Stand back everyone, this baby's going to blow.

Factual Questions
1

So the other day I was thinking about how big dogs can breed with little bitches and when the puppies are born they are of a size that allows the she-dog to ‘lay’ them without the need to have pull-cords and toggles fitted to their parts at a later date so that if a sudden gust of wind blew up their behind it would turn them ‘inside-out’ like a rabbit in a butchers window.

So, my question - If we ignore all the moral implications, imagine a new-born baby that was born with a fully functioning womb, ovaries etc. Basically it had the ability to become pregnant.
If some evil scientists (aren’t they all?) fertilised the subject pretty much a couple of days after it was born the what size of baby would it produce?
Would the offspring grow to be of normal size or would it live a life in ‘The Land of the Giants’ ?

2

All concerns about actual maturity of the organs aside… Say, you grew that baby to adulthood without increasing its physical size?

In humans, the size of offspring is determined somewhat by the womb environment. Genetics plays a role, but there is such a thing as uterine growth restriction (IIRC), which suppresses development of the fetus to some degree in a uterus that is too small for its genetic potential. This is not generally considered to be good for the fetus, though. Young pre-teens who have kids with their not-yet-fully-grown organs tend to have small babies. Humans, in general (~90% of the time, based on the WHO expected ‘normal’ c-section rate of no more than 15%, only some of which would be due to cephalo-pelvic disproportion), do not grow babies beyond their capacity to birth. BUT, that’s the ‘norm’, not all, and because of modern medicine, there are women who are birthing babies they could not have birthed before.

Horses are much the same. Small mare, no problem, she will only grow the fetus to the size she can birth. You can cross a mongolian pony mare (actually a small horse) with a Tennessee Walker (very tall/large horse) and have small foals that grow to be bigger than their dams. Cattle, on the other hand, have to be bred carefully because fetal size is determined to higher degree by the size of the bull, genetically, and the fetus can easily outgrow the maternal capacity to birth it. Breed a small heifer to a large bull, and you may well kill the heifer, and the calf. (Any vets can correct me if I’m wrong on that, it is just what I remember from talking to my sister, akdogdoc, probably a decade ago.)

Does that answer the question?

3

On review, are you asking about mature size of the offspring after it grows up? Because that’s determined by genetics more than womb environment, assuming it was carried to term in the first place.

4

Small womb=smaller baby (realized that kind of got buried in there). But in this case, baby would probably not survive, because womb is TOO small.

5

So small womb=smaller baby, does that mean that if the process is repeated we could create a tiny race of humans? Not like dwarves but like scaled-down proportioned people.

If we could create a race of 12 inch tall humans and put them in a factory manufacturing walkmans, computers, mobile phones etc. just imagine how small and scaled down the products they manufacture would become.

6

Um, in short, NO.

Genetics determine what size you are ‘supposed’ to be. If you deny nutrients to a plant, and it makes seeds, and then you plant them, those seeds will attempt to grow the same size plant as the parental plants tried to grow to. It might be stunted from lack of nutrients in the seed development stage, but if you get seeds from that one, and the next generation, and the next, each will still try to grow the same sized plant as the source genotype.

If you want a smaller plant, you have to actually BREED for it (crossing in new genes from smaller plants, say), rather than just starving it and hoping it will somehow ‘remember’ that is was supposed to be smaller. Genes don’t have memories. It would be like having the mom cut her hair and assuming that all offspring would only grow short hair. Or giving mom not enough protien so her hair stops growing, and assuming that the offspring’s hair wouldn’t grow, either. The environment of one generation does not apply to the next, or not to significant degree.

Same here. Adult size is a combination of factors. Genetics gives you your ‘aimed for’ potential, where you’ll end up with proper nutrition and environment. Before birth, your environment affected the expression of those genes. After birth, your environment also affects development. No matter how small you were at birth, your body will try to acheive the genetic size ‘goal’, assuming you have adequate nutrition. Even if you manage to stunt another generation, you have to start all over each and every time, because each generation will grow to its genetic potential if given the chance. (Plus, if you go below a certain margin, you simply cease to have viable offspring.)

Example: a preemie child grows into a slightly-smaller-than-her-genes-aimed-for mom. She has a baby who is, due to maternal small size, slightly smaller than average. But the father is 6’ 4" and 280 lbs. Child is born slightly smaller than average, but proceeds to grow to be 6’2", because their genes (half from dad) said so. Only a tiny bit of the setup for growth is before birth. Prematurity or other forms of non-optimal growth cause smaller babies, and human development produces babies that meet the maternal size requirements for birth. But you’ve got a good 18 years beyond that to ‘make up for’ any growth issues as much as biologically possible. Most kids end up equalizing in size by age 4, actually (going up to the average range, or dropping down to the average range).

Take my family, for example. Gabe was born at 8 lbs 12 oz. He was rather on the large size. But his genes and his environment are developing him toward slightly taller than average. He will probably be (according to the best estimates of growth I can find) about 5’11". His birth size has little to do with how he grows after birth, his genes and his diet are the main factors. I, myself, was a 6 lb something kid, but am much taller than average (a smidge shy of 5’10"). My neice (who is the same age as Gabe) was about 7 lbs something, and is two inches taller than Gabe, at all of 5 years old. She got all the tall genes, apparently, and her smaller birth size has little to do with how tall she is now. I have an acquaintance who is rather tiny due to slight prematurity herself, who had an 8 lb 12 oz son (whom she birthed fairly easily - apparently those organs developed just fine). The son is looking to be about average, again, as he develops (her husband is also average).

As you can see, where they start is not necessarily where they end up. And each generation, you’d have to start over with the growth retardation to keep getting small babies at all - and that functionally prevents you from going below a certain margin on smaller ones, too - you would just have to get better at stunting them without killing them, and then it would be the environment, not the kids, that are changing. You can’t actually get them to be smaller babies and smaller babies and smaller babies by this method. Genetics is the template, environment before birth is just one set of inputs, and only has limited impact on lifetime growth. Without tinkering with the genes, you won’t be able to create micro-humans.

Does that make sense?

7

Yes, yes it does. You know your stuff. Thank-you.

8

This is completely anecdotal, but I think gives another good illustration of what’s being discussed:

My best friend is ~6’ 7" and his wife is ~4’ 8". Their first child was 4lbs, 7oz carried to full term, completely healthy and delivered naturally. She is now almost 4 years old and is less than a foot shorter than her mom. Her doctor is guesstimating that she’ll wind up a good piece north of 6’ tall. So the mother was a very small person and delivered a very small baby, but said small baby quickly “ramped up” on the growth, and will wind up almost as tall as her father! (supposedly - I’ve never really trusted the accuracy of doctors’ “predictions” of eventual height…)

critter42