Understanding Baby Weight Percentiles

At every well-child visit, your pediatrician weighs your baby and plots the number on a growth chart. The resulting percentile can feel like a grade, but it is not. Growth percentiles are a tracking tool, not a judgment, and understanding what they actually mean takes the anxiety out of those numbers. A baby at the 15th percentile can be just as healthy as one at the 85th, provided their growth follows a consistent pattern.

This guide explains what percentiles represent, how to read growth charts, the differences between WHO and CDC charts, and when a change in percentile actually warrants a conversation with your healthcare provider.

What Are Weight Percentiles?

A weight percentile tells you how your baby's weight compares to other babies of the same age and sex. If your 6-month-old daughter is at the 40th percentile for weight, it means that 40% of girls her age weigh less than she does, and 60% weigh more. The percentile is derived from large population studies that measured thousands of healthy children at regular intervals to establish what typical growth looks like.

Growth charts display smooth curves representing the 3rd, 5th, 10th, 25th, 50th, 75th, 90th, 95th, and 97th percentiles. Your baby's weight is plotted as a point on the chart at each visit, and over time these points create a line that ideally follows one of the standard percentile curves. The specific percentile your baby tracks along matters far less than whether they follow a consistent trajectory.

Percentile vs. Percentage

A common source of confusion is the difference between percentile and percentage. A baby at the 30th percentile does not weigh 30% of what they should weigh. It means they weigh more than 30% of babies their age. The actual weight difference between percentiles is often small. For example, the difference between the 25th and 50th percentile weight for a 6-month-old boy is only about 1 to 1.5 pounds. Percentiles describe rank order within a population, not a proportion of some ideal weight.

WHO vs. CDC Growth Charts

Two main sets of growth charts are used in the United States, and understanding the difference helps you interpret your baby's numbers correctly.

The World Health Organization charts, published in 2006, are based on data from approximately 8,500 children across six countries who were raised under conditions associated with optimal growth: breastfed for at least 12 months, introduced to complementary foods after 4 months, non-smoking environments, and adequate healthcare. These charts describe how children should grow under ideal conditions and are recommended by the CDC for children from birth to 24 months.

The CDC growth charts, last updated in 2000, are based on a representative sample of American children from national health surveys. They describe how American children actually grew, including a mix of breastfed and formula-fed infants. The CDC recommends using their charts for children aged 2 to 20 years, while the WHO charts are preferred for children under 2.

The practical difference is that breastfed babies may appear to fall on the CDC charts after 4 months because those charts included a large proportion of formula-fed infants who tend to gain weight faster in the second half of the first year. Using the WHO charts for infants provides a more accurate representation of healthy breastfed growth.

Average Baby Weight by Age

The following table shows approximate 50th percentile weights for boys and girls from birth through 24 months based on WHO growth standards. Remember that the 50th percentile is the median, not a target.

Notice the wide range between the 5th and 95th percentiles at every age. At 12 months, the difference between the lightest and heaviest babies in the normal range spans about 10 pounds. This enormous variation is completely normal and reflects the natural diversity in human body size.

How to Read a Percentile Chart

Reading a growth chart involves three steps. First, find your baby's age on the horizontal x-axis. Second, find their weight on the vertical y-axis. Third, plot the point where these two values intersect and see which percentile curve it falls closest to.

The curves on the chart represent the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles. If your baby's point falls between the 25th and 50th curve lines, their percentile is somewhere in that range. Over multiple visits, the collection of plotted points should form a line that roughly parallels one of the standard curves. This consistent tracking pattern is the most important thing to look for.

A healthy growth chart shows a line that follows the general shape of the printed curves, even if it wobbles slightly between visits. Some variation is normal because a baby may be weighed right before or after a feeding, may have recently been ill, or may be experiencing a growth spurt. The overall trend across many data points matters more than any single measurement.

Breastfed vs. Formula-Fed Growth Patterns

Breastfed and formula-fed infants follow recognizably different growth trajectories. In the first 3 to 4 months, breastfed babies often gain weight faster than formula-fed babies. After 4 months, this pattern reverses: formula-fed babies tend to gain weight more rapidly while breastfed babies' weight gain slows.

By 12 months, breastfed babies are typically leaner than their formula-fed counterparts. This difference is reflected in the WHO growth charts, which were developed primarily from breastfed babies and show slightly lower weight curves after 6 months compared to the CDC charts. If your breastfed baby appears to drop percentiles on a CDC chart after 4 months, switching to the WHO chart may show they are actually growing normally for a breastfed infant.

Neither growth pattern is inherently better. The key is using the appropriate chart for your baby's age and feeding method, and evaluating growth in the context of their overall health and development.

Real-World Percentile Examples

Example 1: 50th Percentile Baby

Oliver was born at 39 weeks weighing 7 lbs 8 oz, placing him at the 50th percentile for weight. At his well-child visits, his weight tracked as follows:

• 1 month: 10.0 lbs (50th percentile)
• 4 months: 15.0 lbs (48th percentile)
• 6 months: 17.5 lbs (50th percentile)
• 9 months: 20.2 lbs (52nd percentile)
• 12 months: 21.5 lbs (50th percentile)

Oliver's growth is a textbook example of consistent tracking. His percentile barely moves from one visit to the next, staying right around the 50th curve. His pediatrician has no concerns because the pattern is stable and he is meeting all developmental milestones. Oliver is breastfed and transitioned to complementary foods at 6 months.

