Corrected age for a baby born 13 weeks early (27 weeks)
A baby born 13 weeks early (at 27 weeks) has a corrected age thirteen weeks, about 3 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 15 months, twelve months and thirteen weeks after birth.
Updated
Corrected age for a baby born 13 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
- about 15 months, counted from the due date, July 15, 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 13 weeks early means born at about 27 weeks of pregnancy, so the corrected age runs 13 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 13 weeks early at a glance
| Born at | 27 weeks of pregnancy |
|---|---|
| Weeks early | 13 weeks = 91 days ≈ 3 months |
| 12-month checklist | from 15 months after birth |
| Correction stops | 24 months (corrected then: 21 mo) |
Corrected age by month, born 13 weeks early
Age since birth minus thirteen 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 9 wk | – |
| 2 | due in 4 wk | – |
| 3 | at due date | – |
| 4 | 4 wk | – |
| 5 | 9 wk | – |
| 6 | 3 mo | – |
| 7 | 4 mo | – |
| 8 | 5 mo | – |
| 9 | 6 mo | – |
| 10 | 7 mo | – |
| 11 | 8 mo | – |
| 12 | 9 mo | – |
| 13 | 10 mo | – |
| 14 | 11 mo | – |
| 15 | 12 mo | 12 |
| 16 | 13 mo | 12 |
| 17 | 14 mo | 12 |
| 18 | 15 mo | 15 |
| 19 | 16 mo | 15 |
| 20 | 17 mo | 15 |
| 21 | 18 mo | 18 |
| 22 | 19 mo | 18 |
| 23 | 20 mo | 18 |
| 24 | stop correcting | 24 |
When each CDC checklist applies, born 13 weeks early
Each corrected-age checklist starts thirteen weeks after its plain age; from the second birthday, calendar age. Dates: the example baby below (born 21 February 2025, due 23 May 2025).
| CDC checklist | Compare from (age since birth) | Example baby: in use |
|---|---|---|
| 12-month | 12 months + 13 weeks (about 15 months) | 23 May 2026 to 22 August 2026 |
| 15-month | 15 months + 13 weeks (about 18 months) | 23 August 2026 to 22 November 2026 |
| 18-month | 18 months + 13 weeks (about 21 months) | 23 November 2026 to 20 February 2027 |
| 24-month | 24 months: correction stops | 21 February 2027 to 20 August 2027 |
| 30-month | 30 months (calendar age) | 21 August 2027 to 20 February 2028 |
| 36-month | 36 months (calendar age) | from 21 February 2028 |
Worked example: a baby born 13 weeks early
A made-up baby, born on 21 February 2025, thirteen weeks early, with exact calendar dates.
-
Find the due date
21 February 2025 plus thirteen weeks (ninety-one days) is 23 May 2025.
-
Age since birth
On 21 August 2026 the baby is exactly eighteen months old by the calendar.
-
Corrected age
From 23 May 2025 to 21 August 2026 is fourteen months and twenty-nine days: the corrected age.
-
Pick the checklist
Fourteen months and twenty-nine 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 23 August 2026.
-
Stop correcting
21 February 2027 is the second birthday (corrected: twenty months and twenty-nine days); from then on, calendar age only.
Before the due date: the first three months
For the first thirteen 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 three monthly rows. Corrected-age milestones start to mean something once the due date has passed.
The thirteen-week gap matters most early on: at six months it is fifty percent of the baby's age, at twelve months twenty-five percent, and at twenty-three months only thirteen percent. Hence correction for the first two years only.
Corrected age at the AAP screening visits, 13 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 (21 November 2025 for the example baby): about six months corrected, so an earlier CDC checklist (before 12 months).
- 18 months (21 August 2026): about fifteen months corrected, matching the 15-month checklist.
- 24 months (21 February 2027): correction stops, so the 24-month checklist by calendar age.
- 30 months (21 August 2027): no correction; the 30-month checklist.
When to stop correcting for 13 weeks early
The AAP advises corrected age for the first 2 years, so for a baby born thirteen weeks early correction stops at 24 months chronological, when the corrected age is about twenty-one months; the comparison then jumps to the 24-month checklist.
So the 18-month list is used for about 3 months (from eighteen months and thirteen weeks to the second birthday), not six.
Corrected age and real concerns
Being born thirteen 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 13 weeks early?
Subtract thirteen weeks (or count from the due date). At six months: about three months corrected; at twelve months: nine months; at eighteen: fifteen months.
When does a baby born 13 weeks early reach the 12-month milestones?
From twelve months and thirteen weeks after birth (about 15 months); then the 15-month checklist at about 18 months and the 18-month one at about 21 months.
When do you stop using corrected age for 13 weeks early?
At 24 months chronological (AAP: first two years). On the second birthday the corrected age is about twenty-one months; from then on, calendar age and the 24-month checklist.
What is 13 weeks early in weeks of pregnancy?
A baby born thirteen weeks before a forty-week due date is born at twenty-seven weeks of pregnancy. The gap of ninety-one days, about 3 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.