More on pregnancy: the nitty gritty

One major thing that distinguishes us from primates is the knowledge and ability to choose whether or not to become pregnant.  When my husband and I were ready, we chose to “start a family” (stop using birth control), and I was grateful when it happened. I didn’t enjoy the nausea, the back pain, the stomach constriction, the reflux, but I did want to be pregnant. And though not everyone becomes pregnant on purpose, the methods or preventing pregnancy are readily available to people in this country.

As far as I can tell through my reading (mostly Frans de Waals primate behavior books), female chimps don’t really choose to become pregnant. When they ovulate, they get this enormous swelling on their bottoms that is essentially a biological advertisement that they are fertile. They cannot hide it, it is a blatant invitation to males to come and cash in on their biologic destiny. Then they mate, with multiple males, or are mated with, I’m not sure who is doing the asking but it sounds like female choice doesn’t play a huge role. Male chimps are pretty dominant. Multiple males can mate with a fertile female (leading to paternity questions, more on this in later posts). In bonobos, another close relative of ours, the situation is different, and it sounds like females are sexually receptive much of the time, and sex is a very regular part of the social dynamics. Even so, though ovulation and sex are not as linked, sex and pregnancy are related as in any other animal, and nothing I’ve seen indicates that females have a choice to be anything other than pregnant if they are sexually active during ovulation.


A female primate (chimp, I think) in estrous, found at a cool primate behavior blog, here . Never have I been more grateful for concealed ovulation. 

So already, the modern woman is pretty different in being able to choose to be pregnant. (Thanks, birth control!) Maybe it’s why we celebrate it so much, buy books about it (I still remember buying my first pregnancy book, appropriately titled “I’m pregnant!”) In chimps, anyway, pregnancy is apparently a totally normal effect following female fertility cycles. When a female chimp finds herself pregnant, does she feel joy? Annoyance? Fear? Does she complain? That I cannot answer.

Humans who choose to become pregnant often do so when it is a relatively good, stable time. Stable jobs, stable housing, a partner to help, all of these things if and when possible. Other primates don’t have this luxury. And, biologically, females are very much on the hook for gestating the fetus. Is there evidence that other primates get help feeding themselves, escaping predators, during this vulnerable time of increased energy needs and decreased mobility?

How extreme are the energy demands of a gestating female? How many extra tacos, so to speak, must one eat daily to make that beautiful baby after 266 or some odd days?

In the paper “Energy requirements during pregnancy and lactation” (Butte and King) the authors use much available data and several different modeling techniques to come up with a total figure for energy cost of pregnancy. They make the general points that energy costs will differ depending on your health/nutritional status pre-pregnancy, your body size, BMI, and metabolic rate, etc. Makes sense. They break down where this extra energy has to go, besides simply to the fetus: amniotic fluid, placenta, extra blood, bigger boobs, extra fat tissue that will reside permanently like a cottage cheese pillow around your navel. They combine data from things like skinfold tests, BMR tests , studies of protein and fat composition, and some molecular labeling tests that were beyond me. But the interesting thing is, using two different methodologies, they arrive at about 320,000-370,000 kJ of energy for the total cost of pregnancy (this range describing different weight gains for pregnancy).

What is amazing is that I understood only about half of the paper, but when I did a back-of-the-envelope style calculation, using an average pregnancy gain of 25 pounds, and the old diet benchmark of 3500 kcal/pound of flesh, which, when multiplied, gives you a total of 87,500 kcal. This converts to 366,100 kJ. Which is right in the same ballpark, and which is easy for a dummy like me to understand and is perhaps, if the similarities are an indication, not so far off the mark. Dividing that total 87,500 kcal total pregnancy cost over 266 days of pregnancy, you get an extra 330 ish kcal required per day. (Which is not news; any basic pregnancy book has guidelines for calorie intake.) As you might imagine, this average is not evenly spread, but is lower initially and increases as the baby grows in the second and third trimesters. So when you are “eating for two”, an idea most OBs now try to counsel you out of, you are eating for two, it’s just that the second is a lot, lot smaller.  The Academy of Nutrition and Dietetics breaks down the three trimesters in this way: they say no calorie increase is needed during the first trimester, an increase of 340 cal/day in the second trimester, and 450 extra cal/day in the final trimester. So these are all in the ballpark.

In evolutionary terms, this extra 300 something cals a day is significant. I mean, in today’s terms, eating that extra slice of pizza solves the problem, and feels awesome. (I still remember going, after a full dinner, to a school advisor function at my high school. There was free pizza, and I embarrassed myself by eating multiple pieces while we discussed upcoming school dances. I literally could not resist.) But to hunter gatherer humans? That’s a significant fraction of daily calories. To a primate eating leaves and fruit? That’s a challenge for the female who has no access to or appetite for free pizza. To a pregnant chimp, who is more or less making a living on her own, that is a serious energetic burden to bear.

What does this all mean? How has evolution prepared the female body for dealing with this energetic hardship? Has social structure favored help, in the form of male partners (in the form of monogamy, and yes, this shows up not just in certain mammals but in birds) or has it favored strong female-female bonds, like mother-daughter bonds (certain primate societies have matrilineal social structures). Or are our bodies equipped to store extra energy, in the form of fat reserves and lower metabolism (thanks, cellulite!) Do we mobilize energy differently, and are different different appetites or eating strategies selected for? Really I am just looking for something to justify my dark chocolate/peanut butter addiction.


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