In today’s information age, parents are more aware of (and consequently, more concerned about) the developmental milestones children “should be” reaching than ever before. To compound this stress further, many of the apps and online resources that parents use to track their children’s development don’t always convey just how normal it is to see individual variability within the first few years. This article hopes to diffuse some of the stress surrounding monitoring your child’s development by discussing why developmental milestones should be viewed as a range, and why it’s perfectly normal to be a little different. 

Infant and Toddler Development: Learn. Change. Grow. Repeat.

The first few years of development are a period of functional chaos, with infants and toddlers undergoing significant physical, motor, cognitive, social, and emotional changes on a monthly (and sometimes weekly!) basis.¹ While these changes and transformations can be a joy to behold, tracking whether a child is reaching developmental milestones at the “normal” age, or along the “typical” trajectory can be a uniquely stressful experience for parents. On one hand, having a child who walks or talks early can be incredibly affirming (and something that just naturally finds its way into every conversation you have that month). On the other hand, having a child who is slow to roll over or reluctant to crawl can be unsettling, and even a little exasperating at times. (“Crawl like the other babies, or no more Gymboree.”) Don’t worry, you’re not alone—and we doubt the other Moms heard. 

Developmental milestones for children: A useful resource, but not an exact science 

Developmental milestones refer to the behaviors and physical abilities that infants and young children typically acquire at certain ages, such as rolling over at 5 months, uttering a first word at 11 months, and engaging in pretend play at 18 months.^1–3 Milestones provide a research-based framework for tracking children’s development, and have been established for a number of developmental domains including: 1) physical and motor development, 2) cognition and problem-solving, 3) receptive and expressive language, and 4) social-emotional development. For a comprehensive list of milestones and developmental domains, refer to the CDC website. 

Although a useful resource for understanding normal development and recognizing the signs of atypical development, there are several reasons why milestones in isolation are not a perfect measure of child development. First of all, they are based on the average age children acquire new skills and behaviors. This suggests, for example, that for every child who takes their first steps “early”, another will take their first steps “late.” Secondly, milestones don’t account for how biological and environmental factors affect a child’s developmental trajectory, making it difficult to draw meaningful conclusions based on how they compare to children with different genetics, personalities, and social environments.

Thirdly, children don’t all come into this world on their due date (many are born preterm or post term), yet for the most part, their birthdate is used to conceptualize their age. And finally, because development across different domains is very interrelated in early childhood, a subtle delay in one domain (e.g., motor development) can lead to lags in other domains (e.g., social development). We discuss these reasons, and why milestones should be viewed as a range, further in the next few sections. 

Effects of environmental and personality differences on child development 

While physical growth and neurodevelopment tend to proceed along a predictable timeline, other aspects of development tend to vary considerably between children.² This is because many of the developmental changes occurring during early development are rooted in biology but shaped by the environment. In other words, while there are certain behaviors that are important for humans to learn (like walking, talking, forming emotional attachments, etc.), precisely how and when these behaviors develop are subject to influence from the child’s environment and their unique personalities.  

A classic example of environmental influence comes from a research study dating back to the 1970s. This study found that, on average, babies reared in rural Africa achieved motor milestones significantly earlier than babies in the United States. Rather than inferring that American babies are delayed relative to African babies, the researchers attributed these differences to the fact that babies in Kenya are explicitly taught how to sit up on their own and walk from an earlier age.⁴ Conversely, other lines of cross-cultural research have reported that African babies acquire language later than American babies, presumably because they are spoken to less frequently and encouraged to verbally communicate less often than American babies.⁵ Taken together, these studies show that the timing of milestone achievement is a reflection of the experiences and environmental input children receive. And importantly, research finds that many early differences in development naturally resolve themselves with time and subsequent experience.^1,6

Another important but often overlooked environmental reason for variation in early development is differences in nutrition. Whereas some children are “good eaters” and content to eat a variety of nutritious foods, others are far less willing to incorporate new and healthy food items into their diet. Considering that virtually every area of early development is somehow affected by the nutrients children receive (or don’t receive), it stands to reason that differences in nutrition would also lead to differences in the overall rate of children’s development.⁷

For example, a toddler who consumes a diet rich in important nutrients like iron, choline, omega-3 fatty acids, and B vitamins will have more of the nutritional building blocks needed for neurodevelopment and energy metabolism than a same-aged peer who eats a diet low in nutritional value. Consequently, we would expect to see the child receiving superior nutritional support to make physical, motor, and cognitive gains at an advanced rate relative to the child with inferior nutrition. Does this mean that the “good eater” will maintain a developmental advantage throughout childhood? Not necessarily; however, it is important to emphasize the importance of adequate nutrition in early development and how it may affect a child’s developmental trajectory. 

