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Pregnancy Weight Gain: What is the Value of Being Goldilocks?

Whenever I think about pregnancy weight gain, I think of Renée Zellweger playing Bridget Jones, the role that catapulted her to true stardom—and for which she had to gain some 30 lbs.

After losing the weight and then regaining it for the second Bridget Jones film, Zellweger was fed up with the yo-yo-ing. She griped to the Daily Mail about the difficulty of shedding her Bridget Jones weight: “It was horrible. On one side it’s fantastic because you get to go back to taking care of yourself, so you feel wonderful, but the first lap around the track, I felt like I had a toddler on my back.” 

Which nicely sums up how I felt after giving birth, except that I literally had a toddler on my back.

The Stress and Distress of Pregnancy Weight Gain

For most women, gaining weight during pregnancy is normal, natural, and yes, necessary. But that doesn’t make it any easier. As Zellweger illustrates, even for women with a team of celebrity personal trainers and diet gurus at their beck and call, gaining and losing a major amount of weight is daunting. 

And the monthly pregnancy weigh-ins at prenatal check-ups are certainly no help.

At about 24 weeks, I went in for a routine prenatal check-up, and was shocked to discover I had gained 5 lbs. in the last month.

My doctor did not brush this aside. Instead, she sat me down and looked at me intently. I needed to slow down my rate of weight gain, she said. At the rate I was gaining, I would exceed the recommended amount. She pulled out a chart showing different rates of steady weight gain, and showed me that, at the rate I was going, I would gain a total of 39-40 pounds by the time I reached 40 weeks. (With my normal weight BMI, the guidelines stipulated a gain of only 25-35)

“Eat only half a sandwich”, she told me. “Don’t eat for two, that’s a myth. You only need a couple hundred calories extra a day.” 

I wracked my brain for an explanation. Nothing had changed. No late night fridge raids. No pastry add-ons with my morning coffee, or Friday donut at the office. If anything, I was being more active and eating less than in my first trimester. My near-constant nausea had finally faded, and I no longer needed to melt a saltine on my tongue every fifteen minutes to get through my day. 

Most shocking, though, was my doctor’s cavalier delivery. I felt more or less okay about being told I was gaining too quickly, but at a different point in my life, she would have reduced me to tears. 

A friend with a history of eating disorders received a similar lecture a year ago. In response she kept a detailed daily journal of food and calorie intake over the next four weeks. At her next prenatal visit, she presented it to her doctor to prove that she only ate 1800 calories a day, and that her “excess gain” was not her fault. Thankfully, her doctor finally cottoned on to the distress she was causing and gently told her to put the lists, calorie counts, and the scale aside, and just let her body do its thing.

Is This Emphasis on Weight Gain Justified?

Nope.

As I’ve written about before, a sudden bump up in weight gain in the second trimester is normal, mostly water weight. It does not imply you will continue to gain weight quickly.

But, even bigger picture, what’s with all this emphasis on gaining the right amount? Does it actually matter if you go over or under by a few pounds, as most women in the U.S. do?

(Note: Very rapid weight gain  – more than 2 lbs in a week – can also signal the onset of preeclampsia, a serious pregnancy disorder involving high blood pressure. Unlike sticking to the weight gain recommendations, this is a good reason to keep a close eye on your weight gain.)

If the emphasis were merited, two things would have to be true: (1) Medical organizations would have to know the “right” amount of weight gain, and (2) Your health, or your baby’s health, would have to hinge on gaining the “right” amount. 

But for most women, neither is true.

managing back labor

The Current Pregnancy Weight Gain Guidelines: Where Did We Get These Numbers?

Before we get into a new study that captures why both assumptions about the right amount of weight gain are wrong, let’s go back to what these assumptions were built on in the first place.

In 2009, the Institutes of Medicine (now the National Academy of Medicine (NAM)) released their current guidelines for weight gain during pregnancy

Their goal was to minimize four negative health outcomes linked with pregnancy weight gain: 

  • Holding on to excess weight (defined 10+ lbs.) after pregnancy
  • Unplanned Cesarean delivery
  • Large for gestational age (LGA) babies
  • Small for gestational age (SGA) babies

NAM Current Guidelines (for Singleton Pregnancies)

But ten years ago, the NAM simply didn’t have enough data.

They couldn’t evaluate how pregnancy weight gain affects women’s risk of developing complications like gestational diabetes or preeclampsia. 

Fortunately, we now have better data.

A New Large Meta-Analysis Will Likely Result in Changes to the Current Guidelines

This year, however, a new, large meta-analysis of 29 prior studies, was published in JAMA, with data from over 190,000 pregnant women. The largest study of its kind, it will likely lead (eventually) to a revision of the existing guidelines, and perhaps a shift to focusing on pre-pregnancy weight over weight gain during pregnancy.

