At 6 months, using the WHO weight-for-age standard:
| Percentile | Boys | Girls |
|---|---|---|
| 3rd | 14.1 lb (6.4 kg) | 12.8 lb (5.8 kg) |
| 15th | 15.4 lb (7.0 kg) | 14.1 lb (6.4 kg) |
| 50th | 17.4 lb (7.9 kg) | 15.9 lb (7.2 kg) |
| 85th | 19.6 lb (8.9 kg) | 18.1 lb (8.2 kg) |
| 97th | 21.4 lb (9.7 kg) | 20.1 lb (9.1 kg) |
A 6-month-old boy at 15.4 lb is on the 15th percentile. That is a completely normal weight, and the sentence "he's only on the 15th" contains a word that does not belong there.
What a percentile is
The 15th percentile means 15 out of 100 healthy babies of that age and sex weigh less. It is a position in a distribution of healthy children, not a score.
Someone has to be on the 15th percentile. Someone has to be on the 3rd. The curve was built by measuring thousands of healthy, well-fed babies, and its whole width describes normal.
What paediatricians actually read
Not the number. The direction.
A baby who tracks along the 15th percentile from birth to a year is growing exactly as expected. A baby who was on the 75th at two months and is on the 25th at six months has crossed two major lines downward, and that is worth a conversation — even though 25th is a perfectly ordinary percentile to be on.
The same is true upward. Crossing rapidly up through lines is also a pattern rather than an achievement.
Crossing one line is usually nothing, especially in the first few months when babies commonly settle onto their genetic curve after a birth weight that reflected the pregnancy rather than the child.
Crossing two lines, in either direction, sustained over more than one visit is the point at which the trend gets looked at.
Which chart, and why it matters
Two different charts are in common use and they are not interchangeable.
WHO standards (0–24 months) describe how healthy, predominantly breastfed children should grow under good conditions. It is a prescriptive standard.
CDC charts (2 years and up) describe how American children did grow in a reference population. It is a descriptive reference.
Because of that difference, breastfed babies commonly appear to "drop percentiles" after about three months on a CDC chart while tracking perfectly on a WHO one. The CDC itself recommends WHO charts for under-twos in the United States for exactly this reason.
If a chart at a clinic is not labelled, it is worth asking which one it is before reading anything into a shift.
Weight alone is the least useful measurement
Three numbers are plotted, not one:
Length-for-age and weight-for-age together give weight-for-length, which is the one that says whether a baby is proportionate. A baby on the 10th for both is small and proportionate. A baby on the 75th for length and the 10th for weight is a different picture.
Head circumference tracks brain growth and is the measurement most likely to be quietly checked and least likely to be mentioned.
A single weight, plotted once, tells you almost nothing. That is why the chart has a line on it and not a dot.
Weighing at home
Same scale, same time of day, undressed, no nappy. Weigh weekly at most — daily weights vary by several ounces with feeds and nappies, and the variation swamps the trend.
Feeding volumes sit underneath all of this: see how many ounces at each age, and note that clothing size follows length rather than weight.
Sources: World Health Organization Child Growth Standards, weight-for-age (0–24 months); U.S. Centers for Disease Control and Prevention, Growth Chart Recommendations for Children Under 2.
Reference tables reproduced for general information. Any concern about your child's growth belongs with your paediatrician, who has the full history this page does not.