Baby age, week by week
21 Weeks Old — How Many Months?
A 21-week-old baby is 147 days old — about 4 months and 25 days.
This week
Some babies sit propped up on their hands now - the tripod pose.
The math
21 weeks = 147 days ≈ 4 months + 25 days
A month is longer than 4 weeks — on average 30.44 days — so you can't just divide by 4. The exact answer shifts a little with the calendar, so for the day-perfect number, use the calculator with your baby's real birthday.
Get the exact age →What babies do around week 21
At 21 weeks your baby is near the 5 Months mark. Around this age, many babies:
- Sits alone for a moment or two
- Checks out toys with mouth, hands, and eyes
- Deeply attached to their favorite people