% of babies are born on their exact due date, and a normal, full-term birth can happen any time from 37 to 42 weeks. A dating scan is more precise.
▶How many weeks pregnant am I?
Pregnancy is counted from the first day of your last period, so it's the number of weeks since then. Enter that date above and the calculator shows how far along you are today and your trimester.
▶What is Naegele's rule?
It's the standard method for estimating a due date: take the first day of your last menstrual period and add 280 days (40 weeks). This calculator uses it, adjusting for your cycle length.
▶How is the due date adjusted for a longer or shorter cycle?
The standard 280 days assumes a 28-day cycle. If yours is longer or shorter, the estimate shifts by the difference — for a 32-day cycle, ovulation is later, so the due date moves about 4 days on.
▶When do the trimesters start and end?
Broadly, the first trimester runs to about week 13, the second from weeks 14 to 27, and the third from week 28 to birth. The calculator shows which one you're currently in.
▶Is the due date from conception or my last period?
From your last period. Because conception happens about two weeks after that, 40 weeks from your last period is roughly 38 weeks from conception — which is why full term is counted at 40 weeks.
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Open calculator →Disclaimer: Pregnancy Due Date Calculator results are estimates for general information and education only, and are not financial, tax, legal or medical advice. Verify important decisions with a qualified professional.