How this calculator works
Corrected age subtracts the interval between birth and the 40-week due date from chronological age. Before the due date the signed result is negative. The AAP describes using corrected age for developmental expectations during the first two years. This tool does not change vaccination schedules. Month and year unit readouts use fixed average durations; milestone dates instead add calendar months, clamping to the last day of a shorter month. The result is an estimate for information only and is not medical advice.
Formula
Prematurity = max(0, 280 - gestational days at birth). Corrected days = days since birth - prematurity. In due-date mode, prematurity = max(0, due date - birth date).
Worked example
At 12 weeks since a birth at 32 weeks gestation, subtract 8 weeks: corrected age is 4 weeks. At 100 days since a birth at 35 weeks, subtract 35 days to obtain 65 corrected days.
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Frequently asked questions
Why is the answer negative?
The assessment precedes the original due date. A negative corrected age gives the remaining time until that date.
What about a baby born after 40 weeks?
The correction is zero. This tool never adds extra developmental age for a later birth.
Why do calendar milestones differ from months in the unit selector?
The selector uses an average month of 30.4375 days. Calendar milestones follow the actual lengths of months.