How to use

This app counts percentiles from the LMS growth reference published by Korea's National Health Insurance Service and counts months by the same rule as the Korean government's age calculator. The numbers are exact, but no label is attached to them. Below is what it does and what it deliberately leaves alone.

What a percentile is

A percentile says roughly where your baby stands among 100 babies of the same sex and month lined up from the smallest. Percentile 50 is the middle, percentile 10 is about the tenth, and percentile 90 is about the ninetieth.

A growth chart is a distribution built from measuring many babies. There are babies at every percentile. A small number or a large one does not by itself mean any particular state. That is why this app attaches no verdict beneath the number.

The z score is the same thing on a different scale. 0 is the middle, 1 is roughly percentile 84 and -1 roughly percentile 16. When the percentile is below 1 or above 99, the z score is the more detailed of the two.

How it is computed

The reference is the LMS growth chart published by Korea's National Health Insurance Service on the public data portal. Weight and height cover 0 to 84 months, head circumference 0 to 81 months and body mass index 24 to 84 months, with separate rows for boys and girls.

Each month's row carries L, M and S. The measured value is turned into z with those three values, and z into a percentile with the cumulative bell curve. The formula is written out on the reference page.

The percentile is rounded down to a whole number by the same rule as the NHIS percentile table. Below that table's 1 it is written as below 1, and above 99 as 99. For anything beyond, look at the z score.

See the reference tables

How months are counted

Months are completed months. Count the months from the birth date to the measuring date, and subtract one if the day of the month is earlier than the birthday's day. This rule was checked against 24 samples sent to the Korean Ministry of Government Legislation's age calculator.

So a baby born on 31 January is still 0 months on 28 February and turns 1 month on 1 March. A baby born on 29 February turns one on 1 March in a common year.

The growth chart treats the month as a whole number. Six months and five days and six months and twenty-nine days are both compared with the same six-month row. The percentile can jump a little around the day the month changes; the baby did not change, the row being compared did.

Open the months calculator

Corrected age for preterm babies

A baby born before the due date has a second age counted as if born on the due date. That is the corrected age. Enter the due date and this app shows that age alongside, and computes a second set of percentiles at that age.

If the due date is on or before the birth date, no correction is applied. If the measuring date is still before the due date, the corrected age is reported as not yet available. Negative months are never produced.

Up to what age corrected age should be used is not decided by this app. The examining doctor decides for that baby. Here the two results are simply shown side by side.

Rules specific to this app

The calculation runs inside this browser when you press Calculate. What you enter is not sent to a server, not kept in browser storage, and not placed in the address bar. Reload the page and the fields are empty again. A share feature was deliberately left out.

Leave the measuring date empty and today is used. Only the measurements you enter are counted. Body mass index appears only when both weight and height are given and the age is 24 months or more. Outside the table's range the app says so and stops.

No verdict is attached beneath a number. Growth is not predicted, next month's value is not guessed, and no notifications are sent. Vaccination schedules and check-up timings are linked to their original pages rather than copied.

Open the percentile calculator

Where people get stuck

A home scale and a clinic scale differ. Clothes, a nappy, before or after a feed and the time of day move the reading by a few hundred grams, and that alone can shift the percentile by more than ten places.

Height measured lying down differs from height measured standing. The table does not separate the two, so compare values measured the same way.

A single number does not show a trend. A baby's growth is read as movement over time, and that reading belongs to the doctor at the check-up. This app does not take that place.

Sources