If humans were altered so that pregnancy lasted only 3 months, what other biological challenges would this cause? Obviously it would have be at substantial increase in calorie intake to support the accelerated growth of the baby but by how much? What other challenges what would arise?
Not being female, I'm at a disadvantage to answer, but just to get the ball rolling...
- You've mentioned caloric intake. Broadly speaking, the calories needed for the child must increase 3X. A quick Google search reveals:
If you are pregnant with one fetus, you need an extra 340 calories per day starting in the second trimester (and a bit more in the third trimester). That's roughly the calorie count of a glass of skim milk and half a sandwich. (Source)
In other words, women would need very little change of diet for the first month (though they might lose a little weight), but they would need about 1,000-1,200 more calories a day thereafter.
Women would have back and spine problems. Even with nine months, many women by the end of their term have discomfort-to-real-pain carrying the nearly-born child. You're pushing all that time of mind and muscle development to hold up the baby into just three months. A human body probably wouldn't have the time necessary to adjust to the change and distribution of weight unless the woman were athletic to begin with.
There is a lot of chemistry going on in the female body that humanity is only just beginning to understand. It includes the mother's psychological bond with the child and the development of immunities. Remember how women are stereotyped as moody during pregnancy? That stereotype comes from very real behavioral swings due to these chemical changes — and you just accelerated them 3X.
And I'm not going to think about what might happen if you 3X morning sickness, but I'll give you two words: hyperemesis gravidarum.
Finally, a woman's body is designed to stretch with the growth of the child. A 3X acceleration in the birthing process may create substantial skin and muscle damage as the body is forced to accommodate the rapid growth of the child. And the ability to recover from that condition might be compromised, too. Have you ever seen a senior citizen who successfully lost a lot of weight? The reason they have huge flaps of skin is because at their age the body can't reabsorb the mass like it could in its youth. I could easily imagine that kind of problem due to too rapid expansion.
To add to the answers: Cancer and Higher miscarriage rate.
The increased metabolic activity of development creates more heat. This increased heat generation will cause problems for the mother, in terms of just being hotter. More significantly it will up mutation rate, especially in rapidly dividing cells. The increased mutation rate will have increase rate of miscarriages.
This will be stacked upon the increase of the mothers metabolic rate to digest more food to provide nutrients for the child.
Evidence for this is higher rates of miscarriage during prolonged heat waves.
Its probably impossible realistically, 2X as fast is the theoretical limit.
Some great work on what factors limit human developmental speed shows humans can only manage a sustained metabolism of around twice their normal metabolism, this holds true of human mothers regardless of condition. near the end of nine months is when the metabolic demand reaches 2.1 times base metabolic rate, which is when human end gestation, when the body can no longer keep up with demand.
fetal demand in black, mothers demand in grey.
Now of course modern humans can supply more nutrients, but they can't push their metabolism much further, the hypothetical limit on metabolic output is around 4 times Base Metabolic Rate which is how much the the highest output athletes can sustain for extended periods (Grand Tour athletes). note some mammals achieve higher sustained BMR multipliers but humans have remarkably high caloric needs so I will stick to what humans can do in practice. This will leave the mother almost completely helpless, just resting, her metabolic rate is as high as the most extreme athlete's maximum, never mind if they actually had to do anything. These athletes are consuming 7-8000 Kcal per day, so the mother should also be consuming at least this much to be realistic near the end of the pregnancy.
A realistic number for 6 month development would be 3000 KCal/day in the first trimester, 6000 in the second, and 8000 kcal in the last trimester. This is because in the first trimester the limiting factor is how fast fetal cells can divide, while by the end how fast the mothers body can digest, move and metabolize nutrients is the limiting factor.