Understanding growth charts

What’s a growth chart?

A growth chart is a visual aid to help parents and  health-care providers understand how a baby is growing compared with other similar babies. The charts are created using measurements of many healthy babies at different ages. The babies are then divided into groups called percentiles. To understand percentiles, consider 100 babies. If a baby is the tenth-lightest, the baby is on the 10th percentile for weight. Generally, as the baby grows, the baby should stay close to the same percentile. A group of babies born with a similar weight after pregnancies of a similar length usually grow at roughly the same rate. In other words, they stay with their group. Not keeping pace with the group may mean something is wrong.

A) Describing a growth chart

A growth chart is a visual aid based on the measurement of healthy babies at different ages. There are charts for boys and for girls. Babies’ growth can be measured by the change in:

  • Weight.
  • Length.
  • The distance around the head (head circumference).

For breastfeeding purposes, weight is the most useful measurement. A growth chart measuring weight is called a weight-for-age growth chart. We do not use length-for-age or head circumference-for-age charts in this website.

Growth charts can be a little confusing. The following is one way to explain them.

Babies of a similar age, gender, and weight at birth will grow at roughly the same rate. In other words, they stay with their group. For example, a healthy baby who is born weighing less than most other babies generally ends up being a smaller adult.

How much a healthy baby weighs at birth and how the baby grows is often determined by the height of the family. Taller families will tend to have babies who are heavier at birth and will weigh more in the following weeks and months than babies born to families who are shorter.

B) Describing percentiles

Birth percentiles

Growth charts for the weight of breastfed babies are created from the weights of many healthy, breastfed babies at different ages. These babies are divided into groups called percentiles.

Think of taking 100 typical newborn baby girls, all healthy and at 40 weeks in the womb (gestation), and weighing them and then weighing your own newborn baby. If your baby weighs the same as:

  • The third smallest, your baby is on the third percentile.
  • The baby exactly in the middle, your baby is on the 50th percentile.
  • The third heaviest, your baby is on the 98th percentile.

Babies can be weighed at different ages and these different weights can be connected by lines called growth curves. Taken together, these become growth charts.

C) Using percentiles

Healthy babies will follow a growth curve, with some weights slightly above or below their curve.

In general, a baby:

  • Should stay roughly on the same percentile curve.
  • Can move to the next lower curve without causing concern as long as the baby is showing the other signs of taking in enough milk.
  • Can move to the next higher curve without causing concern as long as the baby is not being over-fed.
  • Should not move up or down more than one curve.  
  • Should not cross above the 97th percentile or below the 15th percentile curve for weight unless the parents are very tall or very short.

You and your health-care providers can use this information to ensure your baby is healthy and getting the right amount of milk. Not keeping pace with the rest of the group may mean something is wrong. Babies who are not taking in enough milk will gain too slowly and usually show other signs. Babies can also get too much milk if they are inappropriately supplemented with milk and will grow too quickly.

Growth charts should not be the only tool used to assess a baby or make changes in how the baby is fed. Rather seeing weight gain that is too fast or slow is a reason for reviewing the baby’s health and breastfeeding.