Pregnancy Due Date Calculator
Estimate your due date from your last period.
The start of your last menstrual period (LMP)
Results update as you type.
This calculator provides estimates based on population research. Results are not medical advice. Consult a qualified healthcare provider before making health or nutrition decisions.
How it works
This pregnancy due date calculator estimates when a baby is likely to arrive using Naegele's rule, the standard method that adds 280 days, or 40 weeks, to the first day of the last menstrual period. You enter that date and it returns both the estimated due date and an approximate conception date, which falls about two weeks after the last period in a typical 28-day cycle. Pregnancy is conventionally dated from the last menstrual period rather than the moment of conception because the period date is usually known, while the exact day of conception often is not. It is important to understand that the due date is an estimate, not a fixed appointment: only about one in twenty babies arrives on the predicted day, and a full-term birth can happen any time from three weeks before to two weeks after. Cycle length, irregular periods, and early ultrasound measurements can all shift the estimate, and a doctor or midwife may revise the date based on a scan. Expectant parents use the due date to plan appointments, prepare for the birth, and track pregnancy milestones by week. This tool is for general information only and does not replace professional prenatal care. Enter the first day of your last period to see the estimate.
due date = LMP + 280 days; conception ≈ LMP + 14 days
Naegele's rule estimates the due date by adding 280 days, or 40 weeks, to the first day of the last menstrual period. This assumes a regular 28-day cycle with ovulation and conception around day 14, which is why the estimated conception date is the last period plus about two weeks. Pregnancy is dated from the last period rather than conception because that date is usually known more precisely.
Source:Naegele's rule (ACOG)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter first day of last period.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Estimated due date updates instantly as you type — no button to press.
Frequently asked questions
How is a due date calculated?
The most common method, Naegele's rule, adds 280 days, which is 40 weeks, to the first day of your last menstrual period. This dates the pregnancy from the last period rather than from conception, because the period date is usually known precisely while the conception date is not. The calculator applies this rule and also estimates a likely conception date around two weeks later.
How accurate is the due date?
A due date is an estimate, not a guarantee. Only about five percent of babies are born on their exact due date, and a normal full-term delivery can occur anywhere from about three weeks before to two weeks after it. Factors like cycle length and early ultrasound findings can shift the estimate, and healthcare providers often refine the date with a first-trimester scan.
Why is pregnancy dated from the last period?
Pregnancy is measured from the first day of the last menstrual period because that date is typically known and easy to record, whereas the precise day of conception usually is not. This convention means the first two weeks of the dated pregnancy occur before conception actually happens. It gives a consistent starting point that clinicians worldwide use to track progress in weeks.
What if my cycle is not 28 days?
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter, ovulation and conception occur earlier or later, which shifts the true due date accordingly. For irregular or non-standard cycles, an early ultrasound gives a more reliable date, so treat this estimate as a starting point and confirm it with your provider.
Should I rely on this instead of a doctor?
No. This calculator provides a general estimate for information and planning only. Prenatal care from a doctor or midwife, including ultrasound dating and regular check-ups, is essential for an accurate due date and a healthy pregnancy. Always follow professional medical advice, and use the estimated date simply as a helpful guide for preparing and tracking milestones.