Adjusted age calculator for preemies

What is adjusted (corrected) age?

For a baby born early, it is the age the baby would be if born on the due date: the actual age in weeks minus the number of weeks the baby was born early (American Academy of Pediatrics).

How do I calculate it?

Take the actual age and subtract how many weeks early the baby came, counting from 40 weeks. The AAP example: a baby born at 32 weeks is 8 weeks early, so at 16 weeks (4 months) old the corrected age is 2 months.

How long is adjusted age used?

The AAP says to use the corrected age during the first 2 years when checking growth and development.

Which age counts for vaccines?

Vaccines usually follow the actual age from the birth date, not the adjusted age. Your pediatrician will guide you.

What if my baby was born after 40 weeks?

Then there is nothing to subtract; the adjusted age is the same as the actual age.

Is the adjustment done in weeks or in months?

Weeks are more exact. A baby born at 30 weeks is 10 weeks early. At 20 weeks old, the adjusted age is 20 − 10 = 10 weeks. Converting to months is only approximate, because a month is a little over four weeks.

Is every preemie corrected, even one born at 36 weeks?

Not always. A baby born only a week or two early would have an adjusted age within days of the actual age. The ASQ-3 developmental screening guide, for example, adjusts for children born at least 3 weeks early and stops at 24 months of actual age. Ask your pediatrician which rule applies to your child.

Does the due date or the weeks at birth give the correction?

They give the same answer when the due date is a standard 40 weeks after the start of pregnancy. The calculator takes the weeks at birth and counts the gap to 40 weeks. If your doctor revised the due date, use the weeks your doctor recorded at birth.

How the calculation works

A full-term pregnancy is counted as 40 weeks. The calculator subtracts the weeks at birth from 40 to find how early the baby arrived, then subtracts that gap from the actual age counted from the birth date. The result is the age the baby would have if it had been born on the due date. The baby's real age (the chronological age) does not change; the adjusted age is only a reference point for judging growth and development against typical milestones.

Worked example

A baby is born at 30 weeks. That is 40 − 30 = 10 weeks early. When the baby is 20 weeks old by the calendar, the adjusted age is 20 − 10 = 10 weeks, about two months. While the calendar age is under 10 weeks, the adjusted age is still before the due date. The same baby reaches an adjusted age of 12 weeks (3 months) at a calendar age of 22 weeks.

Common mistakes

• Using the adjusted age for everything. Vaccines and similar schedules follow the actual age from the birth date. The adjusted age is used for growth charts and developmental milestones.
• Entering weeks of pregnancy at birth incorrectly. Use completed weeks of gestation at birth, as recorded in the hospital papers, not the number of weeks until the due date today.
• Stopping or continuing too early or late. There is a standard cutoff in the first two years, but your pediatrician may use a different one for babies born very early. Follow the clinician's advice for your child.

Sources and related calculators

Method: the American Academy of Pediatrics description of corrected age (subtract the weeks born early from the actual age, used during the first 2 years), as quoted in the answers above. The 3-week and 24-month rule of thumb comes from the ASQ-3 user materials on screening premature children. Related: due date calculator, baby growth percentile calculator and age calculator.

For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.