Due date calculator
The standard estimate: 280 days from the first day of the last menstrual period. This page does the arithmetic and tells you how much weight the result can actually bear.
What this estimate is
The calculation is Naegele's rule: 280 days (40 weeks) from the first day of the last menstrual period, adjusted for your average cycle length. It is the same first-pass arithmetic printed in obstetric textbooks, and it is what most period apps use.
Its limits are well known and worth stating plainly:
- It assumes you know the date of your last period with confidence
- It assumes ovulation at a predictable point in your cycle, which varies
- An early ultrasound is more accurate, and when the two disagree meaningfully, clinicians generally go with the ultrasound - the due date in your chart may legitimately differ from this arithmetic
- A due date is the middle of a distribution, not an appointment - a minority of babies arrive on it
Questions for the first prenatal visit
- How will you date the pregnancy, and when would an ultrasound adjust it?
- What does my insurance cover for prenatal care, and is the hospital I would deliver at in-network for my plan?
- Which of my current medications and supplements should we review?
- How does this practice handle questions between visits?
The delivery-cost question deserves asking early, not at 38 weeks - our guide on finding what things cost covers how to get real numbers out of a hospital, and a pregnancy appointment log keeps the visits organized.
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