Growth charts are an essential clinical tool for lactation consultants. They help assess how well an infant is growing and whether that growth is in line with biological expectations for health and nutrition. However, using these charts effectively requires a deep understanding of what they represent, how they were developed, and how to interpret them in the context of breastfeeding support.
From Reference to Standard: A Shift in Paradigm
Historically, many practitioners used the NCHS/WHO growth reference charts, developed in the 1970s based on a primarily formula-fed U.S. population. These charts described how children grew, not how they should grow under optimal conditions.
Recognizing the limitations of this approach, the World Health Organization (WHO) launched the Multicentre Growth Reference Study (MGRS) between 1997 and 2003. The resulting WHO Child Growth Standards were based on children from Brazil, Ghana, India, Norway, Oman, and the USA—selected for their diverse ethnic backgrounds and access to health-promoting environments. Importantly, all infants in the sample were exclusively or predominantly breastfed for at least four months and continued breastfeeding up to 12 months, with no maternal smoking (WHO, 2006).
The new standards were designed not just to describe growth, but to prescribe how children should grow under ideal conditions, including optimal nutrition and care practices. In this way, they emphasize breastfeeding as the normative model for infant growth (WHO, 2006).
Understanding Percentiles
Percentile curves show how an individual child’s measurement compares to a reference population—in this case, the WHO Multicentre Growth Reference Study, which involved healthy, breastfed children across six countries.
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Percentiles are rank-based. For example:
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A child at the 50th percentile is right at the median—half the population is smaller and half is larger.
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A child at the 3rd percentile is smaller than 97% of children their age and sex.
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A child at the 97th percentile is larger than 97% of children their age and sex.
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Important note: Being at a low or high percentile is not necessarily a problem if the child tracks along that percentile over time and is otherwise thriving. Concerns arise when there’s a significant drop or jump across percentiles, especially more than two major lines.
Understanding Z-Scores
Z-scores (also known as standard deviation scores) provide a more mathematically precise way of expressing how far a child’s measurement deviates from the average (median) of the reference population.
How Z-scores Work:
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The mean (average) is 0.
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A z-score of -1 means the measurement is 1 standard deviation below the mean.
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A z-score of +1 means the measurement is 1 standard deviation above the mean.
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Every unit of z-score represents a set distance from the mean, making it easy to detect small changes and use statistically.
Why Use Z-Scores?
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They allow comparison across different ages and indicators (weight-for-age, length-for-age, etc.).
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They make it easier to identify and track subtle changes over time.
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They’re ideal for clinical audits, population studies, and monitoring at-risk infants.
When to be Concerned:
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A z-score below -2 is considered moderate undernutrition or growth faltering.
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A z-score below -3 is considered severe.
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A decline of more than 0.67 z-scores (approximately one major percentile line) between visits may indicate growth concern.
Appropriate Growth in the First Year of Life
Breastfed infants tend to gain weight more rapidly in the first 2–3 months and then slow down compared to formula-fed peers. The WHO standards reflect this natural pattern. Here are approximate average weight gain expectations derived from the standards and velocity data (WHO, 2006).
Weight Velocity (Boys and Girls Combined)
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Birth–1 month: ~25–30 g/day
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1–3 months: ~20–25 g/day
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3–6 months: ~15–20 g/day
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6–9 months: ~10–15 g/day
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9–12 months: ~8–12 g/day
Length Velocity
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0–3 months: ~3.5 cm/month
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3–6 months: ~2 cm/month
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6–12 months: ~1.5 cm/month
Remember: infants should double their birth weight by around 4–6 months and triple it by their first birthday.
Using the WHO Growth Charts in Practice
As a lactation professional, you can use the WHO growth charts to:
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Track growth over time, rather than focusing on one single measurement.
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Identify red flags, such as crossing two major percentile lines downward, which may suggest inadequate intake or an underlying condition.
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Support breastfeeding families by reassuring them that slower weight gain after 3–4 months is expected and normal in breastfed infants.
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Advocate for exclusive breastfeeding, using growth charts to show parents how their baby is thriving on breastmilk alone.
Practical Tips for Chart Use
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Always use WHO charts for breastfed infants up to 2 years of age (CDC recommends switching at age 2).
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Plot both weight-for-age and weight-for-length to assess proportionality.
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Don’t panic over single low percentiles—focus on growth trajectory.
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Use z-score charts when available for a more precise look at growth deviation.
Practical Example
A breastfed baby is consistently at the 15th percentile (z ≈ -1). Parents worry the baby is “small.” You reassure them that:
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The baby is growing along their own curve.
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They’re within the healthy range (between -2 and +2 z-scores).
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There’s no indication of growth failure unless there’s a sharp drop or other concerns.
However, if that baby drops to the 3rd percentile (z ≈ -2) at the next visit, you would:
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Review feeding history.
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Assess milk transfer.
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Consider weight gain trends (e.g., g/day).
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Possibly refer for pediatric evaluation if the cause is unclear.
Centering Growth Standards in Breastfeeding Support
The WHO Growth Standards are more than just tools for charting numbers—they are a framework for understanding what healthy growth looks like when infants are nurtured under optimal conditions. As lactation professionals, these standards empower us to advocate for breastfeeding as the biological norm, reinforce the value of human milk, and guide families through the nuances of infant development with evidence-based confidence.
By understanding and applying percentile curves and z-scores, we can provide nuanced, culturally responsive care that recognizes individual variation while identifying potential red flags. We can distinguish between true growth concerns and normal variation, avoiding unnecessary supplementation or weaning based on misinterpreted data. This approach not only supports better health outcomes, but also helps build trust with families by centering compassion, accuracy, and context in every conversation.
Ultimately, growth monitoring should not be about fitting babies into a narrow mold—it should be about supporting their unique growth journey with informed, skilled, and respectful care. The WHO Growth Standards help us do exactly that.
References
WHO Multicentre Growth Reference Study Group. (2006). WHO Child Growth Standards based on length/height, weight and age. Acta Paediatrica, Supplement, 450, 76–85.
World Health Organization. (2009). WHO Child Growth Standards: Growth velocity based on weight, length and head circumference: Methods and development. https://www.who.int/tools/child-growth-standards/standards
