Quick answer: Divide your baby’s age in weeks by 4.35 to get months. 13 weeks is about 3 months, 26 weeks is 6 months and 52 weeks is 12 months.
Baby age in months chart
| Age in weeks | Age in months | What many babies are doing |
|---|---|---|
| 4 | about 1 | Follows faces, feeds often |
| 8 | about 2 | Social smiles, coos |
| 13 | 3 | Smiles more, tracks objects with eyes |
| 17 | about 4 | Laughs, holds head steadier |
| 22 | about 5 | Reaches for and grabs toys |
| 26 | 6 | Rolls over, babbles |
| 35 | about 8 | Sits with little support |
| 39 | 9 | Crawls, responds to own name |
| 43 | about 10 | Pulls to stand |
| 52 | 12 | Waves bye, says first words |
How do you convert weeks to months?
Quick answer: Divide the number of weeks by 4.35. For example, 20 weeks ÷ 4.35 ≈ 4.6, so the baby is 4 months and about 3 weeks old.
Why isn’t 4 weeks the same as 1 month?
Four weeks is exactly 28 days, but an average calendar month is about 30.4 days. That is why 4 weeks is slightly less than one month. Read the full explanation in How Many Weeks Are in a Month?
Does this chart work for premature babies?
Babies born before 37 weeks are usually compared using corrected age, not age since birth. See how to calculate corrected age, or tick “Born early?” in our baby age calculator.
Key takeaways
- One month averages 4.35 weeks, not 4.
- Count in weeks for the first 12 weeks, then in months until 24 months.
- Premature babies are usually compared using corrected age.
Frequently asked questions
How many weeks old is a 6-month-old baby?
A 6-month-old baby is about 26 weeks old.
How old is a baby at 12 weeks in months?
A 12-week-old baby is about 2.8 months old, just under 3 months.
At what age do you stop counting a baby’s age in weeks?
Most parents and doctors use weeks for the first 12 weeks, then switch to months until about 24 months, then years.
Written by the Baby Age Calculator editorial team. Last updated October 28, 2026. General information only, not medical advice. See our medical disclaimer.