Premature babies should not be measured against the calendar alone. A baby born two months early has had two fewer months of intrauterine development, so comparing its skills with a same-chronological-age peer understates its progress and risks an unnecessary developmental delay diagnosis. Corrected age — also called adjusted age — fixes this by counting from the original due date.
How it works
The calculation has two steps: work out how early the baby arrived, then subtract that gap from the time since birth.
weeks premature = 40 − gestational age at birth (in weeks)
corrected age = chronological age − weeks prematureThe tool computes the chronological age precisely from the birth date and assessment date, converts the gestational age (entered as completed weeks plus any additional days) into a decimal weeks-premature figure, and reports the corrected age broken into months, weeks, and days.
Worked example
A baby born at 30 weeks and 2 days is approximately 9 weeks and 5 days premature — a hair under 10 weeks early.
Assessed at a chronological age of 4 months (about 17.4 weeks):
- Chronological age: ~17.4 weeks
- Weeks premature: ~9.7 weeks
- Corrected age: ~7.7 weeks, or about 1 month and 3 weeks
Developmental milestones — smiling, tracking, reaching — should be expected at the corrected age, not the chronological age. In this example, the baby is not “behind” at 4 months; it is developmentally where a 7-week-old is expected to be.
When to stop correcting
Correction is conventionally applied until approximately 24 months of age. By 2 years, the developmental gap between preterm and term children typically narrows to the point where chronological age becomes the appropriate reference. Some neonatology and developmental paediatrics services continue correction slightly longer for very preterm infants (born before 28 weeks), particularly for motor milestones.
A baby born at full term (40 weeks) carries no correction at all — weeks premature is zero.
Clinical context
Corrected age is used for growth charts as well as developmental milestones. Weight, length, and head circumference for a preterm infant should be plotted against the corrected age when using standard centile charts designed for term infants. Health visitors and neonatologists in the UK typically use corrected age explicitly on clinical records to avoid confusion.
For developmental screening tools, follow the guidance of the specific tool — some are validated for corrected age, some for chronological age, and using the wrong one is a common source of false concerns.
This calculator is a clinical reference aid. Always interpret results in the context of the full developmental assessment and the clinical team’s guidance.