The Statistical Saga of Baby’s Weight

Minitab Blog Editor 03 March, 2015

Many things have shocked me since having my first baby back in August. I didn’t think it was possible to be so tired that it actually hurt, and I also didn’t think that changing 10+ diapers a day would actually be the norm (or that needing to perform 10+ outfit changes was even possible, let alone necessary). I also didn’t think that we’d fall in love so hard with the little guy. What a wonderful, rewarding experience it is to be a parent!

That’s enough mushy talk for now. Let’s get back to the surprises involved in having a newborn. Another shock we experienced those first few days stemmed  from the weight loss our son experienced. I certainly didn’t imagine that my perfect newborn would lose so much weight those first couple of days! After all, he was born at a very healthy 8 pounds 3 ounces, and I was doing all I could those first couple of days to ensure he was fed every 2 hours, on the dot. I didn’t know that newborn weight loss was even a thing, let alone a very common thing. 

Here’s where things get cloudy and pretty crazy (please be sure to imagine my very ugly cry here, due to the aforementioned sleep deprivation). We took our son to his first doctor’s appointment a few days after his birthday, which included a weight check. According to the doctor, things weren’t looking good and he had lost “too much” weight. Our pediatrician followed what is known as the “10 percent rule of thumb” for breastfed babies, which basically means that a 7-10 percent weight loss after birth is considered normal. Our son had 12 ounces of weight loss, or about 9.2 percent of his total weight—the higher end of “normal.” But in my sleep-deprived mind, that 12 ounces became more than 1 pound of lost weight, and I was calling in all the troops to assess what was going wrong.

I only wish that one of the troops I called on had been this cool newborn weight tool, known as NEWT. Folks at the Penn State College of Medicine and the Penn State Hershey Children’s Hospital developed a “growth chart” for infant weight loss in the first few days of a baby’s life that mimics the percentile-approach commonly used by pediatricians for plotting the height, weight, and head circumference of children. (Before making the tool, the doctors knew they needed a large set of data for NEWT to be statistically sound. You can read more about how they got this data and implemented NEWT here.)

Let’s take a look at where his weight loss fell on NEWT’s continuum:

Now, I can definitely see our doctor’s cause for concern. After all, according to NEWT, results that tend toward higher percentile levels may provide early identification of adverse weight loss conditions. Our son's weight loss at about 61 hours after birth (see the light blue dot) fell just outside the 75th percentile.

However, since our son is a breastfed baby, his weight loss of 9.2% at three days old was still considered normal by most pediatricians, albeit on the higher end of being normal (which NEWT also shows nicely). The doctors who created NEWT brought up a good point in the article regarding the “10 percent rule of thumb”: a weight loss of 10 percent can matter a lot, or not all, depending on when and at what rate it occurs.

But…at 3 days postpartum, I was convinced I heard the doctor say our son had lost 16 ounces of weight, which equates to a much scarier 12.2% weight loss. Yikes! Sleep deprivation does crazy things to people. Like most first-time parents, I wanted my baby to be, above all things, healthy and normal. The 12.2% weight loss my tired brain had fabricated wasn’t normal, but his actual weight loss (9.2%) wasn’t far from normal at all. 

This all ended quite well, as two days later we headed back to the doctor for another weight check, and our son ended up gaining a whopping 9 ounces—putting his weight almost back to his birth weight. Our doctor likes to see breastfed babies reach their birth weight again about one week after their birthday. So we were right on track!

Since his weight has been a sore spot for me, I’ve been charting it using a Time Series Plot in Minitab in time increments that have followed his doctor appointment schedule (2 weeks, 2 months, 4 months, etc.):

Moving past his initial newborn weight loss, I’m monitoring for no dips moving forward, and hoping for a steady climb. You can see that the little guy has been doing just fine gaining weight so far, and we may even want to call him the “big” guy now!

As a parent, I’m very thankful for statistics and statistical tools like NEWT and Minitab!