24 weeks into my first pregnancy, I was feeling smug. I had been eating right, continuing to exercise, and the last dregs of first trimester nausea and fatigue had faded. I was feeling good.

That is, until my OB took me down a few notches at my 6 month appointment.

She told me I had gained “too much weight”. She passed the chart. It was all there in black and white.

Although I had been on target to gain the “correct” amount of weight by the end of my pregnancy, I had gained a whopping 8 lbs in a month. I was now on a trajectory to gain over the upper limit of 35 pounds. She began what was clearly her set lecture. “Eat less bread. Eat only half of the sandwich…”

But as she spoke, my mind began to wander. I was pretty taken aback. I silently reviewed my recent eating habits. No, I did not think they had changed. Certainly not enough to cause a gain of 8 lbs in a month. And yes, I had been exercising my normal amount. If anything, I had become more active in my second trimester. My energy levels had gone up in the last couple of months, not down.

Convinced that I had done nothing dramatically different in the last month, I started to wonder whether we could accurately or reasonably expect a steady rate of weight gain. Maybe it’s normal to gain a lot of weight in the middle of pregnancy, and less towards the end. Maybe pregnancy weight gain looks more like a curve than a line.

So I went home and did a little searching.

I found the 2009 Institute of Medicine (IOM) research review on pregnancy weight gain, the source of the current weight gain guidelines: 25-35 lbs for normal weight women, 28-40 lbs for underweight women, and 15-25 lbs for overweight women. (Most pregnancy-oriented sites repeat these guidelines, but provide no explanation of why those amounts are recommended.)

To be clear: I don’t have a beef with guidelines. Guidelines are useful for medical practice. But we have to remember that guidelines derive from messy data, incomplete information, conjecture, and compromises made by a committee.

All too often, once codified, guidelines become elevated above all their underlying messiness and instead become rules.

So, let’s go back to the actual data, to what we actually know about pregnancy weight gain.

1. First of all, a sudden bump up in weight in the second trimester, like the one I experienced, is not only common, it is the norm.

“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.

2. Pregnant women do not gain weight at a consistent rate from week to week. Not during a whole pregnancy, nor from the second trimester on. Studies that track the rate of weight gain find a pattern of gain that looks more a side-lying S than it does a straight line.

This is because women usually gain the the largest amount of weight in their second trimester. And this in turn is because…

3. Most of the weight gained in the second trimester is water. Compared to fat, water weight can come on very fast. In the second trimester, blood volume increases by almost 50%. This is why many women experience extreme thirst in their second trimesters.

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

By the third trimester, the ramp up in blood volume and amniotic fluid is largely behind you. Weight gain tapers off. In fact, in the last month of pregnancy, many women gain almost nothing or even lose a couple of pounds.

Chart Title: Where the Weight Gain Comes From, Week by Week

*Note that this chart shows a linear weight gain, which is inaccurate. I include it because it is the best chart of the components of weight gain I have seen, despite this misrepresentation.

As shown in the chart above, much of the gain in the third trimester is fat. But there is an important caveat. 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 third trimester weight gain. In the third trimester, women may gain anywhere from one to nine (nine!) additional lbs of water weight from swelling. And this is completely normal.

“Total body water accretion is largely under hormonal control and is highly variable during pregnancy…. For a reference 12.5-kg GWG, 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 (4,697 g) with generalized edema (Hytten and Chamberlain, 1991).” – IOM 2009 report, pg. 78

In addition to swelling, the amount of amniotic fluid present in the third trimester varies by up to 2.2 lbs.

“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

Adding this up, in the third trimester alone, women with perfectly normal, healthy pregnancies can vary in their water weight gain by up to 11 lbs.

Why do I bring this up? For me, the wide range of water weight gain underscores how hard it is to determine 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, would translate into an additional ten or eleven pounds of fat for another.

I often think on this 1-11 lb. range in reference to my own postpartum weight loss. Despite gaining within the recommended amount for both of my pregnancies (30 lbs), each time in the first month after delivery, I shed only about 15 lbs. 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 they, of course, had swelled up like water balloons in their last few months of pregnancy, while I had not.

This realization, more than anything, persuaded me that we–medical establishment included–all need to relax about the number on the scale, and the rate of weight gain.

What was your experience with gaining weight in pregnancy?