> Instead, the mother delivers and the neonate’s growth rate slows relative to its fetal growth rate.

Worth noting how massive the growth rate is I think: Even after birth, the baby gains weight faster than it ever will again in its life. In absolute terms! In relative terms it's even more insane.

A 12-year-old boy going through puberty will go from 90lb to 100lb over a year, an increase of 10lb or ~10%.

The average baby boy will be born at 7.5lb and weigh 20lb at one year old, an increase of 12.5lb or ~270%. Nearly triple the size!

Diminishing returns as the numbers of scale increase.

The growth might seem impressive when you do percentages of a very small number, but it's still a very small number overall.

It's not though, that's my point. Even in absolute terms, weight gain from growth in the first year is still more than any other time in the average person's life.

Putting on 6kg in a year is not that fast, people do sometimes gain weight faster than that in adulthood.

That's not growth, just fat accumulation which is a much simpler process.

How are you defining growth? Are you defining it by volume? Ie assuming the body mass density remains the same, how much does the weight increase (which means the body is volumetrically growing)?

If not--I am very lightweight right now, and I'm trying to put on body mass by eating more and exercising. While my volume will increase, my main growth is in density. My growth is not in fat, but actual muscle mass. Is this growth? If so, I have gone from 125lb to 140lb in ~2 months, which in absolute terms is obviously more than the baby (my goal is 160lb by the end of the year).

A few years ago, I went from ~130lb to 165 lb in three months--blame the free food at FAANG cafes during an internship. But this might be closer to what you were saying earlier--putting on fat.

Edit. I guess in the original statement saying that a person will never gain weight at this rate again there should be the explicit assumption that we're talking about natural growth, not growth due to external factors (ie more eating--intentional or not). In that case, that statement would be true.

I wouldn't say that growth through eating is unnatural though.

I guess what I might have implied by "natural" is growth from growing older as opposed to growth from nutrition.

But in that sense, of course a baby will have more growth than an adult--isn't it reverse exponential?

And a child puts on 100% bone and muscle?

No, but I'm not sure what point you're trying to make?

I was just pointing out that a baby does not gain weight faster than it ever will again in its life, in absolute terms. That's all. Not sure why I got the downvotes and no-true-scotsman arguments in return.

Plus fat and some water, yes. And furthermore, to go from a 10 pound baby to a 100 pound adult, I will make the claim that a baby has to gain 900% body mass.

I'm going by average weight gain from normal growth. But let's look at height instead then.

Average height at age 12: 59". At age 13: 62". 3" of growth.

At birth: 19". At one year: 29". 11" of growth.

Try matching that just by eating more.

(obviously, yes, it's easier for a small thing to grow than a large thing. But it's still crazy fast. Imagine being 50% taller in a year)

You talked about weight gain, in absolute terms. A novice powerlifter can easily put on both more absolute weight and absolute strength in a year than a newborn.

The image of a baby trying to outlift the weights that a novice weightlifter is lifting is now ingrained in my mind.

What I was trying to say is the baby is growing faster. If you stay at average weight throughout your life, like you're tracking along the 50% line on the growth chart, then your fastest weight gain is in your first year. On average that's when you gain the most weight (even in absolute terms), despite the fact that you're much much smaller than, say, a growing teenager.

But putting on 100% or even 200% of weight in a year is not normal or even possible for most adults without crippling their health. All babies do this.

I believe the GP was contesting the "In absolute terms!" part of the comment.

> Interestingly, the EGG hypothesis postulates that the constraining factor is not pelvis size, but rather that neonates consume so much energy that we give birth just before a women's metabolic limits would otherwise be overrun (leading to both maternal and infant death). Based on this, the increased energy demands of our larger brains mean that we have to sacrifice neonatal motor skills or drastically improve the metabolic efficiency of adult women.

This seems weird. Fully breastfed babies require even more calories from the mother. (They are bigger than unborn babies, and they lose more heat.)

Having gone through it, while it's true you need more calories after the baby is born (I breastfed and pumped exclusively and was told it's about 500 calories per day vs 300 or so extra prior to birth) the strain on your body is lessened because you are producing a single item (milk) going to one part of your body that's drained periodically.

In contrast, by the third trimester of pregnancy, your total blood volume has increased by approximately 50% (3-4 pounds of additional blood), not to mention an extra 2-3 pounds of retained fluid, 2 pounds of amniotic fluid, and 1.5 pounds of placenta. All told, this is another ~10 pounds on top of the baby. Additionally, the mother / placenta is performing all respiratory and blood filtration functions on behalf of the fetus (the kidneys don't really do much prior to birth other than produce urine).

> https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-...

Personally, while breastfeeding and newborn care was exhausting, it felt more akin to training for an athletic event vs having another entity symbiotically tied to my system like pregnancy.

Yes, lungs, heart, kidney and liver seem like more obvious bottlenecks than calorie intake alone.

I was mostly just remarking on the energy-demand hypothesis advanced in the comment above.

My first daughter is just in the process of weaning at the moment.

Perhaps it’s more than just calories or energy intake, but as well doing the breathing, immuno responses, digesting, filtering blood etc. for both mother and baby?

Maybe. But now we are just speculating.

> Instead, the mother delivers and the neonate’s growth rate slows relative to its fetal growth rate, keeping both the offspring’s and the mother’s energy requirements in check.

Growth rate slows after birth, so that would account for some of that. Mother’s also would not need to carry around the weight of the baby, and would be more able to provide for the child.

Women in Africa (in traditional societies), carried their babies on their backs while they worked the fields, carried water, etc.

Baby-wearing is pretty popular in modern times and other parts of the world too.

Though it's not 24/7.

> consume so much energy that we give birth just before a women's metabolic limits would otherwise be overrun

Since babies traditionally breast feed and thus survive solely on the mother’s energy after birth, this explanation doesn’t seem to hold water.

Breast feeding is quite hard on women and many women struggle to sustain the child on the breast alone. There’s a history of communal feeding which adds new energy into the equation and also mixed feeding once the child is old enough. I would say it’s probably a continuation at the peak metabolic rate described in the parent post.

Most women I know who struggled with breastfeeding were due to inability to have the baby correctly latched. Once the baby is latched, women shouldn’t have a problem with supply. Mother spends at most 500 calories extra when breastfeeding which most of the time comes from the weight that’s gained during pregnancy. I myself was able to exclusively breastfeed for 6 months after which we started introducing solids.

The truncated gestation is sort of taking that into account, at least according to this paper https://www.pnas.org/content/109/38/15212

> constraining factor is not pelvis size

I kind of doubt that, because if baby is after term, the risk is supposed to be childbirth itself being too dangerous. And they induce birth if the baby is too big too.

Large baby is considered a risk on itself.

Yes. And for even larger babies, c-sections are common.