Corrected age for a baby born 12 weeks early (28 weeks)
A baby born 12 weeks early (at 28 weeks) has a corrected age twelve weeks, about 2.8 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 14.8 months, twelve months and twelve weeks after birth.
Updated
Corrected age for a baby born 12 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 7 days
- about 15 months, counted from the due date, July 8, 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 12 weeks early means born at about 28 weeks of pregnancy, so the corrected age runs 12 weeks (about 3 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 12 weeks early at a glance
| Born at | 28 weeks of pregnancy |
|---|---|
| Weeks early | 12 weeks = 84 days ≈ 2.8 months |
| 12-month checklist | from 14.8 months after birth |
| Correction stops | 24 months (corrected then: 21 mo 1 wk) |
Corrected age by month, born 12 weeks early
Age since birth minus twelve 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 8 wk | – |
| 2 | due in 3 wk | – |
| 3 | 1 wk | – |
| 4 | 5 wk | – |
| 5 | 10 wk | – |
| 6 | 3 mo 1 wk | – |
| 7 | 4 mo 1 wk | – |
| 8 | 5 mo 1 wk | – |
| 9 | 6 mo 1 wk | – |
| 10 | 7 mo 1 wk | – |
| 11 | 8 mo 1 wk | – |
| 12 | 9 mo 1 wk | – |
| 13 | 10 mo 1 wk | – |
| 14 | 11 mo 1 wk | – |
| 15 | 12 mo 1 wk | 12 |
| 16 | 13 mo 1 wk | 12 |
| 17 | 14 mo 1 wk | 12 |
| 18 | 15 mo 1 wk | 15 |
| 19 | 16 mo 1 wk | 15 |
| 20 | 17 mo 1 wk | 15 |
| 21 | 18 mo 1 wk | 18 |
| 22 | 19 mo 1 wk | 18 |
| 23 | 20 mo 1 wk | 18 |
| 24 | stop correcting | 24 |
When each CDC checklist applies, born 12 weeks early
Each corrected-age checklist starts twelve weeks after its plain age; from the second birthday, calendar age. Dates: the example baby below (born 11 January 2025, due 5 April 2025).
| CDC checklist | Compare from (age since birth) | Example baby: in use |
|---|---|---|
| 12-month | 12 months + 12 weeks (about 14.8 months) | 5 April 2026 to 4 July 2026 |
| 15-month | 15 months + 12 weeks (about 17.8 months) | 5 July 2026 to 4 October 2026 |
| 18-month | 18 months + 12 weeks (about 20.8 months) | 5 October 2026 to 10 January 2027 |
| 24-month | 24 months: correction stops | 11 January 2027 to 10 July 2027 |
| 30-month | 30 months (calendar age) | 11 July 2027 to 10 January 2028 |
| 36-month | 36 months (calendar age) | from 11 January 2028 |
Worked example: a baby born 12 weeks early
A made-up baby, born on 11 January 2025, twelve weeks early, with exact calendar dates.
-
Find the due date
11 January 2025 plus twelve weeks (eighty-four days) is 5 April 2025.
-
Age since birth
On 11 July 2026 the baby is exactly eighteen months old by the calendar.
-
Corrected age
From 5 April 2025 to 11 July 2026 is fifteen months and six days: the corrected age.
-
Pick the checklist
Fifteen months and six days is between 15 and 18 months: use the 15-month CDC checklist (the earlier one, as CDC advises) until the 18-month checklist on 5 October 2026.
-
Stop correcting
11 January 2027 is the second birthday (corrected: twenty-one months and six days); from then on, calendar age only.
Before the due date: the first two months
For the first twelve 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 twelve-week gap matters most early on: at six months it is forty-six percent of the baby's age, at twelve months twenty-three percent, and at twenty-three months only twelve percent. Hence correction for the first two years only.
Corrected age at the AAP screening visits, 12 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 (11 October 2025 for the example baby): about six months and one week corrected, so an earlier CDC checklist (before 12 months).
- 18 months (11 July 2026): about fifteen months and one week corrected, matching the 15-month checklist.
- 24 months (11 January 2027): correction stops, so the 24-month checklist by calendar age.
- 30 months (11 July 2027): no correction; the 30-month checklist.
When to stop correcting for 12 weeks early
The AAP advises corrected age for the first 2 years, so for a baby born twelve weeks early correction stops at 24 months chronological, when the corrected age is about twenty-one months and one week; the comparison then jumps to the 24-month checklist.
So the 18-month list is used for about 3.2 months (from eighteen months and twelve weeks to the second birthday), not six.
Corrected age and real concerns
Being born twelve 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 12 weeks early?
Subtract twelve weeks (or count from the due date). At six months: about three months and one week corrected; at twelve months: nine months and one week; at eighteen: fifteen months and one week.
When does a baby born 12 weeks early reach the 12-month milestones?
From twelve months and twelve weeks after birth (about 14.8 months); then the 15-month checklist at about 17.8 months and the 18-month one at about 20.8 months.
When do you stop using corrected age for 12 weeks early?
At 24 months chronological (AAP: first two years). On the second birthday the corrected age is about twenty-one months and one week; from then on, calendar age and the 24-month checklist.
What is 12 weeks early in weeks of pregnancy?
A baby born twelve weeks before a forty-week due date is born at twenty-eight weeks of pregnancy. The gap of eighty-four days, about 2.8 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.