Can we have a little chat about averages?

Hands on pregnant belly with feet on scale in the distance

I see so many women worrying because they are not “average” or trying to figure out if some small variation from average holds meaning. It takes several forms, among them:

  • New mom wants to know how she can get her baby to sleep for an extra hour every day, because 3 month olds are “supposed” to sleep for 15 hours each day and her baby only sleeps 14.
  • Expectant mom wants to know if her baby is going to be too big to birth vaginally, because at her last visit her baby is measuring a week ahead.
  • Expectant mom freaking out because in the third trimester she is “supposed to” be gaining 1 pound per week and last week she gained 3, this week she had a stomach bug and lost 1. Wants to know if her baby will be permanently damaged.
  • New moms who are worried about their baby being “only” the thirtieth percentile pretty consistently and are not going up. Or who have pressure from well meaning friends and family members to supplement their breastfed baby with formula to get their baby over the fiftieth percentile.

These examples all have one thing in common: stress over being different from the exact average! Which comes from a belief that you have to be exactly at – or above – average to be “normal.” Thankfully, this is far, far from the truth. Let’s take a closer look:

What does “Average” mean?


Let’s revisit math class for a bit here. (I know, I know, you thought you’d NEVER use math once you graduated. But stick with me for a little bit. It’s not the complicated math.)

Most of the time when we use the term “average” we refer to the “mean” – which is calculated by taking all of the values, adding them up, and dividing by the number of values. Let’s do a little example with weight gain in the third trimester as an example.

When we say “The average pregnant woman will gain about one pound a week in the third trimester” where does that one pound number come from? They take a whole bunch of normal, healthy pregnant women and add up the weight they gained each week. I made a super small sample of fictional moms and some sample weight gains. You can see these in the chart on the right. Just like real life, some moms will be gaining more, some less. The amount will vary from week to week. Some weeks moms might gain a lot of wait, some weeks none. There might even be a week or two where mom *loses* weight. Rarely did any of these moms gain exactly one pound in a week, though it did happen a few times. All of these variations are perfectly normal. None of these expectant parents needs to be concerned about weight gain, or do anything to change it.

Let’s look at an example graph showing each of these fictional mom’s average weekly weight gain:


Each one has a different weekly weight gain. Each one is healthy and normal. And when you take an average of these varying weight gains, you get….about a pound a week. And most of these moms didn’t actually gain exactly a pound a week every week in the third trimester.

But wouldn’t it be better to be at average than a variation?

Not necessarily. For each area of pregnancy, birth or parenting where there is an average, there’s also a normal range. That range can be quite wide. It’s probably better to be within the normal range, but for specifics talk to your care provider about what is the normal range and what risks there might be to being outside of that range.

There is, however, a risk to trying to be exactly average. A risk of stressing yourself out unnecessarily, a risk of wasting time and energy trying to figure out what it all means (when it doesn’t usually mean anything), and a risk that you could stress out your child trying to make them be different than they are, without medical reason. If your baby sleeps 14 hours a day when average is 15, trying to force a newborn to conform to averages is pretty much an unneeded exercise in futility.

If you’re concerned, some simple questions to ask your care provider might be:

  • What is the normal range of variation for this?
  • Is there reason for concern?
  • Should I do anything about this?
  • What are the risks if I am outside the normal range?

Some parents (and grandparents who just love to share their opinion) worry because the baby is below the fiftieth percentile. By it’s very nature, the fiftieth percentile is where half the babies will be bigger and half the babies will be smaller. It’s impossible for every baby to be above the fiftieth percentile! Unless there are dramatic changes in the pattern of growth, or the baby is outside the normal range, it’s perfectly fine for your baby to hang out under the fiftieth percentile.

As long as you and your baby are within the range of normal, take the words of Paul McCartney and just let it be.



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