Corrected age for premature babies

Chronological age is the age from the birth date. It is the age on the birth certificate.

Corrected age is the chronological age minus the weeks your baby was born early. It is the age your baby would be if born on the due date.

A baby born two months early spent those two months growing outside the womb. The brain and body follow their own timetable. At 6 months after birth, this baby is often doing what a 4-month-old does. That is normal.

You need two numbers. The week of pregnancy at birth, and your baby’s age today.

1. Take 40 weeks, a full-term pregnancy.
2. Subtract the week your baby was born. This gives the number of weeks born early.
3. Subtract those weeks from your baby’s age today.

Your baby was born at 32 weeks.

• 40 minus 32 is 8. Your baby was born 8 weeks early. That is about 2 months.
• Today your baby is 6 months old.
• 6 months minus 2 months is 4 months.

Your baby’s corrected age is 4 months. Compare your baby with 4-month-olds.

Another example. A baby born at 28 weeks was 12 weeks early, about 3 months. At 9 months after birth, the corrected age is 6 months.

For growth and milestones, use corrected age. For vaccines, use the birth date.

In the neonatal unit, the team counts in weeks of pregnancy. A baby born at 33 weeks who is 4 weeks old is said to be at 37 weeks. This tells the team how mature your baby is. Many babies go home around the time they reach their due date.

Use corrected age when you:

• check milestones, such as smiling, sitting, crawling, first words and walking
• read your baby’s weight, length and head size on the growth chart
• think about when to start solid foods, together with your doctor

Your doctor plots your baby’s growth by corrected age too. Ask to see the chart.

Most premature babies catch up with other children of the same birth age. Doctors use corrected age until about 2 years. After that, the gap usually matters very little. A baby born very early may need corrected age a little longer.

Vaccines are the exception. Premature babies get their vaccines at the same chronological age as other babies. Do not wait for the corrected age. Do not delay them.

A baby born two months early still gets the 2-month vaccines two months after birth. Premature babies are more fragile against infections, so on-time protection matters even more. Some babies need a different timing for one vaccine, depending on weight or health. Your doctor will tell you if that applies. Read our guide to vaccines.

Call your doctor if your baby:

• has lost a skill they already had, such as smiling, babbling or sitting
• is behind on milestones even by corrected age
• does not react to loud sounds or does not follow your face with their eyes
• seems very stiff or very floppy
• grows away from their curve on the growth chart

You can also call if you are unsure. Our guide to milestones lists what to look for at each age.