Skip to content
Lizely

Pregnancy Due Date Calculator

Estimate your baby's due date and current gestational age from your last period

Privacy: your files never leave your device. All processing happens locally in your browser.

How to use

  1. 1.Select the first day of your last menstrual period (LMP) in the date field.
  2. 2.Adjust the average cycle length if yours differs from the default 28 days (valid range 20–45).
  3. 3.Read your estimated due date and current gestational age, updated instantly as you type.

About Pregnancy Due Date Calculator

The pregnancy due date calculator estimates when your baby is likely to arrive using Naegele's rule, the standard obstetric dating method taught in medical and midwifery practice worldwide. You provide the first day of your last menstrual period (LMP) and, optionally, your average cycle length; the tool instantly returns an estimated due date and your current gestational age in weeks and days, recalculating as you type. Naegele's rule works by adding 280 days — exactly 40 weeks — to the first day of your last period. This assumes a regular 28-day menstrual cycle with ovulation around day 14 and fertilization shortly after. Because pregnancy is conventionally dated from the LMP rather than from the moment of conception, gestational age runs about two weeks 'ahead' of the actual age of the embryo: a full-term pregnancy of 40 weeks corresponds to roughly 38 weeks since fertilization. That two-week offset is why your provider counts you as, say, six weeks pregnant when the embryo itself is only about four weeks old. If your cycle is consistently longer or shorter than 28 days, ovulation shifts accordingly, so this calculator adjusts the estimate by (cycle length − 28) days. A 30-day cycle pushes the due date two days later; a 21-day cycle moves it a full week earlier. The tool accepts cycle lengths from 20 to 45 days, covering the vast majority of regular menstrual patterns while rejecting values that fall outside a plausible physiological range. Every calculation runs on UTC calendar arithmetic rather than your device's local clock, so the result never drifts by a day because of your time zone, daylight-saving changes, or travel across regions — a subtle bug that trips up many simpler calculators. It helps to understand what a due date actually represents. It is a single statistical midpoint, not a deadline: large studies consistently find that only about 4% of babies are born on their exact estimated date, and a normal, healthy delivery can occur anytime between 37 and 42 completed weeks. First-trimester ultrasound dating is generally more accurate than an LMP-based estimate, particularly when your cycles are irregular, when you are unsure of your last period, or when you conceived while breastfeeding or shortly after stopping hormonal birth control. Many clinics will revise your official due date if an early scan differs meaningfully from the LMP estimate. Use this calculator as a convenient planning aid for tracking trimesters, prenatal appointments, screening windows, and milestones such as when you enter the second and third trimesters. It is not a diagnosis and not a replacement for professional prenatal care. Always confirm your dates and discuss any concerns — bleeding, irregular cycles, or uncertainty about your last period — with your obstetrician, midwife, or healthcare provider.

Methodology & sources

This calculator applies Naegele's rule, the standard obstetric dating method: estimated due date = first day of the last menstrual period (LMP) + 280 days (40 weeks), assuming a regular 28-day cycle with ovulation near day 14. For cycles other than 28 days, the estimate is adjusted linearly by (cycle length − 28) days to reflect the shifted ovulation date. Gestational age is computed as the whole number of days between the LMP and today, expressed as weeks and days. All calculations use UTC epoch-day arithmetic so results are independent of the user's time zone. This tool provides a statistical estimate only and is not medical advice, not a diagnosis, and not a substitute for prenatal care — first-trimester ultrasound dating is more accurate, and you should confirm all dates with a qualified obstetrician, midwife, or healthcare provider.

Frequently asked questions

How is my due date calculated?
It uses Naegele's rule: the tool adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle. If your cycle differs, the estimate shifts by (your cycle length − 28) days to account for a later or earlier ovulation.
How accurate is a due date estimate?
A due date is a statistical midpoint, not a guarantee — only about 4% of babies are born on their exact estimated date, and a normal term delivery can happen anytime between 37 and 42 weeks. First-trimester ultrasound dating is usually more precise than an LMP-based estimate, especially with irregular cycles.
What is gestational age and why is it 'two weeks ahead'?
Gestational age is measured from the first day of your last period, not from conception. Because ovulation typically happens about two weeks after your period starts, gestational age runs roughly two weeks ahead of the embryo's actual age since fertilization.

Related tools

Calculators guides

View all