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Child Growth Lab

Pediatric growth calculators based on CDC and WHO growth standards.

Preterm timeline arithmetic

Corrected Age Calculator for Preterm Babies

Build a corrected-age timeline for a baby born before 37+0 weeks using the actual birth date and either the original estimated due date or a gestational age from 20+0 through 36+6 weeks recorded at birth. Choose an assessment date or a future planning date.

The result explains chronological age, the born-early adjustment, the 40-week reference date, corrected age, and postmenstrual age. It does not diagnose development, predict milestones, choose a growth chart, or change a birthday or vaccine timeline.

Before you calculate

  • Use the actual date of birth.
  • Use the original estimated due date from the pregnancy record when available.
  • Otherwise use the recorded gestational age at birth: 20–36 completed weeks plus 0–6 days.
  • Do not estimate gestational age from current size, weight, milestones, appearance, or discharge date.
  • Dates are processed as YYYY-MM-DD calendar dates in whole days; time of day and timezone are not used.
  • Select an assessment date or a future planning date. The result is date arithmetic only.

Review the four age terms before using the result. If records disagree, ask the clinical team which pregnancy-dating record to use.

Changing methods clears the due-date or gestational-age entry so the two records are not mixed.

A future date is a planning date, not the baby's current age and not a prediction.

Use the due date originally assigned during pregnancy, not a date reconstructed after birth. For this calculator, it must be 22–140 days after the birth date.

Choose the input record you have

Original estimated due date

Use the due date assigned during pregnancy. The date becomes the 40-week reference date, and the accepted 22–140-day interval from birth to that date is the born-early adjustment.

Recorded gestational age at birth

Use a recorded value from 20+0 through 36+6 weeks, entered as completed weeks plus 0–6 days. The calculator derives the adjustment and 40-week reference date.

For consistent records, the two methods produce the same timeline. If they conflict, this page cannot decide which pregnancy-dating record is clinically authoritative.

The 20+0 lower limit is a technical data-entry guard for this calculator. It is not a survival, prognosis, resuscitation, treatment, or medical-decision boundary; earlier or uncertain records must be handled by the clinical team.

The four age terms in this result

Chronological age
Time since the actual date of birth.
Corrected age
Chronological age minus the born-early adjustment after the 40-week reference date.
Gestational age at birth
How far the pregnancy had progressed when birth occurred.
Postmenstrual age
Gestational age at birth plus chronological age.

For deeper education about these terms, read What Is Corrected Age for Premature Babies?.

The three timeline states

  • Before the reference date: postmenstrual age and time remaining are shown; no negative corrected age is displayed.
  • At the reference date: corrected age is 0 days and postmenstrual age is 40 weeks.
  • After the reference date: corrected age, chronological age, the adjustment, and reference date remain visible.

Current assessment versus future planning

A current or past date describes the timeline on that assessment date. A future target date is allowed for planning and is labeled clearly in the result.

A future calculation is not the baby's age today and does not predict development, growth, medical stability, discharge, or milestone timing.

Growth, development, and chart choice

Preterm means birth before 37+0 weeks. AAP parent guidance and NICE commonly use correction through about 2 years for early developmental context, while current AAP growth guidance may use corrected age through 3 years. These are purpose-specific practices, not one universal calculator cutoff.

This calculator does not decide whether correction should be used or select Fenton, Olsen, WHO, CDC, or another chart. The Baby Growth Percentile Calculator uses the actual birth date; never enter a fictional adjusted birth date. Confirm the chart and age basis with an appropriate healthcare professional.

Vaccines use chronological age

CDC guidance says most clinically stable preterm infants are vaccinated at the same chronological age and on the same schedule as full-term infants. Specific hepatitis B circumstances can depend on birth weight and maternal hepatitis B status.

Do not shift vaccine dates with corrected-age arithmetic. This page does not calculate vaccine eligibility or replace the current immunization schedule.

Worked examples

After the reference date: a baby born January 1, 2024, at 32+0 weeks has a February 26, 2024, reference date. On July 1, chronological age is 6 months, corrected age is 4 months, 5 days, and the baby was born early by 8 weeks.

