We know very very little about the long-term effects of 0.166g on human health, because it's never been done. (best guess: also not good).
https://www.space.com/international-space-station-mouse-embr...
Skepticism is good, but ignoring related work weakens an argument.
But we don't know anything about the long term effect of 0g on human fetuses, which live in a very different environment than the humans we have tested. They live in an environment that combines fluid immersion and surface support, with buoyancy playing a major role -- which could (or could not -- absence of evidence etc) seriously change the importance of gravity for development.
I'd be more concerned about the impact of zero and low gravity on newborns than fetuses.
I agree with that. If (and it's if) it turns out that zero and low gravity are OK for foetal development, then there's the around 20 years of development that comes after birth and before adulthood, where "fluid immersion" is not part of the normal development process.
That's how reason works. "You can't prove there isn't a silver, bubblegum-farting unicorn living on the asteroid belt" doesn't make it true. Nor more plausible.
But the idea that this is "Nor more plausible" is wrong - there are good reasons to believe that human health and development that has occurred under 1g throughout our entire evolutionary history could go wrong when deprived of that, is in fact a highly plausible idea.
The "extraordinary claim" would be that there is no impact on human health and development from living entirely in (for example) lunar 0.16g.
I'm all for looking into it getting the data, but no space agency has yet constructed the required fractional G rotating structure. Nasa has made plans, see e.g. Nautilus-X (2) and AGOS study (3) but nothing has yet been built "due to budget constraints".
1) https://en.wikipedia.org/wiki/Russell%27s_teapot
2) https://en.wikipedia.org/wiki/Nautilus-X
3) https://www.researchgate.net/publication/319164912_AGOS-Arti...
> By changing the rotation rate AGOS Stage 1 can be used as a testbed for different g-levels and their influence on human health.
1. The twisting and folding of the heart tube is highly dependent on gravity and micro-pressures of circulating blood in the embryo. I learned that from Dr. Larry Taber at Wash U in St. Louis. In microgravity, there's a very strong chance the heart forms incorrectly or if it does form correctly, it conditions itself for zero-g life, so it will have reduced pumping strength because it never needs to move blood from toe to head against gravity. So, even if you gestate a kid correctly in microgravity, the transition to an environment _with_ gravity could be extremely stressful on the body or possibly fatal.
2. Other phases of gestation _depend_ on gravity. The "baby dropping" around the beginning of third trimester is important to kick start the body to prepare for birth. The baby presses on the cervix to stimulate dilation during the process of birth, etc.
