Corrected age for a baby born 4 weeks early (36 weeks)
A baby born 4 weeks early (at 36 weeks) has a corrected age four weeks, about 0.9 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 12.9 months, twelve months and four weeks after birth.
Updated
Corrected age for a baby born 4 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
- 17 months 2 days
- about 17 months, counted from the due date, May 13, 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 4 weeks early means born at about 36 weeks of pregnancy, so the corrected age runs 4 weeks (about 1 month) 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. See what is emerging at 17 months.
When each CDC checklist applies for this child
Born 4 weeks early at a glance
| Born at | 36 weeks of pregnancy |
|---|---|
| Weeks early | 4 weeks = 28 days ≈ 0.9 months |
| 12-month checklist | from 12.9 months after birth |
| Correction stops | 24 months (corrected then: 23 mo) |
Corrected age by month, born 4 weeks early
Age since birth minus four weeks, in 30.4-day months; a dash means an earlier CDC checklist. The checklist changes in month thirteen, sixteen and nineteen, then at twenty-four.
| Age since birth (months) | Corrected age (mo, wk) | CDC checklist (months) |
|---|---|---|
| 1 | at due date | – |
| 2 | 5 wk | – |
| 3 | 9 wk | – |
| 4 | 3 mo | – |
| 5 | 4 mo | – |
| 6 | 5 mo | – |
| 7 | 6 mo | – |
| 8 | 7 mo | – |
| 9 | 8 mo | – |
| 10 | 9 mo | – |
| 11 | 10 mo | – |
| 12 | 11 mo | – |
| 13 | 12 mo | 12 |
| 14 | 13 mo | 12 |
| 15 | 14 mo | 12 |
| 16 | 15 mo | 15 |
| 17 | 16 mo | 15 |
| 18 | 17 mo | 15 |
| 19 | 18 mo | 18 |
| 20 | 19 mo | 18 |
| 21 | 20 mo | 18 |
| 22 | 21 mo | 18 |
| 23 | 22 mo | 18 |
| 24 | stop correcting | 24 |
When each CDC checklist applies, born 4 weeks early
Each corrected-age checklist starts four weeks after its plain age; from the second birthday, calendar age. Dates: the example baby below (born 18 February 2024, due 17 March 2024).
| CDC checklist | Compare from (age since birth) | Example baby: in use |
|---|---|---|
| 12-month | 12 months + 4 weeks (about 12.9 months) | 17 March 2025 to 16 June 2025 |
| 15-month | 15 months + 4 weeks (about 15.9 months) | 17 June 2025 to 16 September 2025 |
| 18-month | 18 months + 4 weeks (about 18.9 months) | 17 September 2025 to 17 February 2026 |
| 24-month | 24 months: correction stops | 18 February 2026 to 17 August 2026 |
| 30-month | 30 months (calendar age) | 18 August 2026 to 17 February 2027 |
| 36-month | 36 months (calendar age) | from 18 February 2027 |
Worked example: a baby born 4 weeks early
A made-up baby, born on 18 February 2024, four weeks early, with exact calendar dates.
-
Find the due date
18 February 2024 plus four weeks (twenty-eight days) is 17 March 2024.
-
Age since birth
On 18 May 2025 the baby is exactly fifteen months old by the calendar.
-
Corrected age
From 17 March 2024 to 18 May 2025 is fourteen months and one day: the corrected age.
-
Pick the checklist
Fourteen months and one 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 17 June 2025.
-
Stop correcting
18 February 2026 is the second birthday (corrected: twenty-three months and one day); from then on, calendar age only.
The first months after a birth at 36 weeks
For the first four weeks after birth the baby has not reached the due date, so the one-month row shows the due date still ahead. From the second month on the corrected age is positive and climbs one month at a time.
The four-week gap matters most early on: at six months it is fifteen percent of the baby's age, at twelve months eight percent, and at twenty-three months only four percent. Hence correction for the first two years only.
Corrected age at the AAP screening visits, 4 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 (18 November 2024 for the example baby): about eight months corrected, so an earlier CDC checklist (before 12 months).
- 18 months (18 August 2025): about seventeen months corrected, matching the 15-month checklist.
- 24 months (18 February 2026): correction stops, so the 24-month checklist by calendar age.
- 30 months (18 August 2026): no correction; the 30-month checklist.
When to stop correcting for 4 weeks early
The AAP advises corrected age for the first 2 years, so for a baby born four weeks early correction stops at 24 months chronological, when the corrected age is about twenty-three months; the comparison then jumps to the 24-month checklist.
So the 18-month list is used for about 5.1 months (from eighteen months and four weeks to the second birthday), not six.
Corrected age and real concerns
Being born four 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 4 weeks early?
Subtract four weeks (or count from the due date). At six months: about five months corrected; at twelve months: eleven months; at eighteen: seventeen months.
When does a baby born 4 weeks early reach the 12-month milestones?
From twelve months and four weeks after birth (about 12.9 months); then the 15-month checklist at about 15.9 months and the 18-month one at about 18.9 months.
When do you stop using corrected age for 4 weeks early?
At 24 months chronological (AAP: first two years). On the second birthday the corrected age is about twenty-three months; from then on, calendar age and the 24-month checklist.
What is 4 weeks early in weeks of pregnancy?
A baby born four weeks before a forty-week due date is born at thirty-six weeks of pregnancy. The gap of twenty-eight days, about 0.9 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.