Other sources of variability include differences in personality, or temperament. For example, a child with a quieter disposition may be less inclined to verbally communicate but possess perfectly “normal” receptive language skills. However, based on their expressive language output, this child might be flagged for a language delay. Conversely, a sociable but risk-averse child might prefer to hold off on walking and focus more energy on communicating verbally with parents and caregivers. Using strictly developmental milestone standards, this child might be flagged for a motor delay simply because they were more disposed to talking than exploring their environment. 

In order to accommodate for the range of variables influencing development, milestones should be interpreted within a range of possible ages. Take walking, for example. The average child takes their first steps at 12 months; however, it’s not uncommon for children to begin walking anywhere between 8 and 16 months.^1,2 The reason for such a large age range is because, in addition to receiving the nutrients needed for physical and motor development, walking is dependent on children mastering a series of skills, including pulling to stand, “cruising” furniture, and balancing on two feet unassisted. The time it takes for children to master all of these individual components of walking influences when they can successfully coordinate them all into their first steps. And naturally, that can vary. 

The “normal” age ranges associated with other developmental milestones varies; however, a good rule of thumb is that ranges broaden as development progresses. This is because later milestones typically require the coordination of a larger number of individual components, as well as greater muscle coordination, attention, perception, and cognition. 

Differences in gestational age can skew timing of milestone achievement 

People tend to look at birth as the “great equalizer” when it comes to development. However, birth doesn’t expedite maturity for babies born preterm (< 37 weeks) or roll back the clock for babies born post term (> 40 weeks). In other words, we can’t expect a baby born 36 weeks to reach the same early milestones at the same chronological age as a child who was born at 42 weeks, and consequently came into the world with 6 more weeks of neurodevelopment under their belt. And yet, because developmental milestones are typically based off a single chronological age (e.g., 2-months, 9-months, or 18-months after birth, etc.), many people are in effect doing that.⁸

Fortunately, many physicians will use a preterm child’s “corrected age” (i.e., their gestational age) when gauging development; however, there are no official guidelines for when and how to apply a gestational age correction, and no consensus regarding how long a gestational age correction should be applied.⁹ Needless to say, if the parents of preterm children are monitoring their development by chronological age, this could lead to a lot of unnecessary anxiety. And given that roughly one in ten babies in the United States (12%) are born premature each year, this is a relevant consideration for many.¹⁰

At this point you might be wondering, “But does a few weeks difference really matter?” (Great question!). In early development, the answer is yes. This is because many of the behaviors and skills babies acquire early on are facilitated by the complex and significant nervous system development that occurs towards the ends of the third trimester. Because children born before 37 weeks do not undergo the same degree of neurodevelopment within the protective environment of the womb, they may have difficulty meeting early milestones relative to children born at term, or post-term.¹¹ Fortunately, however, research finds that that most children with transient delays related to prematurity have caught up with their full-term peers by the time they enter preschool.¹

Development builds on development—and so does variability

Another reason why milestones shouldn’t be associated with a single age is because development builds on itself. In other words, individual differences in one area can influence development in other areas. Going back to the example of the risk-averse child who was slow to establish motor skills—if this child’s wariness and reluctance to explore their surroundings were to affect their ability to approach people and engage in social interactions, this lag in motor development could inadvertently lead to a lag in social and emotional development as well. Does this mean that their parents should be concerned that something is “wrong” with their child? No. Chances are, this child will catch up to his peers in both areas in time. 

Your child is a unique individual

While developmental milestones are helpful for tracking a child’s growth, judging development by a single number on a chart can be problematic given the number of sources of variability influencing the rate and trajectory of typical development. That being said, if you notice that your child is experiencing significant delays in a specific area or multiple areas, trust your instincts and consult their pediatrician when appropriate. However, next time you click on an email with the subject “what to expect from your baby this month,” remember that milestones are based on averages, whereas your child is a unique individual.