This is one of those things where doctors tell you that you have to gain weight then are all "Whoa, easy there Horton" if you gain too much.
There seems to be two camps of women where this one is an issue: ones that are scared to gain any weight because they are so accustomed to keeping it off and women who use pregnancy as an excuse to unhinge their jaw and eat 20 buckets of chicken in a sitting (wafer thin mint, sir?).
So since the 1970s, the powers-that-be have advised women in North America to gain 25 - 35 pounds during their pregnancies. Women who are over weight are advised to gain 5-10 lbs less and women who are under weight are advised to gain 5-10 lbs more. Here is an approximate breakdown of where it's all going if you're carrying a single baby:
Baby - 8 pounds
Placenta - 2-3 pounds
Amniotic fluid - 2-3 pounds
Boobs - 2-3 pounds
Blood - 3-4 pounds
Fat & water - 5-9 pounds (or, if you're like me, 60 pounds)
Uterus increase - 2-5 pounds
Total: 25 to 35 pounds
As for the risks associated with tipping the scales either way:
If you gain too much weight: The only clear cut thing I could find on this one is that you are more likely to have a big baby, which can cause labour complications and puts you at a higher risk for having a cesarean delivery. There is also some talk of your giant baby becoming obese as an adult but the studies I read were a little wishy washy on whether it was because of pregnancy weight gain or just because obese people tend to feed their kids obese foods.
The other issues stem from going into your pregnancy over weight. This includes not taking off the weight from your previous pregnancies. It seems that if you start pregnancy overweight you are at higher risk for complications including gestational diabetes and preeclampsia and it increases the risk of breast cancer and type 2 diabetes later in life. But as far as I'm concerned that's a whole lot of fat chat that we already know about that has less to do with pregnancy and more to do with the risks associated with being over weight so they aren't shaking the donut free from my hand yet.
If you don't gain enough weight: You at higher risk of delivering a preterm infant or a low-birth-weight baby (under 5.5 pounds). But lets say you're a relentless barfing machine from morning sickness and we're not just talking about once here and there, but constant vomiting which is clinically called hyperemesis gravidarum. No long-term follow-up studies have been conducted on children of hyperemetic women but they don't appear to have any greater risk of complications or birth defects than other children. That said, it's treated fairly aggressively because it's damn unpleasant and because of the risks listed above.
On an interesting note, in England, women are weighed only at the beginning and end of the pregnancy, with medical experts citing the lack of evidence that monitoring weight promotes healthier births. Yet another reason I would move to Europe if I was going to give birth again – acceptable wine drinking on occasion being the first reason.
As for myself, I gained 65lbs in both of my pregnancies. The first time around I was meticulous about making sure I was eating all the right things then added some ice cream and a ton of french fries on top of it because I was HUNGRY. Not just peckish, but knock-over-the-kid-with-the-cookie-a-steal-it hungry. The second time around, I felt like shit so I ate Mr. Noodle and whatever my toddler dropped on the floor. Yet I still gained the same amount of weight.
There are a few people that think you are a little pre-disposed to gaining a certain amount of weight regardless of what you eat or don't eat. I like these people. They can move to Europe with me.
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