Chronological Age Calculator

gestational age at birth = 40 weeks minus the days between the birth date and the due date, divided by 7; adjusted age = the chronological age minus 7 days for each of those weeks, and the correction is never subtracted below zero

birthDate

The date the baby was actually born. A date in the future is rejected, because a baby who has not arrived has no age yet

dueDate

The estimated due date. The page assumes 40 weeks of gestation on that day, which is the anchor the whole calculation hangs on

chronologicalDays

Days from the birth date to today, counting the leap days in between

chronologicalWeeks

The same age in weeks and a fraction of a week, to one decimal place

chronologicalMonths

Whole calendar months from birth; months that have not finished do not count

adjustedDays

The chronological age with the weeks of prematurity taken off, so it measures from the due date instead of the birth date

adjustedWeeks

The adjusted age in weeks and a fraction of a week

adjustedMonths

Whole calendar months of the adjusted age, counted from the due date or the birth date, whichever is later

gestationalWeeks

How far along the pregnancy was at birth: 40 on the due date, lower for a baby born early, higher for one born late

weeksPremature

The correction actually applied. It is capped at zero, so a baby born after the due date gets none

Use it for any baby who arrived before the due date, and use it whenever two people are comparing dates: the paediatrician asking how old the baby is, the parents checking a milestone chart, and the immunisation schedule are each working from a different one of the two ages. It also settles the question of when a premature baby's growth and development should be judged against full-term norms, which is the due date, not the birth date. When the due date and the birth date are the same day the two ages come out identical, so the page degrades cleanly for a full-term baby rather than needing a second tool.

1. Born at 35 weeks: birth 1 December 2025, due 5 January 2026

   1. The due date is 35 days after the birth date, and 35 divided by seven is 5, so the baby was 5 weeks early
   2. 40 weeks minus 5 is 35 weeks of gestation at birth
   3. 1 December 2025 to 15 January 2026 is 45 days, which is the chronological age
   4. The correction is 5 weeks, or 35 days, so the adjusted age is 45 minus 35, which is 10 days
   5. 45 divided by seven is 6.4 weeks; 10 divided by seven is 1.4 weeks

   Late preterm is the largest band and the easiest to dismiss, and this example is why it should not be: a baby read as six weeks old is developmentally ten days old. Read the adjusted age when checking milestones, and the chronological age when the vaccination schedule asks.

2. Born at 26.9 weeks: birth 10 October 2025, due 10 January 2026

   1. The due date is 92 days after the birth date, and 92 divided by seven is 13.1 weeks
   2. 40 minus 13.1 is 26.9 weeks of gestation at birth, which is the extremely preterm band
   3. 10 October 2025 to 15 January 2026 is 97 days, which is the chronological age
   4. The corrected age is 97 minus 92, which is 5 days
   5. 97 divided by seven is 13.9 weeks; 5 divided by seven is 0.7 weeks

   This is the case the page exists for: three months old by the birth certificate and five days old by the due date. Every milestone chart read as though the first figure were the real age would be about thirteen weeks wrong, which at this age is the difference between a newborn and a baby who tracks a face.

Today is taken as the UTC calendar day, so in time zones ahead of UTC the ages can be a day behind the date on your wall. The two dates must be plausible together: a pair implying a gestational age under 20 weeks or over 45 weeks is rejected rather than reported as an extreme band. The correction only ever removes time, so a baby born after the due date gets an adjusted age equal to the chronological one. The page assumes a single baby and a stated 40-week due date, and knows nothing about twins, IVF dating or an ultrasound-adjusted due date.

What is the difference between chronological and adjusted age?

Chronological age counts from the day the baby was born; adjusted age counts from the due date. A baby born five weeks early is six weeks old chronologically and one week old adjusted. Both are correct, they answer different questions, and this page shows them next to each other rather than making you choose.

Which age should I use for milestones?

The adjusted age, for a baby born before 37 weeks, and usually until about age two. Growth charts, motor milestones and language milestones are all built from full-term babies, so a premature baby measured by the birth date looks behind by exactly the number of weeks they were early. The chronological age is the one to give at the clinic desk.

How do I work out the gestational age at birth?

Count the days from the birth date to the due date, divide by seven, and subtract that from 40 weeks. A baby born 35 days before the due date is 5 weeks early, so 35 weeks of gestation. This treats the due date as exactly 40 weeks, which is the convention the figure is normally quoted under.

What if the baby was born after the due date?

Then there is nothing to correct and the two ages are identical. The page caps the correction at zero instead of subtracting a negative, because removing time would produce an adjusted age larger than the age the baby actually is. The lateness is still visible in the gestational age row, which reads above 40 weeks, and the badge says post-term.

Why was my pair of dates rejected?

Because the two dates imply a gestational age outside 20 to 45 weeks, which no pregnancy reaches. That almost always means the birth date and the due date went into the wrong boxes, or one of them has the wrong year. The band would otherwise be computed and shown as an extreme one, which is a wrong answer wearing a confident face.

• Preterm birth — World Health Organization
• ICD-10 Version: 2019 — World Health Organization