Corrected age for a baby born 9 weeks early (31 weeks)
A baby born 9 weeks early (at 31 weeks) has a corrected age nine weeks, about 2.1 months, below the calendar age. The AAP advises comparing development by corrected age for the first 2 years, so the 12-month CDC checklist applies from about 14.1 months, twelve months and nine weeks after birth.
Updated
Corrected age for a baby born 9 weeks early
Weeks early = 40 minus the weeks of pregnancy at birth. Everything is calculated in this browser; nothing is sent or saved.
Example: a child born April 15, 2025, checked on October 15, 2026 (18 months old). Enter your child's birth date.
- Corrected age
- 15 months 28 days
- about 16 months, counted from the due date, June 17, 2025
- Chronological age
- 18 months
- about 18 months, counted from the birth date
- Stop correcting on
- April 15, 2027
- the second birthday (24 months chronological)
Born 9 weeks early means born at about 31 weeks of pregnancy, so the corrected age runs 9 weeks (about 2 months) behind the age since birth. Until April 15, 2027, compare skills with the corrected age: between two checklist ages CDC says to use the earlier one, which is the 15-month checklist here.
When each CDC checklist applies for this child
Born 9 weeks early at a glance
| Born at | 31 weeks of pregnancy |
|---|---|
| Weeks early | 9 weeks = 63 days ≈ 2.1 months |
| 12-month checklist | from 14.1 months after birth |
| Correction stops | 24 months (corrected then: 22 mo) |
Corrected age by month, born 9 weeks early
Age since birth minus nine weeks, in 30.4-day months; a dash means an earlier CDC checklist. The checklist changes in month fifteen, eighteen and twenty-one, then at twenty-four.
| Age since birth (months) | Corrected age (mo, wk) | CDC checklist (months) |
|---|---|---|
| 1 | due in 5 wk | – |
| 2 | at due date | – |
| 3 | 4 wk | – |
| 4 | 8 wk | – |
| 5 | 13 wk | – |
| 6 | 4 mo | – |
| 7 | 5 mo | – |
| 8 | 6 mo | – |
| 9 | 7 mo | – |
| 10 | 8 mo | – |
| 11 | 9 mo | – |
| 12 | 10 mo | – |
| 13 | 11 mo | – |
| 14 | 12 mo | – |
| 15 | 13 mo | 12 |
| 16 | 14 mo | 12 |
| 17 | 15 mo | 12 |
| 18 | 16 mo | 15 |
| 19 | 17 mo | 15 |
| 20 | 18 mo | 15 |
| 21 | 19 mo | 18 |
| 22 | 20 mo | 18 |
| 23 | 21 mo | 18 |
| 24 | stop correcting | 24 |
When each CDC checklist applies, born 9 weeks early
Each corrected-age checklist starts nine weeks after its plain age; from the second birthday, calendar age. Dates: the example baby below (born 10 September 2024, due 12 November 2024).
| CDC checklist | Compare from (age since birth) | Example baby: in use |
|---|---|---|
| 12-month | 12 months + 9 weeks (about 14.1 months) | 12 November 2025 to 11 February 2026 |
| 15-month | 15 months + 9 weeks (about 17.1 months) | 12 February 2026 to 11 May 2026 |
| 18-month | 18 months + 9 weeks (about 20.1 months) | 12 May 2026 to 9 September 2026 |
| 24-month | 24 months: correction stops | 10 September 2026 to 9 March 2027 |
| 30-month | 30 months (calendar age) | 10 March 2027 to 9 September 2027 |
| 36-month | 36 months (calendar age) | from 10 September 2027 |
Worked example: a baby born 9 weeks early
A made-up baby, born on 10 September 2024, nine weeks early, with exact calendar dates.
-
Find the due date
10 September 2024 plus nine weeks (sixty-three days) is 12 November 2024.
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Age since birth
On 10 December 2025 the baby is exactly fifteen months old by the calendar.
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Corrected age
From 12 November 2024 to 10 December 2025 is twelve months and twenty-eight days: the corrected age.
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Pick the checklist
Twelve months and twenty-eight days is between 12 and 15 months: use the 12-month CDC checklist (the earlier one, as CDC advises) until the 15-month checklist on 12 February 2026.
-
Stop correcting
10 September 2026 is the second birthday (corrected: twenty-one months and twenty-nine days); from then on, calendar age only.
Before the due date: the first two months
For the first nine weeks after birth the baby has not reached the due date, so the corrected age is below zero ("due in" in the table) for the first two monthly rows. Corrected-age milestones start to mean something once the due date has passed.
The nine-week gap matters most early on: at six months it is thirty-four percent of the baby's age, at twelve months seventeen percent, and at twenty-three months only nine percent. Hence correction for the first two years only.
Corrected age at the AAP screening visits, 9 weeks early
The AAP recommends developmental screening at 9, 18 and 30 months and autism screening at 18 and 24 months. Ask your pediatrician which age they use; by corrected age:
- 9 months (10 June 2025 for the example baby): about seven months corrected, so an earlier CDC checklist (before 12 months).
- 18 months (10 March 2026): about sixteen months corrected, matching the 15-month checklist.
- 24 months (10 September 2026): correction stops, so the 24-month checklist by calendar age.
- 30 months (10 March 2027): no correction; the 30-month checklist.
When to stop correcting for 9 weeks early
The AAP advises corrected age for the first 2 years, so for a baby born nine weeks early correction stops at 24 months chronological, when the corrected age is about twenty-two months; the comparison then jumps to the 24-month checklist.
So the 18-month list is used for about 3.9 months (from eighteen months and nine weeks to the second birthday), not six.
Corrected age and real concerns
Being born nine weeks early is a reason to compare by corrected age, never a reason to wait on a worry: talk to your pediatrician if your baby loses a skill, misses milestones for their corrected age (at eighteen months by the calendar that means the 15-month checklist) or anything else concerns you. Many babies born early catch up by about age 2 (AAP). In the U.S. you can contact your state's Early Intervention program for children under 3 yourself; no doctor's referral is needed.
Frequently asked questions
How do you calculate corrected age for a baby born 9 weeks early?
Subtract nine weeks (or count from the due date). At six months: about four months corrected; at twelve months: ten months; at eighteen: sixteen months.
When does a baby born 9 weeks early reach the 12-month milestones?
From twelve months and nine weeks after birth (about 14.1 months); then the 15-month checklist at about 17.1 months and the 18-month one at about 20.1 months.
When do you stop using corrected age for 9 weeks early?
At 24 months chronological (AAP: first two years). On the second birthday the corrected age is about twenty-two months; from then on, calendar age and the 24-month checklist.
What is 9 weeks early in weeks of pregnancy?
A baby born nine weeks before a forty-week due date is born at thirty-one weeks of pregnancy. The gap of sixty-three days, about 2.1 months, is what corrected age subtracts.
Related
Milestones by corrected age
Neighbouring weeks
Corrected age tools
By corrected age
Monitoring, not medical advice. Talk to your pediatrician about any concern. Our sources and method.