Pregnancy Due Date Calculator

A due date is an estimate — not a deadline. Understanding how it is calculated, what the calculation assumes, and why the actual delivery date often differs helps set realistic expectations and reduces anxiety as your pregnancy progresses.

How Due Dates Are Calculated

The most common method uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. It dates the pregnancy from the beginning of the last period, not from conception — so at a positive pregnancy test (about 2 weeks after conception), you are already considered "4 weeks pregnant" by clinical convention.

Ultrasound dating is considered the most accurate method, especially in the first trimester. An ultrasound measures the crown-rump length (CRL) of the embryo, which correlates closely with gestational age. First-trimester ultrasounds are accurate to within 3–5 days. Later ultrasounds (second and third trimester) are less accurate due to variations in fetal size.

IVF dating uses the embryo transfer date as the reference point, adding the embryo's developmental stage to calculate the estimated due date. This is the most precise method when the conception date is known.

Why Due Dates Are Estimates

Only about 4% of babies are born on their exact due date. Approximately 80% are born within 10 days before or after the estimated date. The normal range for a full-term pregnancy is 37–42 weeks, meaning there is a 5-week window during which delivery is considered normal. Factors that influence actual delivery timing include genetics, maternal health, fetal position, and whether this is a first pregnancy.

Trimester Breakdown

• First trimester (weeks 1–12): The embryo develops major organs, the heart begins beating, and the foundation for all body systems is established. This is the period of highest risk for miscarriage.
• Second trimester (weeks 13–26): Often called the "golden period." The fetus grows rapidly, movement becomes noticeable, and many early pregnancy symptoms subside.
• Third trimester (weeks 27–40): The fetus gains weight rapidly, lungs mature, and the body prepares for birth. Weekly prenatal visits typically begin in the third trimester.

Important Considerations

Due dates calculated from LMP may be inaccurate if you have irregular cycles, are uncertain about the date of your last period, or if ovulation occurred earlier or later than day 14. An ultrasound in the first trimester provides the most accurate dating. Always confirm your due date with your healthcare provider, and remember that the due date is a target, not a guarantee — babies arrive on their own schedule.