Baby · Age
Baby Age — Chronological & Corrected
How old is your baby? Chronological age from birth date. Corrected age for preterm babies (chronological minus weeks of prematurity), used for developmental milestones until 2 years.
Last reviewed 2 June 2026
How old is my baby?
Baby Age Calculator
Enter your baby’s date of birth to see their current age and the next milestone.
Why ages in months?
Babies change dramatically month-to-month until age 2. Milestones, vaccines, growth charts all run monthly. After 2 years: years + months.
Corrected age (preterm)
Chronological age minus weeks of prematurity. Example: born 32 wk, now 6 months chronological = ~4 months corrected.
Used for developmental milestones until 2 years. Then chronological.
Vaccines
Use chronological age. Preterm babies benefit from on-time vaccinations at standard intervals (8 wk, 12 wk, 16 wk, 1 yr, etc.).
Growth charts
- Preterm: corrected age until 2 years.
- Fenton chart 22-50 weeks corrected GA.
- Switch to WHO / UK-WHO at 50 wk corrected (= term).
- Chronological after 2 years.
First year milestones (chronological for term babies)
- 0-3 mo: smiling, head control, recognising parents.
- 3-6 mo: rolling, reaching, babbling, sitting with support.
- 6-9 mo: sitting independently, crawling, finger food, stranger awareness.
- 9-12 mo: pulling to stand, waving, first words.
- 12 mo: typical walking starts (range 9-15 months).
Age categories
- Newborn 0-3 months.
- Infant 3-12 months.
- Toddler 12-36 months.
- Pre-schooler 3-5 years.
- School-age 5+.
Different scenarios
Scenario 1: Born 28 wk, now 6 months chronological
Corrected age ~3 months. Use for milestones + growth.
Scenario 2: 14-month-old not walking
Normal range (9-15 months). Recheck at 18 months if still not.
Scenario 3: 25-month-old preterm, still using corrected age
Switch to chronological now — 2-year mark.
Scenario 4: Vaccines at 8 weeks, was preterm 30 wk
Vaccinate at 8 weeks chronological, not corrected. NHS standard.
Scenario 5: 6-month milestones not met chronologically
If preterm, check corrected age first.
Care guidance
- Corrected age for preterm until 2 years.
- Chronological for vaccines always.
- Wide normal variation in milestones.
- Health visitor checks at standard ages.
Sources
- NHS. Baby age and milestones.
- BAPM. Preterm follow-up.
- WHO Growth Standards.
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Frequently asked questions
Why are baby ages given in months instead of years?
Babies change so dramatically month-to-month that months is the most useful unit until age 2. Most paediatric milestones (sitting, crawling, walking, first words) tracked in months; vaccinations timed in months; growth charts run monthly. After 2 years, conversion switches to years + months.
What's corrected age?
For PRETERM babies: chronological age minus weeks of prematurity. Example: born 32 wk, now 6 months old = 6 months - 8 weeks (40 wk full-term) = ~4 months CORRECTED. Used for developmental milestones until 2 years. After 2, growth + development converge to chronological. /calculators/fenton-growth /calculators/new-ballard.
When do I stop using corrected age?
At 2 YEARS chronological. By then, preterm catch-up generally complete. Some children with extreme prematurity / complications continue at corrected age longer (specialist guidance). MOST: switch to chronological at 24 months.
How is age calculated for vaccines?
CHRONOLOGICAL age (from birth). Even preterm babies get vaccinations at standard chronological intervals (8 wk, 12 wk, 16 wk, 1 yr, etc.). Preterm babies have weaker immune system; benefit from on-time vaccination. NHS UK routine schedule applies regardless of gestation at birth.
Adjusted vs chronological — which on growth charts?
PRETERM: use CORRECTED age on growth charts until ~2 years. SOME charts (Fenton for very preterm) corrected to ~50 weeks gestational age then switch to WHO/UK-WHO. CHRONOLOGICAL after 2 years. ASK paediatric team for the right approach for your baby's specific situation.
What about international birthday differences?
Some cultures count age differently. Western: 0 at birth + 1 every birthday. Korean traditional: 1 at birth (counted from conception roughly) + 1 every New Year (most are 1-2 years 'older' than Western age). Chinese traditional: similar but +1 at Lunar New Year. MEDICAL records use chronological from birth date.
Why does my baby's age matter for development?
Developmental milestones (smiling, sitting, walking, talking) follow age-dependent patterns. KNOWING right age informs: which milestones expected; when to worry about delays; vaccination timing; feeding stages (solids, cow's milk timing); when to introduce activities. Pediatric care plans tied to age.
What's a 'baby's first year' broken down?
(1) 0-3 months: smiling, head control, recognising parents; (2) 3-6 months: rolling, reaching, babbling, sitting with support; (3) 6-9 months: sitting independently, crawling, finger food, stranger awareness; (4) 9-12 months: pulling to stand, waving, first words; (5) 12 months: typical walking starts (range 9-15 months). MASSIVE individual variation.
When does baby become 'a toddler'?
Generally from 12-18 months — once walking established. 'Toddler' = walking but not fully steady. By 3 years: pre-schooler. ROUGH categories: NEWBORN 0-3 months; INFANT 3-12 months; TODDLER 12-36 months; PRE-SCHOOLER 3-5 years; SCHOOL-AGE 5+.
What age is my baby in months?
MONTHS from birth date. Round simply: < 2 wks = 'newborn'; 2 wks-8 wks = '6 weeks old' typically; 8 weeks = 2 months; 12 weeks = 3 months. Many parents say '12 weeks' until baby is ~3 months chronological. After 3 months, easier to use months. After 2 years, years + months ('2 years and 4 months').
What about adjusted ages for IVF?
Same as for natural conception — chronological from birth date. IVF DUE DATE calculation different from natural conception (uses embryo transfer date — /calculators/ivf-due-date). But after birth, age proceeds normally.
How does this relate to other calculators on BumpBites?
Companion: /calculators/baby-percentile (growth); /calculators/milestone-tracker; /calculators/fenton-growth (preterm); /calculators/new-ballard (gestational age estimate); /calculators/vaccine-scheduler; /calculators/sleep-schedule; /calculators/food-intro-tracker.