Measure Height 
How it works
The calculator uses the WHO Child Growth Standards (2006), built from the WHO Multicentre Growth Reference Study, which followed healthy breastfed babies in Brazil, Ghana, India, Norway, Oman and the United States. Since 2010 the CDC has recommended these standards for every child under 2 in the US, so they are the charts most pediatricians plot on.
For each month of age the WHO publishes LMS values for length and weight. The calculator interpolates between months using the extra weeks you enter, converts the measurement to a z-score and then to a percentile. Weight uses the full skewed LMS curve, which matters because baby weight is not symmetric: the heavy end of the chart stretches further than the light end.
Worked examples
Computed with the same code as the calculator above.
| Baby | Length | Pct | Weight | Pct |
|---|---|---|---|---|
| Boy, 2 mo | 56 cm (22.0 in) | 11th | 5 kg (11 lb 0 oz) | 20th |
| Girl, 6 mo | 64 cm (25.2 in) | 22nd | 7.3 kg (16 lb 1 oz) | 50th |
| Boy, 12 mo | 73 cm (28.7 in) | 12th | 9.6 kg (21 lb 3 oz) | 48th |
| Girl, 15 mo | 73 cm (28.7 in) | 5th | 8.9 kg (19 lb 10 oz) | 27th |
| Girl, 18 mo | 80 cm (31.5 in) | 40th | 10 kg (22 lb 1 oz) | 43rd |
Look at the 12-month-old boy: 12th percentile for length, 48th for weight. On paper that reads as a stocky baby. In practice the first thing to check is the length, because a baby who curled his knees on the board loses length. Measured 2.5 cm longer, at 75.5 cm, he would sit on the 46th percentile, right in step with his weight.
Why baby length is so hard to measure
Weight comes from a scale and is reliable. Length depends on how straight the baby lies, and babies resist. At home it is worse:
- A tape along a wriggling baby follows the curve of the body and reads long.
- Marks on a sheet at head and heel move as soon as the baby does.
- A bent knee or a tilted head takes a centimetre or more off.
Clinics use a length board with a fixed headpiece and a sliding footpiece, and two people: one holds the head against the board, the other straightens the legs and slides the footpiece. If your home number disagrees with the clinic's, trust the clinic's.
When percentiles change
Between birth and 2 years many babies move up or down a band. Birth size reflects the pregnancy more than the genes, and the first two years are when a baby shifts toward their family's size. What pediatricians watch for is a fall across two major lines (for example from the 50th to below the 10th) or weight and length heading in opposite directions. From age 2, the height and weight percentile calculator takes over on the CDC charts. And from 2 the height predictor can project adult height.
Questions
What is a normal length percentile for a baby?
Anything between roughly the 3rd and 97th percentile is within the normal spread, and a baby who stays near the same percentile from visit to visit is growing as expected. A 10th percentile baby with 10th percentile parents is simply a small baby.
Why did my baby's percentile drop?
Shifts in the first two years are common. Babies born big to smaller parents often drift down, and small newborns of taller parents drift up, as they move toward their genetic size. Home length measurements are also unreliable. A sustained fall across two major lines, especially with weight falling too, is the pattern to raise with your pediatrician.
Why WHO charts and not CDC charts?
Since 2010 the CDC has recommended the WHO growth standards for children under 2 in the US. They describe how healthy breastfed babies grow under good conditions, rather than how a particular population happened to grow. From age 2 the CDC charts take over.
Should I correct for prematurity?
Yes. For a baby born early, pediatricians usually use corrected age (age from the due date, not the birth date) until about 2 years. Enter the corrected age to compare with term babies.
Is length the same as height?
Not quite. Under 2, babies are measured lying down, which is called length. Standing height reads a little shorter, which is why the WHO and CDC charts switch measurement at 24 months.
Source: WHO Child Growth Standards, length-for-age and weight-for-age LMS tables (WHO Multicentre Growth Reference Study Group, 2006). CDC recommendation: MMWR Recomm Rep 2010;59(RR-9). For curiosity and screening, not diagnosis.