Quick answer
Count complete calendar months from the birth day of the month. Month lengths vary, so month-based age and week-based age drift apart from the start.
How month age actually advances
A baby born on the 12th turns one month old on the 12th of the following month, whether that month has 28 days or 31. The count follows the day number, not an elapsed quantity of days, which is why two babies born a fortnight apart can have month anniversaries separated by a different number of days each time.
This is the same arithmetic used for adult age, just at a scale where the irregularity is visible. Over a year it averages out; over the first six months it does not, and a month-based figure can differ from a 30-day-based one by several days.
Use this guide with the right tool: Open the Baby Age in Months Calculator for a baby’s chronological age in calendar months and days. If the question shifts, compare it with the Age in Months Calculator or the Corrected Age Calculator.
The end-of-month problem
A baby born on the 31st has no monthly anniversary in February, April, June, September or November. Something has to give, and the standard treatment is to clamp to the last day of the shorter month — so a 31 January birth reaches one month on 28 or 29 February.
The alternative, rolling forward into the next month, also exists. Neither is wrong; what causes confusion is switching between them. Pick one and keep it, because a baby's month age is often recorded repeatedly over a short period and inconsistency shows up quickly.
Months versus weeks, again
Month age and week age are different measurements and will not agree. At 7 calendar months a baby is about 30 weeks, not 28. Anyone converting by multiplying months by four will be short by roughly two weeks at this point, and further out as the age grows.
Use whichever the form or conversation calls for and calculate it from the birth date. Forms that ask for age in months mean calendar months; developmental material that works in weeks means weeks. Converting between them is where the error enters.
Prematurity and month counts
For a baby born preterm, calendar-month age from the birth date is chronological. Corrected age subtracts the weeks born early — and because that subtraction is in weeks, it does not land neatly on a month boundary.
That is expected rather than a problem. A corrected age of "about 5 months" is a legitimate way to express it in conversation, provided the record also carries the precise figure and the label. Keeping the chronological month age alongside it means forms and appointments both have what they need.
Worked example
The example is there to show the shape of the working. Your own dates will behave the same way; the rule that governs them may not.
Common mistakes to avoid
- Multiplying weeks by seven and dividing by 30. This is not how calendar months work. Count months from the birth day of the month.
- Changing the end-of-month rule between records. Clamping and rolling both exist. Inconsistency makes a sequence of records irreproducible.
- Blending corrected and chronological month ages. Label each. They differ by however many weeks early the baby arrived.
Use the right calculator
These are the practical next step. Each opens with the calculator, states its method, and links onward when the question turns out to be a different one.
Frequently asked questions
What about a baby born on the 31st?
Clamp to the last day of any shorter month — so one month from 31 January is 28 or 29 February. Record which convention you used.
Why does my app show a different number of months?
Usually an end-of-month convention, or a tool converting from days rather than counting calendar months. Compare the methods.
Which unit do forms want?
Forms asking for age in months mean calendar months. Give the calendar figure rather than a converted week count.