Family Dynamics

Can Birth Order Put Children at Risk for Overweight?

Birth order and weight problems linked in new studies.

Posted Jun 16, 2016

As if the list of only-child stereotypes wasn’t long enough, a new one has been brewing: Children without siblings are at a higher risk for being overweight and obese, as are daughters and youngest children. Can family makeup tell you which of your children will be overweight?

The latest findings, published in Pediatrics, titled “Effect of Sibling Birth on BMI Trajectory in the First 6 Years of Life” found a suggested link between the Body Mass Index (BMI) of a first-born child and its sibling status. The study looked at the prevalence of obesity for those who had siblings by the time they were age six and those who remained only children.

Researchers sampled 697 families using data from the National Institute of Child Health and the Human Development Study of Early Child Care and Youth Development. They concluded that singletons in the first grade had the “highest prevalence of obesity.” On the other hand, first-borns who welcomed a new sibling earlier in life had a healthier BMI trajectory.

“Children who did not experience the birth of a sibling by the time they were in first grade had the highest prevalence of obesity at 12.8%. Children who experienced the birth of a sibling when they were 36 to 54 months old had the lowest prevalence at 4.8%,” the study states.

This is not the first study in recent years to assert a possible link between family size, birth order and childhood obesity. Several studies on the topic are “fat” with flaws, yet news coverage has hyped many findings that still need a closer look.

For instance, in 2010, Disease Prevention covered a British study from the Institute of Education at the University of London. The headline: Being an Only-Child Increases Chances of Being Overweight by 25 Percent. Looking at weight over the first seven years of a child’s life, this investigation “showed a direct link between parental behavior and children’s weight,” specifically highlighting girls and only children as more prone to being overweight when they are between the ages of five and seven. One theory was that not having siblings results in a less-active lifestyle. Another was that onlies may be “overfed by indulgent parents.” The analysis of the study concluded: “This paper is among the first to show that singleton status is a risk factor for childhood overweight; stressing the importance of family structure and related lifestyle behaviors.” However, the authors added, “Fewer only children had two-parent households; they had less play time outdoors; a higher propensity to consume sugar; and were more likely to have parents supportive of food as a reward and television in the bedroom.”

Two years later, the risk of only children being overweight jumped to 50 percent, according to researchers at the University of Gothenburg. That study, of children ages 2 to 9, looked at data of 12,720 children from eight European countries. A headline covering the story also seemed to broadcast that singletons were locked into a particular state of health simply because they lacked siblings: “Study Finds Only Children are 50% More Likely to Be Overweight.” The investigators specified that “among the six- to nine-year-olds, the likelihood climbed to 70 percent.”  Medical News Today, Science Daily, and the Huffington Post all also weighed in with similar headlines, respectively: “Only Children Have a Higher Risk of Obesity,” “Only children are significantly more likely to be overweight,” and “Children Without Siblings May Have Higher Risk of Overweight and Obesity,” respectively.

Another study, “Number of siblings, birth order, and childhood overweight: a population-based cross-sectional study in Japan,” examined weight problems in elementary school age children. Looking at weight and birth order of 4,026 children ages nine and ten, the findings were similar: The odds that an only child would be overweight were higher when measured against middle children. The summation of the findings were: “Being an only or youngest child was associated with childhood overweight.”

In 2015, a study from the University of Michigan and reported in Appetite Journal,  found, “Higher weight status of only and last born children,” joined in to add a new label to only children. The small sample size (300) and its makeup are noteworthy: low-income 4- to 8-year-olds and their mothers recruited through Head Start programs. After all the finger pointing, the authors wrote, “…while our study only included maternal feeding behavior, the behavior of other family members (e.g. fathers) might contribute to underlying pathways…our findings may not be generalizable to families without these characteristics (low income).”

The bottom line in all the investigations is that if parents don’t want overweight children they need to give their firstborn brothers and/or sisters—but not too many. As family size increases, the overweight problem doesn’t hold up.

Findings “Fat” with Flaws

These studies taken as a group are misleading, and make it impossible to conclude that only children—daughters or youngest children—are at risk for obesity and overweight. The researchers themselves call for more closely sampled and monitored investigation. Among the problems with buying into the idea that siblings keep a child thin is inconsistency. It’s important to remember that results change between and within studies. For instance:

  • Number of siblings in the family altered outcomes.
  • Age difference between siblings made a difference.
  • Mothers’ weight during pregnancy might have influenced a child’s weight.
  • Parents’ disciplinary approaches with onlies, firstborns and later born children affect findings.
  • Whether or not food is used as a reward can be an influencer.
  • Some studies looked at low-income families whose nutrition knowledge and access to healthy foods might be limited.
  • Self-administered questionnaires to parents are not always reliable.

Most striking is the parallel rise in child obesity in the overall population during the last ten years and parents’ awareness. Mirroring my own reaction to these reports, Evan Nadler, MD, co-director of the Children’s National Obesity Institute in Washington, DC, said of the University of Gothenburg study: “the study suggests an association, but does not connect the dots between being an only child and risk of being overweight.”

The driving forces more likely seem to be parents’ attitudes and behavior around food, no matter how many children they have. And yet there’s a strong push to add overweight and obesity to the stigma surrounding only children. University of Michigan’s Julie Lumeng, M.D., one of the authors of the Pediatrics study mentioned earlier, said in a press release: “We need to further study how having a sibling may impact even subtle changes such as mealtime behaviors and physical activity.” Lumeng rightly adds that if the birth of a sibling changes family behavior and alters the home environment in a significant way, there needs to be a greater understanding of the connection.

Weight problems are complicated and trying to say conclusively that only children, girls and youngest siblings are more susceptible to becoming overweight feels like splitting hairs amid much larger issues.

These studies and the news coverage signal to parents of singletons in particular that if they don’t want an obese only child, a solution is having more children sooner than later. Somehow, I don’t think adding a brother or sister is the answer to any child’s overweight problem. Do you?


Does Your Birth Order Actually Matter? 

Only Child Stereotypes: Fact v. Fiction 

Should I Keep My Firstborn an Only Child? 

Standing Up to the Pressure to Have More Children 


Haugaard LK, Ajslev TA, Zimmermann E, Angquist L, Sørensen TIA. “Being an Only or Last-Born Child Increases Later Risk of Obesity.” PLoS ONE, 2013. doi:10.1371/journal.pone.0056357.

Lumeng J, Mosil R, Kaciroti N, Corwyn R, Bradley R. “Effect of Sibling Birth on BMI Trajectory in the First 6 Years of Life,” Pediatrics, April, 2016. DOI: 10.1542/peds.2015-2456.

Mosli R, Lumeng J, Kaciroti N, Peterson K, Rosenblum K, Baylin A, Miller A. “Higher weight status of only and last-born children. Maternal feeding and child eating behaviors as underlying processes among 4–8 year olds.” Appetite, 2015. doi:10.1016/j.appet.2015.05.021.

Ochiai H, Shirasawa T, Ohtsu T, et al. “Number of siblings, birth order, and childhood overweight: a population-based cross-sectional study in Japan.” BMC Public Health. 2012. doi:10.1186/1471-2458-12-766.

Copyright @2016, 2019 by Susan Newman 

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