GenzeePregnancy Due Date Calculator
Enter the first day of the last menstrual period to get the estimated due date, current week of pregnancy and trimester — using the same Naegele's rule doctors use.
Estimated due date
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How due dates are calculated
The standard method, Naegele's rule, dates from the 1800s and is still what obstetricians write in charts: take the first day of the last menstrual period (LMP), add 280 days (40 weeks). It counts from the LMP rather than conception because women reliably know the former and rarely the latter; the convention builds in the assumption that ovulation happens around day 14 of a 28-day cycle. That's also why "40 weeks pregnant" includes roughly two weeks of not actually being pregnant — medical week-counting starts at the LMP.
How to use this calculator
Enter the LMP date and, if cycles are consistently longer or shorter than 28 days, the average cycle length — the tool shifts the estimate accordingly (a 32-day cycle typically means ovulation ~4 days later, so 4 days are added). It shows the estimated due date, how many weeks and days along the pregnancy currently is, and the trimester.
The formula
weeks pregnant = days since LMP ÷ 7
Worked example
LMP on 1 January 2026 with regular 28-day cycles: due date = 1 January + 280 days = 8 October 2026. On 1 April 2026 the pregnancy is 90 days along — 12 weeks 6 days, right at the end of the first trimester and around the time of the routine dating ultrasound. With 33-day cycles instead, the estimate moves five days later, to 13 October.
What "due date" really means
It is the middle of a probability window, not an appointment. Only about 4–5% of babies arrive on the exact date; the practical expectation is "any time between 37 and 42 weeks", with first babies trending slightly later. Early ultrasound (8–13 weeks) measures the embryo directly and is more accurate than any LMP arithmetic — when the two disagree by more than a few days, doctors use the scan date. Use this calculator for planning (leave applications, travel cut-offs, when the trimesters change) and let your antenatal care set the official date. The date difference calculator is handy for counting the days to any milestone, and the age calculator takes over after the big day.
Frequently asked questions
How accurate is a due date from the LMP?
Within about a week for regular cycles — but only ~4–5% of babies arrive on the exact date. An early ultrasound is more accurate and overrides the LMP estimate when they differ.
What if I don't remember my LMP?
Your doctor will date the pregnancy by ultrasound — crown-rump length between 8 and 13 weeks dates it within a few days. The calculator is only as good as the date entered.
Why does cycle length change the due date?
Ovulation, not the period, starts the biological clock. Longer cycles usually mean later ovulation, so each day of cycle beyond 28 pushes the estimate a day later.
When do the trimesters change?
Common convention: first trimester through week 12, second from 13 to 26, third from 27 to birth. Sources vary by a week — your care provider's convention is the one that counts.
When is a baby considered full term?
39 weeks 0 days to 40 weeks 6 days is 'full term'; 37–38 weeks is 'early term' and past 41 'late term'. Delivery anywhere from 37 to 42 weeks is within the normal window.