Few dates matter more than a due date, yet it's fundamentally an estimate. Understanding how it's calculated — and why it shifts — helps you plan without treating it as a guarantee.
Counting from the last period
Pregnancy is dated from the first day of the last menstrual period because that's a date most women know. Adding 280 days reaches the 40-week mark that defines 'term'. Conception actually happens about two weeks later, which is why the conception-based count is 266 days.
Trimester by trimester
The first trimester spans weeks 1–13 and is a period of rapid development; the second (14–27) is when the anatomy scan happens; the third (28–40) is growth and preparation for birth. Knowing your week and trimester makes milestone planning straightforward.
Estimates, not guarantees
Only about 5% of babies arrive exactly on schedule. Providers treat 37–42 weeks as the normal window, so a due date is best used as a planning tool rather than a firm prediction.
Frequently asked questions
The standard method counts 280 days (40 weeks) from the first day of your last menstrual period. If you know your conception date, the due date is 266 days from that.
It's an estimate — most babies arrive within two weeks before or after the calculated date, and only about 5% arrive exactly on it.
An early ultrasound measures the baby's size to date the pregnancy more precisely, and providers often adjust the date if it differs from the LMP estimate.