Before the reference date: on February 20, 2024, the same baby has a postmenstrual age of 39 weeks, 1 day and is 6 days before the reference date. No negative corrected age is shown.

Equivalent inputs: the original due date of February 26, 2024, and recorded gestational age of 32+0 produce the same timeline.

How the calculation works

Born-early adjustment = 40 weeks − gestational age at birth.

Corrected age = chronological age − born-early adjustment.

Postmenstrual age = gestational age at birth + chronological age.

The alternative due-date method uses the interval between actual birth and the submitted original due date. It can derive an arithmetic gestational-age equivalent, but does not independently verify a recorded gestational age. All inputs are YYYY-MM-DD calendar dates, and the calculation uses integer-day arithmetic with no birth, assessment, or due time and no timezone. Exact calendar-day arithmetic and the 280-day reference remain unchanged. Read the Methodology for technical details.

Limitations

  • The page accepts recorded gestational age from 20+0 through 36+6 weeks, or the equivalent 22–140-day due-date adjustment, and cannot validate pregnancy dating.
  • The 20+0 lower limit is a technical data-entry guard, not a survival, prognosis, resuscitation, treatment, or medical-decision boundary. Earlier or uncertain records require the clinical team.
  • It uses chronological age and does not change the actual birth date.
  • It does not decide when correction should be used, diagnose development, predict catch-up or milestones, or select a preterm chart.
  • It does not alter legal, school, or vaccination age.
  • A future result is planning arithmetic, not a forecast.
  • Dates are date-only YYYY-MM-DD values. Time of day and timezone are not used, so very close to the 40-week reference time a clinical hour-level PMA state may differ. This page is not for hour-level newborn intensive care, inpatient, or treatment timing.
  • Conflicting due-date and gestational-age records require review by the clinical team.

Official sources

Sources rechecked 2026-08-03.

Helpful context

Frequently asked questions

Which input method should I use?

Use the original estimated due date from the pregnancy record when it is available. Otherwise, use the gestational age at birth recorded as completed weeks plus 0–6 additional days. Do not combine or estimate the two records.

What counts as the original estimated due date?

Use the due date assigned during pregnancy and retained in the obstetric, birth, or neonatal record. If records conflict, the calculator cannot decide which date is authoritative; ask the clinical team.

Can I calculate with only gestational age at birth?

Yes. Enter a recorded value from 20+0 through 36+6 weeks as completed weeks and 0–6 additional days. The 20+0 lower limit is a technical data-entry guard for this calculator, not a survival, prognosis, resuscitation, treatment, or medical-decision boundary. Earlier or uncertain records require the clinical team.

Why does corrected age use a 40-week reference?

Forty weeks, or 280 days, is the arithmetic reference used by the corrected-age formula. It is distinct from ACOG’s obstetric full-term category of 39+0 through 40+6 weeks.

What happens before the 40-week reference date?

The result shows postmenstrual age and time remaining until the reference date. Corrected age starts at 0 days on that date, so the parent-facing result does not display a negative corrected age.

Can I select a future target date?

Yes. A future date produces a planning timeline for that selected date. All dates are treated as YYYY-MM-DD calendar dates using whole-day arithmetic; times and timezones are not used. Very close to the 40-week reference time, a clinical record with exact times may show a different hour-level PMA state. This calculator is not for hour-level newborn intensive care, inpatient, or treatment timing.

Does corrected age change a birthday or vaccine schedule?

No. It does not change the actual birthday, legal age, school age, or routine vaccine timeline. CDC says most clinically stable preterm infants are vaccinated by chronological age, with specific exceptions such as some hepatitis B circumstances.

Does this calculator choose a growth chart?

No. Corrected-age arithmetic does not automatically select Fenton, Olsen, WHO, CDC, or another chart. Use the actual birth date in the baby calculator and confirm the chart and age basis with an appropriate healthcare professional.

How long should corrected age be used?

There is no universal cutoff for every purpose. Developmental guidance commonly uses correction through about 2 years, while current AAP growth guidance may use corrected age through 3 years. Follow the specific chart, assessment, and healthcare team.

Does this site save the information?

Calculator inputs are processed in your browser and are not saved as a child health record or sent to an external calculation service.