Five little known facts about pregnancy weight gain

Even for the most body-secure among us, gaining anywhere from 20 to 40 pounds during pregnancy is no picnic. Sure, we have days where we are full of energy and glowing and ready to flaunt our adorable baby bumps. But we also have days, especially in the third trimester, where we feel less like mama-goddesses and more like sweaty, frumpy, nothing-fits-anymore messes.

To take the edge off, I turned to the Institute of Medicine’s (IOM) report on pregnancy weight gain, which provides the evidence base for their widely-referenced weight gain guidelines:

• Underweight: Gain 28-40 pounds
• Normal weight: Gain 25-35 pounds
• Overweight: Gain 15-25 pounds
• Obese: Gain 11-20 pounds

Some of what I learned from their report was outright reassuring, like that a sudden bump up in weight in the second trimester is common and does not imply that you will continue to gain weight at a fast clip. Other facts, like that 15-30 percent of the fat gained during pregnancy goes straight to our thighs, were less reassuring.

But ultimately what became clear was this: Nothing in the actual research supports treating these guidelines as hard and fast rules. In fact, the evidence is murky for most outcomes related to pregnancy weight gain. There are complex tradeoffs between gaining too little and gaining too much, and pregnant women are not always in control of how much they gain–their blood sugar control, fat metabolism, and thus weight gain have been hijacked by one very adorable little co-passenger.

To see why, below I list five key–yet often overlooked–facts about pregnancy weight gain, taken from the IOM report itself:

1. Pregnant women do not gain weight at a consistent rate. Your weight gain will not be consistent from week to week, not during your whole pregnancy, nor even just from the second trimester on. This is surprising, because the charts most OBs use to track pregnancy weight gain like the one below typically show an even, steady increase of about a pound per week from the second trimester on.

There’s no nice way to put this. This is just nonsense. Your weight will come on in fits and starts, not at a steady rate.

In my 6th month of pregnancy, I shot up 8 lbs without a change in my exercise habits or diet. Disconcerted, I decided to get to the bottom of these charts. Was it normal to have a large gain in weight followed by what I hoped would be a plateau?

It turns out the the answer is a resounding yes. Studies of actual weight gain in pregnancy show it does not follow a straight line. Instead the pattern of weight gain looks more like a side-lying S, with a slow rate of gain in the first trimester, a more rapid weight gain in the second trimester, and then a slow down during the third trimester. In the last month of pregnancy, many women gain almost nothing or even lose a pound or two.

“The pattern of GWG is most commonly described as sigmoidal, with mean weight gains higher in the second than the third trimester across BMI categories, except for obese women.” – IOM 2009 report, pg. 101.

Do doctors actually expect women to gain weight at a steady rate? As unlikely as this seems, many women, myself included, have been harangued by their doctors for minor deviations from the “correct” trajectory. As Emily Oster describes in her book Expecting Better, she once received a long lecture when, sometime in mid-pregnancy, her trajectory pointed to a final gain of 36 pounds (unacceptable) instead of 35 pounds (acceptable). As she notes, nothing supports this–not the data and not common sense.

Her story is especially absurd, given that a jump up in weight in the second trimester is common and to be expected. This is because (2) most of the weight you gain in the second trimester is water. And water weight, unlike fat, can come on very fast.

The insane, unquenchable thirst that strikes in the second trimester? It allows your blood volume to increase by almost 50%.

“Plasma volume increases progressively to 50 percent by 30-34 weeks of gestation.” – IOM 2009 report, pg. 93

By the third trimester, the increase in blood volume and in amniotic fluid is largely behind you. So your weight gain mainly comes from gaining fat, on the baby, and on you.

(While I love this chart’s breakdown of weight gain components by week, it does show show a linear weight gain, which, as I noted above, is nonsense.)

(3) The fat you gain will not look like your normal pattern of weight gain, instead it will accrue on your back, hips, and thighs. Ugh! Pregnancy hormones drive this unique pattern of fat accrual.

Even worse news? For the average pregnant woman, a fifth or more of the fat she gains goes to her upper thighs. (I experienced this for all my pregnancies; I just had no idea it was so common. I had assumed it was a personal problem.)

“Based on serial measurements of skinfold thickness at seven sites made in 84 healthy, pregnant women, fat appears to be deposited preferentially over the hips, back, and upper thighs… This pattern of fat deposition is unique to pregnancy.” – IOM 2009 report, pg. 79

Bummer!

