Days, weeks and months do not line up
A baby's age gets quoted in three different units by three different sources, and they are not convertible by any simple factor. Days are unambiguous. Weeks are days divided by seven, so a week boundary is exact. Months are calendar months counted from the day of the month the baby was born, so a month is 28, 29, 30 or 31 days depending on which one it is.
The practical consequence is that eight weeks is not two months. Eight weeks is 56 days; two months is somewhere between 59 and 62. The gap is small at first and grows: at twelve months the child is about 52 weeks and one or two days old, not 52 weeks exactly. Feeding and sleep material tends to be written in weeks for the newborn period and months afterwards, and growth charts are usually plotted in months, so it is worth knowing which unit a figure is in before acting on it.
Day zero or day one
Two counting conventions are in circulation. Clinically, the day of birth is day 0, so a baby born on Monday is one day old on Tuesday and seven days old on the following Monday. In everyday speech people often count the day of birth as day one, which puts every figure one higher. This page shows both and labels them, because a mismatch of one day is exactly the kind of thing that goes unnoticed and then quietly disagrees with a chart.
Month ends
A baby born on the 31st has no 31st to turn one month old on in the following month. The convention used here is to clamp to the last day the month has, so a January 31 birth date gives a one-month date of February 28 in a common year and February 29 in a leap year, then March 31, then April 30. The alternative — rolling into the first of the next month — would make the baby a month old a day later than the calendar date suggests and would put the month count out of step with the date of the month for the rest of the year. Clamping keeps them aligned.
Gestational age, corrected age and postmenstrual age
Three terms, frequently confused:
| Term | What it measures | Used for |
|---|---|---|
| Gestational age at birth | Completed weeks of pregnancy at delivery | Classifying the birth as preterm, term or post-term |
| Chronological age | Time since the date of birth | Immunisation timing, the age everyone says out loud |
| Corrected age | Time since the due date | Developmental expectations and growth plotting for a preterm baby |
| Postmenstrual age | Gestational age at birth plus time since | The neonatal period, while still in hospital care |
Corrected age exists because a baby born eight weeks early has had eight fewer weeks to develop than a baby of the same chronological age born at term. Judged on the calendar the preterm baby looks behind; judged from the due date, usually not. Correction is applied for babies born before 37 weeks, and it is commonly used until around two years, by which point the difference has generally closed. Immunisations are a deliberate exception and are normally scheduled from the actual birth date rather than the corrected one. Where and when to stop correcting is a clinical judgement, not a rule this page can settle.
The gestational age here comes from the due date on the standard 280-day convention: a birth 14 days before the due date is taken as 38 weeks 0 days. If the clinical team recorded a different gestational age at delivery — dating scans can shift it — enter that figure directly instead and the due date will be derived from it.
What this page will not tell you
Whether anything is on track. There is no milestone table here and no interpretation of any kind, because ranges for normal development are wide, an age in weeks says nothing on its own, and a page that pairs a number with a list of expected skills invites exactly the comparison it should not. If you have a concern about growth, feeding or development, take it to the clinician who has met your baby.
Questions people ask
Why does another calculator give a different number of months?
Almost always because it divides days by 30 or by 30.44 instead of counting calendar months from the birth date. At 200 days the two methods differ by several days, and the gap widens. Clinical practice counts calendar months, which is what this page does.
Should I use corrected age or actual age when someone asks how old my baby is?
In conversation, the actual age. Corrected age is a tool for reading growth charts and developmental expectations for a baby born preterm, not a replacement for the age on the birth certificate. Which one applies in a given clinical context is for the paediatric team to say.
The baby was born a few days early. Does that need correcting?
Correction is applied for births before 37 weeks. A baby born at 38 or 39 weeks is term, and the calculator says so rather than applying a correction. Whether a borderline case is handled differently is a clinical decision.
I do not know the due date. Can I still get corrected age?
Yes, if you know the gestational age recorded at birth. Switch the gestational age field to entering it directly, put in the weeks and days, and the due date is derived from that instead. If you know neither, only the chronological age is available.
Does it handle a February 29 birth date?
Yes. Yearly anniversaries clamp to February 28 in common years rather than rolling to March 1, and February 29 in a leap year is accepted as a valid date while February 29 in a common year is rejected outright rather than being silently shifted.