Here’s a quick summary of their findings:

1. Women who begin pregnancy at a normal BMI (19.5 -24.9) or high BMI (25-25.9) have more leeway than the current NAM guidelines suggest.

The IOM recommends that women, within a normal BMI range, gain between 25-35 lbs. The new meta-analysis study found a wider range, between 10-18 kg. (22-40 lbs), was linked with the best outcomes.

Similarly, women who begin pregnancy overweight (BMI 25-29.9) have a much wider range that is considered healthy—between 4.5-35lbs (2-16kg)

Optimal Weight Gain for Women with BMI’s 19.5-24.9

For women starting pregnancy in a normal weight range, the new study sets a wider range for weight gain: between 22-40 lbs (10-18 kg).

Optimal Weight Gain for Women with BMI’s 25-29.9

For women starting pregnancy in the overweight range, the new study sets a much wider healthy range for weight gain: between 4.5-35 lbs (2-16 kg).

 

2. But women who begin pregnancy underweight (BMI <19.5) have a narrower optimal weight gain range than the NAM’s.

Underweight women face the highest risk of having a small-for-gestational baby. To help their babies pack on pounds, they do best with some additional weight gain. But this new study actually narrows the range.

Optimal Weight Gain for Women with BMI’s <19.5

The new study found the optimal range was between 30-35 lbs. (14-16 kg), a narrower range than previously set for women considered underweight.

This study found the optimal range was between 14-16 kgs. (roughly 30-35 lbs). The NAM, on the other hand, recommends a wider range of 12-18 kgs. (26-40 lbs).

3. Women who fall into the obese category (BMI of 30 or higher) have the best outcomes when they gain less weight than currently recommended.

The NAM recommends that women with a BMI of 30 or higher gain between 11-20 lbs. But many doctors have questioned whether this amount is too high.

This meta-analysis affirms their concerns. For women with a BMI between 30-35, the best outcomes came with gains of between 2-6 kgs. (4-13 lbs.)

Optimal Weight Gain for Women with BMI’s between 30-35

For women with a BMI between 30-35, the best outcomes came with weight gains between 4-13 lbs (2-6 kg).

This news may be distressing to women beginning pregnancy with BMIs of 30 or above, especially as the majority exceed the current higher guidelines

But, it’s worth noting that although gaining less than 6 kg (~13 lbs) was ideal, the risk for adverse outcomes at higher gains really only rises noticeably at gains of 16 kg or more (35+ lbs).

For women with BMIs over 35 or over 40, gaining no weight at all was perfectly fine, and the researchers found the best outcomes occurred when women limited their weight gain to a max of 4-5 kg (9-11 lbs). 

Optimal Weight Gain for Women with BMI’s between 35 and 39.9

For women above BMI of 35, the researchers found the best outcomes occurred when women limited their weight gain to a max of 9-11 lbs (4-5 kgs).

 

Optimal Weight Gain for Women with BMIs above 40

To be clear—this amount of weight gain actually means fat loss during pregnancy, something doctors have long been wary of recommending. Pregnancy weight includes a 6-9 lbs baby, 2 lbs of amniotic fluid, a 1-2 lb placenta, and at least 4 pounds of extra fluid in your blood and tissues. That’s at least 14 lbs, without even taking into account breast weight gain.

managing back labor

But really—and this is the most important point—we can all relax about pregnancy weight gain.

Yes, the ideal ranges clearly shift in these graphs but here is the other thing to pay attention to: the risk numbers don’t shift much.  Even for gains outside the ideal range, the individual data points don’t jump up or down dramatically. And that’s exactly what this study found: Women’s weight before pregnancy has a much bigger impact than weight gain during pregnancy

Dr. Mary M. McDermott of Northwestern University Feinberg School of Medicine, and Dr. Linda Brubaker of University of California, San Diego drive this point home in an accompanying editorial. From now on, they assert, “resources should be dedicated toward ensuring an optimal BMI for all women of reproductive age rather than on [pregnancy] weight gain.” 

While pregnancy is still not a get-out-of-jail-free card when it comes to the cookie dough ice cream smothered in hot fudge, we can (and should!) stop obsessing over the difference between 34 and 37 pounds.

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About Amy

Amy Kiefer is a researcher by training, and earned her Ph.D. from the University of Michigan. She currently lives in the Bay Area with her husband and three children where she writes about fertility, pregnancy, and breastfeeding. Check out her blog, expectingscience.com, for more great evidence-based pregnancy and parenting info.

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