Ready for some good news? The thigh fat comes off first after birth. The bad news is that belly fat tends to linger. (And, despite the research described below, women did not need full body scans to tell us that.)

“Of the total fat deposition, 46 percent was in the lower trunk, 32 percent in the upper trunk, 16 percent in the thighs, 1 percent in the calves, 4 percent in the upper arms, and 1 percent in the forearms.” – IOM 2009 report, pg. 80

“Postpartum, fat was mobilized more completely from the thighs than the trunk, and non-subcutaneous fat in the upper trunk actually increased postpartum. Evidence obtained with computer tomography from 14 women suggests that childbearing may be associated with acquisition of visceral fat (Gunderson et al., 2008).” – IOM 2009 report, pg. 80

(4) How much fat you gain may not affect the size of your baby. Note I said fat, not weight. Total weight gain does correlate with the baby’s eventual size; fat gain does not. Greater water and protein gain in the placenta, uterus, and amniotic fluid all predict a bigger baby. More fat mass in your thighs, breasts, and stomach does not.

“Birth weight was positively correlated with gains in weight… but not FM [Fat Mass] gain. Lederman et al. (1997) also found that maternal weight and FFM [Fat-free Mass], but not FM [Fat Mass], at term related to birth weight” — IOM 2009 report, pg. 83

Why, then, do we advise women with a slow rate of gain to eat more, in order to avoid having a baby that is too small? There is no evidence that eating past hunger leads to bigger, healthier babies. Instead, these studies suggest that the arrow of causation goes in the opposite direction–healthy babies tend to promote greater weight gain.

Worse, what is there good evidence for? That excess fat gained during pregnancy is hard to shed.

“The extent to which fat mass accretion is critical rather than incidental to pregnancy is not clear, but unrestrained weight gain leads to postpartum weight retention.” – IOM 2009 report, pg. 102

(5) Swelling in your arms, legs, hands and feet, what doctors refers to as edema, varies a lot from woman to woman and can have a big impact on your third trimester weight gain. In your third trimester, gaining anywhere from 1 to 9 additional lbs. of water weight from this swelling is considered normal.

“Total body water accretion is largely under hormonal control and is highly variable during pregnancy…. For a reference 12.5-kg GWG [Gestational Weight Gain], total water gain at term is distributed in the fetus (2,414 g), placenta (540 g), amniotic fluid (792 g), blood-free uterus (800 g), mammary gland (304 g), blood (1,267 g), and ECF (1,496 g) with no edema or leg edema and ECF [Extracellular Fluid] (4,697 g) with generalized edema (Hytten and Chamberlain, 1991).” – IOM 2009 report, pg. 78

I often reflect on this 1 to 9 pounds variable when I think about my postpartum weight loss. Despite gaining within the recommended amount for both of my pregnancies (30 lbs), I shed only 15 lbs. the first month after delivery. My friends who exceeded the guidelines and gained 35, 40 lbs? They all dropped 20 to 25 lbs in the first couple of weeks. But their hands, feet, and ankles had swelled up like water balloons in their last few months of pregnancy; mine had not.

I guess what I would like to tell you is this: If late in pregnancy your wrists and ankles look like tree trunks, take heart. These swollen wrists and ankles probably foretell several additional pounds of rapid post-delivery weight loss!

If this were not enough to convince you to reconsider obsessing over the number on the scale, the amount of amniotic fluid present in the third trimester also varies by up to 2.2 pounds from woman to woman.

“Given the wide range of normal amniotic fluid volume at term, this compartment may affect maternal GWG by as much as 1 kg.” – IOM 2009 report, pg. 92

That’s right: In the third trimester alone, women vary in their water weight gain by up to 11 lbs. To me, this fact underscores the difficulty of determining the “right” amount of weight gain for any woman. What might be a reasonable gain for one woman, consisting of a small amount of fat, could translate to an additional ten pounds of fat for another woman.

This realization, more than anything, persuaded me that we–medical establishment included–should all relax a bit about the number on the scale.

(One important caveat: A sudden bump in weight over the course of a week can signal preeclampsia, a serious pregnancy disorder characterized by high blood pressure and protein in your urine. Unlike obsessing over pregnancy weight gain, checking for preeclampsia is a good reason to hop on the scale regularly throughout your second and third trimesters.)