Example 2: 90th Percentile Baby

Sophia was born at 40 weeks weighing 9 lbs 2 oz, at the 90th percentile. Both her parents are above average height. Her growth follows the 90th curve consistently:

• 2 months: 13.5 lbs (92nd percentile)
• 4 months: 16.8 lbs (89th percentile)
• 6 months: 19.5 lbs (90th percentile)
• 9 months: 21.8 lbs (88th percentile)
• 12 months: 23.5 lbs (90th percentile)

Some caregivers might worry about a baby at the 90th percentile, but Sophia's growth is perfectly normal. She has been tracking consistently along her curve since birth, her length is proportional at the 85th percentile, and she has a genetic predisposition toward a larger frame. Her pediatrician confirms that a high percentile with consistent tracking and proportional measurements is not a cause for concern.

Example 3: 10th Percentile Baby

Kai was born at 38 weeks weighing 6 lbs 1 oz, placing him around the 10th percentile. His mother is 5 feet 2 inches and his father is 5 feet 7 inches. Kai's growth follows:

• 2 months: 10.2 lbs (12th percentile)
• 4 months: 13.0 lbs (10th percentile)
• 6 months: 15.0 lbs (11th percentile)
• 9 months: 17.5 lbs (10th percentile)
• 12 months: 19.0 lbs (10th percentile)

Kai's percentile might initially seem low, but he has been tracking consistently along the 10th curve from birth. Given his parents' smaller stature, this growth pattern is genetically appropriate. He is alert, meeting all developmental milestones, eating well, and producing plenty of wet diapers. His pediatrician reassures his parents that consistent tracking along any percentile curve, even a lower one, indicates healthy growth.

When Crossing Percentile Lines Matters

While consistent tracking along any curve is reassuring, crossing percentile lines can sometimes signal a problem that needs evaluation. Pediatricians pay attention when a baby's growth pattern deviates significantly from their established trajectory.

A drop across two or more major percentile lines, for example from the 75th to below the 25th over a few months, is called faltering growth or failure to thrive and warrants investigation. Possible causes include inadequate caloric intake, difficulty feeding, malabsorption, chronic illness, or metabolic conditions. Similarly, a rapid upward crossing of multiple percentile lines may indicate overfeeding or, rarely, an endocrine condition.

However, some percentile crossing is expected and normal. Many babies experience a shift toward their genetically determined growth trajectory during the first 6 to 18 months. A baby born large who has smaller-statured parents may gradually settle to a lower percentile, and a baby born small who has larger parents may climb. This normal adjustment typically involves crossing one to two percentile lines and stabilizing at a new consistent trajectory.

Premature Babies and Adjusted Age

Premature babies require a different approach to growth chart plotting. Because they missed weeks or months of in-utero growth, plotting their weight against their actual birth date would make them appear significantly underweight. Instead, pediatricians use adjusted age, calculated by subtracting the number of weeks of prematurity from the actual age.

For example, a baby born at 28 weeks gestation (12 weeks early) who is now 6 months old by actual age would be plotted at approximately 3 months adjusted age on the growth chart. This correction is typically used for the first 24 months of life, after which most premature babies have caught up to their full-term peers.

Catch-up growth, a period of accelerated growth where premature babies grow faster than expected to close the gap, is common and expected during the first 2 years. During catch-up growth, crossing percentile lines upward is actually a positive sign that the baby is progressing toward their genetic growth potential.

Tips for Understanding Your Baby's Growth

Focus on the trend, not individual data points. One measurement that seems slightly off is not a cause for alarm. Growth charts are most informative when viewed as a series of measurements over time. Ask your pediatrician to show you the plotted chart so you can see the visual trend yourself.

Consider weight in context with length and head circumference. A baby who is at the 20th percentile for weight but also the 20th percentile for length is proportionally small, which is usually normal. A baby at the 20th percentile for weight but the 70th percentile for length might be underweight for their frame and deserves closer evaluation.

Share your family's growth history with your pediatrician. If both parents were small as infants, it provides helpful context for a baby tracking at a lower percentile. Similarly, a family history of constitutional growth delay, where children grow slowly but reach normal adult height, can explain a growth pattern that might otherwise raise questions.

Common Mistakes to Avoid

• Treating the 50th percentile as a goal. The median is a statistical reference point, not an optimal target. A baby consistently tracking at the 15th percentile is healthy. Trying to push a naturally smaller baby toward the 50th through overfeeding can lead to unhealthy weight gain.
• Comparing your baby's percentile to other babies. Every child has their own genetic growth potential. Comparing your 25th percentile baby to your friend's 80th percentile baby creates unnecessary worry. The babies are different individuals with different genetic blueprints.
• Weighing your baby too frequently at home. Daily or weekly home weigh-ins capture normal fluctuations from feeding, hydration, and diaper status. These minor swings can cause anxiety that is not supported by the overall trend. Trust the schedule of well-child visits for growth monitoring.
• Using the wrong growth chart. Using CDC charts for a breastfed infant under 2 years old, or using WHO charts for an older child, can produce misleading percentile readings. Confirm with your pediatrician which chart they are using and why.
• Panicking over a single measurement. One data point tells you very little. A baby might weigh less if they recently had a stomach bug, were measured on a different scale, or were weighed with different clothing. The pattern over multiple visits is what matters.

Last updated: February 23, 2026