Interpreting infant growth charts
Remember your first visit with your baby’s pediatrician?
I remember that it seemed like a HUGE deal just to leave the house. Did we fasten BabyC in the car seat correctly? Where can we sit in this waiting room where there will be 0% chance of a sick kid coughing on her? OMG, she’s crying! How long will we have to wait? Should I feed her? Yes, let’s try that. (First time nursing in public.) Five minutes later, when I had finally situated the baby and arranged the nursing cover and gotten a proper latch, the nurse called us back to the examining room… That’s what I remember.
Oh, but our visit with the pediatrician – what do I remember about that? Two things. First, he told me that, even though it seemed like my baby was nursing for 45 minutes out of every hour of the day, my milk may not come in for another 2 or 3 or 4 days. And my baby might get a little hungry. Great. Second, the nurse weighed our baby and measured her length and head circumference. Then we got those all-important percentile stats that told us how our baby compared to her peers. So began a lifetime of pretending not to care how our baby measured up.
BabyC was born 8 lb. 9 oz. and 22 inches long: a big baby. This came as a surprise, as neither my husband or I are very big people, and several OB’s had estimated that I was carrying a 6-7 lb. baby. As our pediatrician looked over the neonatal notes from the hospital and BabyC’s measurements from his office, he commented that she was LGA – Large for Gestational Age. She was in the 95th percentile for weight and 100th for length.
I was not happy to hear my baby called LGA. I had just devoted the last two years of my life to researching the effects of gestational diabetes on the fetus. One of the outcomes of diabetic pregnancies is LGA babies, so I’d spent a lot of time reading about the adverse outcomes we see in LGA babies – both at birth and decades later. I had to remind myself that by definition, 5% of all babies will be LGA, and most of these babies will grow up to be perfectly healthy. I had eaten well and exercised plenty during my pregnancy, and luckily I was not diabetic. So I told myself not to worry.
Within a few months, however, I found something else to worry about. At her 4 month checkup, breast-fed BabyC was coming in at around 25th percentile for weight, and her height was dropping off as well. Our pediatrician was a fairly laid-back kind of guy and told us not to worry. He dropped the hint that we might consider trying some solids soonish, but he didn’t make me feel like my milk production was inadequate. BabyC seemed like she was satisfied at the end of a nursing session, so I felt pretty sure that this was just her normal growth trajectory.
This got me wondering though – where do these growth charts come from? Who are these babies with beautiful smooth growth trajectories? This question also came up in the discussion around my last post (New Study: Exclusive Breastfeeding Can Support Infant Growth to 6 Months of Age). In order to conclude that babies grow normally in any experiment, researchers have to compare them to some standards for growth. What is normal growth in an infant?
In the US, there are two sets of infant growth charts in use by pediatricians: one produced by the Centers for Disease Control (CDC) and one produced by the World Health Organization (WHO). Both sets of charts can be downloaded from the CDC’s website. Let’s compare the two:
CDC Growth Curves for children <24 months:
- Released in 2000.
- Include data collected from the National Health and Nutrition Examination Survey (NHANES) in 1971-1974, 1976-1980, 1988-1994, plus national vital statistics (birth weights), Missouri and Wisconsin vital statistics (birth lengths – only states where this is recorded on birth certificates), and the Pediatric Nutrition Surveillance System (lengths for 0.1 to <5 months).
- Cross-sectional data, meaning a bunch of kids of different ages were measured and plotted by age.
- Sample size for 2-23 months = 4,697 children for a total of 4,697 measurements.
- Excluded babies born with very low birth weight (<3 lb, 4 oz).
- Estimated 50% ever breastfed, 33% still breastfeeding at 3 months (not necessarily exclusively). No information beyond that.
The WHO curves for children <24 months:
- Released in 2006, published as this study¹.
- Data were collected between 1997 and 2003 from children in Pelotas, Brazil; Accra, Ghana; Delhi, India; Oslow, Norway; Muscat, Oman; and Davis, CA, USA.
- Longitudinal data, meaning the same kids were tracked at multiple time points, in this case at birth; 1, 2, 4, 6, and 8 weeks; and 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, and 24 months.
