Pregnancy Due Date Calculator
Estimate your due date from your last period, conception date, IVF transfer, or an ultrasound reading.
What is Gestational Age? This is how far along your pregnancy is, counted in weeks and days from your LMP - not from conception. Because of this, a "40-week pregnancy" actually includes about 2 weeks before conception occurred, which is why it feels longer than 9 exact months.
Why 40 weeks? The 40-week standard (called "Naegele's Rule") is the globally accepted medical estimate. It starts from your LMP and accounts for the average ovulation window. Most healthy babies arrive between 38 and 42 weeks.
How many weeks and days pregnant were you measured at during that ultrasound?
This tool is for informational purposes only and is not a substitute for professional medical advice. Due dates are estimates - only about 5% of babies are born on their exact due date. Always confirm your due date with your healthcare provider.
Understanding Your Due Date and Pregnancy Timeline
A due date is not a guaranteed delivery date - it is a statistical estimate of when a pregnancy reaches full term, calculated from a standard 40-week (280-day) gestation period. Doctors track pregnancy progress in weeks rather than months because weekly milestones (such as the end of the first trimester at 13 weeks, or viability around 24 weeks) are far more clinically useful than rough monthly estimates.
Each calculation method on this page approaches the same goal - estimating the 40-week mark - from a different starting point. The Last Menstrual Period (LMP) method is the most common because the first day of a period is usually easy to remember, even though ovulation (and true conception) typically happens about two weeks later. The conception-date and ultrasound methods can be more precise for people with irregular cycles, while the IVF method relies on a known transfer date rather than an estimate.
Original Insight: Why Only 5% of Babies Arrive on Their Due Date
It is a common misconception that a due date marks the single "correct" day for delivery. In reality, full-term birth is defined as anywhere from 37 to 42 weeks of gestation - a five-week window, not a single point. The 280-day estimate assumes a textbook 28-day menstrual cycle with ovulation on day 14, but real cycle lengths vary widely from person to person and even month to month for the same person. A cycle that runs longer than 28 days shifts ovulation - and therefore conception - later, which means the LMP-based due date can be off by a week or more even when the calculation itself is performed correctly. This is precisely why healthcare providers treat the due date as the center of a range rather than a deadline, and why an early ultrasound (which measures the embryo directly) is often considered the most accurate dating method of all.
Glossary of Key Terms
- LMP (Last Menstrual Period)
- The first day of a person's most recent menstrual period before becoming pregnant. It is the standard reference point for dating a pregnancy because it is usually easier to recall than the exact date of conception.
- Gestational Age
- The age of a pregnancy measured in weeks and days, counted from the first day of the LMP - not from the actual date of conception, which is typically about two weeks later.
- EDD (Estimated Due Date)
- The projected date a pregnancy reaches 40 weeks of gestation. Also called the "estimated date of confinement" (EDC) in older medical literature.
- Trimester
- One of three roughly 13-week stages of pregnancy. The first trimester covers weeks 1 to 13, the second covers weeks 14 to 27, and the third covers weeks 28 through delivery.
- Naegele's Rule
- The traditional formula used to estimate a due date from the LMP: add one year, subtract three months, and add seven days. This produces the same 280-day estimate used by this calculator.
- IVF Transfer Date
- The date a fertilized embryo is placed into the uterus during in vitro fertilization. Because the embryo's age is known precisely at transfer, due dates calculated from this date tend to be more accurate than LMP-based estimates.
Frequently Asked Questions
Why do different calculators sometimes give slightly different due dates?
Most discrepancies come down to which dating method is used and how cycle length is handled. A calculator using a strict 28-day-cycle assumption will produce a different result than one that adjusts for a longer or shorter cycle. Ultrasound-based estimates can also differ slightly from LMP-based ones, especially if the scan was performed later in the first trimester. None of these small differences (typically a day or two) are cause for concern - your healthcare provider will use the dating method they consider most reliable for your specific situation.
Can my due date change as my pregnancy progresses?
Yes. It is common for a due date to be adjusted after a first-trimester ultrasound, since direct measurement of the embryo or fetus at that stage is considered highly accurate. Once a due date has been confirmed by an early scan, providers generally do not change it further based on later scans, since growth rates naturally vary more in the second and third trimesters.
What is the difference between "full term" and my due date?
Your due date represents the 40-week mark, but "full term" is defined as 39 weeks 0 days through 40 weeks 6 days. Babies born from 37 weeks 0 days through 38 weeks 6 days are considered "early term," and those born at 41 weeks or beyond are "late term" or "post-term." This terminology reflects research showing that babies born even a week or two before 39 weeks can have measurably different outcomes on average, which is why many elective procedures are scheduled no earlier than 39 weeks when possible.
This calculator provides a statistical estimate for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your doctor or midwife to confirm your due date and discuss your individual pregnancy timeline.