Pregnancy due date calculator
Work out your estimated due date (EDD) from the first day of your last period using the standard Naegele rule — LMP plus 280 days, adjusted for your cycle length.
It also tells you how many weeks pregnant you are today and which trimester you are in. Everything is computed in your browser using AD (English) calendar dates. An estimate only — not medical advice.
Your dates
The first day of your most recent menstrual period, in AD (English) dates.
Days from the start of one period to the next. The textbook default is 28; 20–45 is accepted.
Pick the first day of your last period above to see your estimated due date and current pregnancy week.
Estimated due date
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Enter your last menstrual period to begin
Trimester
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Gestational age
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Stage
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Days pregnant
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Days to go
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An estimate only. The Naegele rule assumes a regular cycle and ovulation around day 14; only about 1 in 20 babies arrive on the exact due date, and most healthy births occur within two weeks either side. An ultrasound dating scan and your clinician's assessment take precedence over any calculator. This tool does not provide medical advice.
From your last period to a due date
Pregnancy is dated from the first day of your last menstrual period. The Naegele rule then counts forward 40 weeks, with a small shift for cycles that are not 28 days long.
Start from the LMP
Enter the first day of your last menstrual period. This is the anchor date for all standard pregnancy dating.
Add 280 days
The Naegele rule adds 280 days (40 weeks) to the LMP to estimate the due date, then adjusts by (cycle length − 28) days.
Weeks & trimester
Gestational age today is the days since the LMP, shown as completed weeks and days, and mapped to a trimester.
Due dates, answered
How is a pregnancy due date calculated?+
The standard method is the Naegele rule. It estimates the due date (EDD) as the first day of your last menstrual period (LMP) plus 280 days — that is 40 weeks. This calculator also shifts the date when your average cycle differs from 28 days, adding one day for each day your cycle is longer than 28 (and subtracting one for each day shorter).
Why 280 days from the last period?+
Pregnancy is dated from the first day of the last menstrual period, not from conception, because the LMP is the date most women can recall reliably. Ovulation and conception usually happen about two weeks after the LMP, so the 280-day (40-week) figure includes those first two weeks before conception.
What if my cycle isn't 28 days?+
The textbook Naegele rule assumes a 28-day cycle with ovulation around day 14. If your cycle is regularly longer or shorter, ovulation shifts accordingly, so this tool adjusts the due date by (cycle length − 28) days. For very irregular cycles the LMP method is less reliable and an early ultrasound dating scan is more accurate.
What are the three trimesters?+
Pregnancy is divided into three trimesters measured in gestational weeks: the first trimester runs from week 0 up 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. This calculator shows which trimester your current gestational week falls in.
How accurate is the estimated due date?+
It is an estimate, not a guarantee. Only about 1 in 20 babies are born on the exact due date, and most healthy term births occur within two weeks before or after it. An ultrasound dating scan in the first trimester is the most accurate way to confirm dates, and your doctor or midwife may revise the due date based on it.
Does this calculator give medical advice?+
No. It performs a simple, well-established calendar calculation for general information only. Always confirm your due date and follow antenatal care guidance with a qualified health professional.
Sources & data note
Based on the Naegele rule: estimated due date = first day of the last menstrual period + 280 days (40 weeks), adjusted by (average cycle length − 28) days, with trimesters at gestational weeks 0–13, 14–27 and 28+. This is a planning estimate computed from the dates you enter and is not medical advice. Confirm your due date with an ultrasound dating scan and a qualified clinician; values are indicative and should be verified against current obstetric guidance.