- Sample size: 882 children with a total of 18,973 measurements.
- Excluded babies if they met any of the following criteria:
- Low socioeconomic status; birth at high altitude; birth at <37 wks or >42 wks; multiple birth; perinatal illness; any condition known to affect growth; mom smoked during pregnancy or lactation; breastfeeding for <12 months; introduction of solid foods before 4 months or after 6 months; weight to length ratios >3 standard deviations above or below study median (i.e. babies that were super chubby or super lean were excluded so as not to skew the data).
- 100% of babies were predominantly breastfed at 4 months, and 100% were still breastfeeding at 12 months. Solids were introduced at a mean age of 5.4 months.
In September 2010, the CDC released a report² comparing the two sets of growth curves and making the recommendation that pediatricians use the WHO curves to assess growth of babies 0-24 months old, while continuing to use the CDC data for older children. The CDC cited the following reasons why the WHO curves are better than their own data:
- The WHO data were specifically collected to define optimal growth. They excluded children that were affected by any circumstance known to potentially inhibit optimal growth. The CDC data included all children except those born very small – they represent average rather than optimal growth.
- The WHO growth curves include more data points, measuring 882 infants per time point. The CDC data have only 72 weight measurements at 2 months (38 boys and 34 girls) and then 200 measurements per month up to 5 years of age.
- The WHO data only include children that were breastfed according to the current American Academy of Pediatrics (AAP) recommendations, while the CDC data include a cross-section of American children from the 1970s-1990s, the majority of which were fed at least some formula. Why does this matter?
- More babies are breastfed now than when the CDC data were collected. In a 2007 CDC survey, 75% of infants had ever been breastfed and 58% were breastfed for at least 3 months.
- Infant formulas have changed in the last 30 years. They look a lot more like human milk in nutrient composition than they did when the CDC data were collected.
- Breastfed babies are known to grow differently than formula-fed babies. Breastfed babies usually gain weight more quickly than formula-fed babies for the first few months of life, but beginning around 3 months of age, they gain slower than formula-fed babies. This means that the CDC growth charts over-diagnose failure to thrive, resulting in expensive medical workups. Use of the CDC charts for breastfed babies may lead to the misconception that they are not receiving adequate nutrition if they are “falling off” the growth curve, triggering advice such as starting solid foods earlier or supplementing with formula (which would likely decrease breast milk production). Even if the doctor is aware of the differences in growth between breast- and formula-fed babies and tries to explain this to parents, it can be demoralizing to see your baby fall off the growth chart and cause unnecessary anxiety in a new parent. (I can attest to that.)
The differences in the CDC and WHO data are illustrated in the figures below, taken from the 2010 CDC report.
In November 2010, an article in the AAP News Magazine³ stated that the AAP supported the shift to the WHO growth curves for kids under 2. However, making this change in pediatric practices may take some time, as it will require education, training, and probably some new software.
I was happy to learn that our pediatrician has already made the switch to the WHO growth charts. As for BabyC, her 10 month stats would put her weight at 10% on the CDC chart and 20% on the WHO chart. Her length is at 5% on the CDC chart and between 2-5% on the WHO chart. It turns out she’s just a little kid. Knowing that she is a hearty, healthy eater now, I’m not worried. In fact, I’m pretty sure she just went through a growth spurt, because all of a sudden she seems HUGE!
Did your baby’s growth percentiles change in the first year or two of life? If so, were you worried? Was your child’s pediatrician worried? Do you know if your child’s pediatrician uses the WHO or CDC growth charts?
WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards based on length/height, weight and age. Acta Paediatr Suppl. 2006 Apr;450:76-85.
- Centers of Disease Control and Prevention. Use of the World Health Organization and CDC Growth Charts for Children Aged 0-59 Months in the United States. MMWR 2010; 59(No. RR-9).
- Greer F.R. and Bhatia J.J.S. CDC: Use WHO Growth Charts for Children Under 2. AAP News 2010; 31:1; doi:10.1542/aapnews.20103111-1.