Baby age
Just the date of birth: completed weeks and months. If born before 37 weeks, also the corrected age used for growth and milestones.
Weeks or months?
In the first months everything is counted in weeks (checkups, vaccines, development): “12 weeks” is more precise than “almost 3 months”. The tool gives both, consistent with how paediatricians count: completed calendar months, not 30-day blocks.
Corrected age for preemies
A baby born at 32 weeks is 8 weeks “early” on term (40): at 3 calendar months, development compares to a 1-month-old. That's corrected age: normally used up to 24 months for growth, percentiles and milestones (smiling, grasping, crawling). Our growth percentile tool uses exactly the age in months.
Note
Worldwide about 1 in 10 babies is born preterm: corrected age is the standard neonatology convention. Informational tool: assessments and follow-up belong to your paediatrician and, for very preterm babies, the neonatal follow-up clinic.
Why weeks and not months
In the first months everything is counted in weeks because months are not the same length: two «three-month-old» babies can be ten days apart, and at that age ten days are an era. Developmental milestones, vaccination boosters and health checks are set in weeks for exactly this reason.
The switch to months usually happens around two years old, when a few days stop mattering. After two it becomes years, and the same question («how old is she?») stops needing a precise answer.
Counting months has an awkward edge
Counting months from a date has one case that defeats most programs: a baby born on 31 January turns one month old on 28 February, not on 3 March. If the count overshoots the end of the month, it has to be clamped to the last available day.
The same goes for babies born on 29 February, who in ordinary years mark the day on the 28th. These are the two edges that, when handled wrongly, produce absurd results like «one month and -2 days», and they are also why it pays to trust a count done properly rather than in your head.
Corrected age applies until two years
For a baby born before term you use corrected age: the weeks of prematurity relative to 40 are subtracted from the chronological age. A baby born at 32 weeks, at 6 months of life, is 4 months corrected, and that is the age to read developmental milestones and growth curves with.
The correction is dropped around two years, by which point catch-up is typically complete: from there chronological age takes over. Comparing a premature baby with chronological peers in the early months is the simplest way to worry about a delay that is not there.
Nearby tools
Measurements are read on the Growth percentiles, which should be used with corrected age. To count days and weeks between two dates there is Difference between dates.