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The Truth about Developmental Milestones

Unfortunate or not, there is something inherently competitive about being a parent. One of the ways this comes to light is when parents compare how old their children were when they finished a whole bottle, slept through the night, or sat up by themselves. It is a source of pride when our children learn to stack blocks or recite the alphabet or master toilet training earlier than the other children we know do. (The things our friends’ and neighbors’ children do earlier than our are all minor or irrelevant, of course!)

But drawing broad conclusions from whether one child passes these developmental milestones, as they are called, earlier than another is fraught with danger and is sure to lead to disappointment and frustration. It is a particular problem during the first year or so for the parents of children who are born prematurely.

Most new parents find themselves glancing at the many charts in books about child development, wondering if their child will be “on time” or “ahead of schedule” in his development. The mere fact that these milestones are in print and in the form of a table or checklist lends them an undeserved aura of precision. It’s that implied precision that gets parents upset if, for example, their 6-month-old son can’t pick up a small block in his hands. After all, that’s the age at which he “should” be able to do that.

And so parents worry. Is he falling behind? Does this mean he’ll have problems in school? Should I spend extra time helping him with his coordination? The answer to all of these questions is: Probably not.

Passing the Stage is Important, Not the Age

There’s one basic rule you should remember about developmental charts that will save you countless hours of worry and heartache in case your child hasn’t been studying them as closely as you have, or is feeling rebellious and has decided to follow his own developmental schedule. The fact that a child passes through a particular developmental stage is always more important than the age of that child when he or she does it. In the long run, it really doesn’t matter whether you learn to walk at ten months, twelve months or fifteen months — as long as you learn how to walk.

When I give speeches to parents in the U.S. and Canada, I’m often asked questions about toilet training by parents (almost always mothers, for they are apparently more comfortable with the topic) who are obviously upset and worried because their child is “behind schedule.” I usually preface my answer to their question by asking two questions of my own: How many adults do you know who have never been successfully toilet trained? Before you married your spouse, did you ask him how old he was when he was toilet trained? Both questions help put their concerns into perspective. Yes, it feels like it will go on forever, but it won’t. And yes, it seems like a big deal right now, but in the long run it won’t matter.

That’s why you really should look at the ages in those charts as a range instead of a single number. In general, the older the child the wider the range. After all, children aren’t magically able to grasp a small object a certain number of months after being born. It’s the culmination of a series of skills involving vision, concentration, muscle coordination, and more. We would naturally expect children to show a significant variation in when they master something that, for them, requires a lot of complicated coordination.

There are two other reasons for this variation or age range. Understanding those reasons takes a bit of arithmetic. This is where the parents of premature babies can get some much-needed reassurance.

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First, when you’re trying to figure out roughly when your baby should reach a particular developmental milestone, it’s a good idea to start counting from your child’s due date instead of the actual day he was born. Remember that being born a month early isn’t the same as getting a high school diploma ahead of your peers because you worked extra hard, went to summer school, and skipped a grade.

Physical and mental development take time. A child who’s born two months premature is no more developed than an embryo who will stay in the womb for another eight weeks. The act of being born doesn’t speed up that child’s growth. Consequently, it’s not surprising to find a 6-month-old baby who was born two months ahead of schedule to be at the same developmental level as a 4-month-old who was carried to term. That’s exactly where he should be. But if you simply looked on a chart for 6-month-olds, you’d be unnecessarily worried.

Second, look at ratios instead of age differences. This is related to the first point, and explains why we don’t worry as much about correcting for premature babies’ age after the first year or two.

The age difference between a 6-month-old and a 9-month-old is the same as the difference between a 30-month-old and a 33-month-old. But the 9-month-old has had 50 percent more time outside the womb to develop, while the 33-month-old has had only 10 percent more time to develop. Consequently, we would expect to see more significant differences in everything from physical coordination to intellectual abilities between the younger children than between the older ones.

That’s why we need to recognize that even though the developmental charts are only rough guidelines for the first year, they are even less precise as our children get older. Older children can be years apart in when they reach certain developmental milestones (such as puberty) and still be completely normal.

There’s one other important variable that sometimes causes parents needless angst: a false belief in the precision of anything associated with numbers. When I taught a graduate seminar in public health, I would tell my students that my son, according to the medical records from his routine physical examinations, was consistently at about the 70th percentile for children his age in both his weight and the circumference of his head. The records also showed that he was taller than only about 30 percent of children his age when he was 3 months old, but bigger than 90 percent of children his age a few months later. I then asked them for the most likely explanation.

I was always amazed by the complexity of their answers, which ranged from rare brain tumors to growth-hormone problems. I then reminded them that babies don’t like to lie still and be measured. They squirm. You can get much more reliable measurements of a baby’s weight and head circumference because he can move around while you’re doing those. The most likely explanation for the difference between the two checkups was that the nurse wasn’t able to get an accurate measurement of his height either time.

The Truth about Developmental Milestones

Lawrence Kutner, Ph.D

Dr. Lawrence Kutner is a nationally known clinical psychologist who teaches at Harvard Medical School, where he's co-founder and co-director of the Harvard Medical School Center for Mental Health and Media. He's the author of five books, including: Parent & Child: Getting Through to Each Other; Pregnancy and Your Baby's First Year; Toddlers and Preschoolers; Your School-Age Child; and Making Sense of Your Teenager. All articles appearing here originally were published on Used with permission.

APA Reference
Kutner, L. (2018). The Truth about Developmental Milestones. Psych Central. Retrieved on December 2, 2020, from
Scientifically Reviewed
Last updated: 8 Oct 2018 (Originally: 17 May 2016)
Last reviewed: By a member of our scientific advisory board on 8 Oct 2018
Published on Psych All rights reserved.