When you attend hospital with a fear that
a) you could die,
b) your baby could be at risk
your adrenaline will be high/you'll be in fight or flight, your immune system will therefore be compromised and you're more likely to deteriorate.
It is my strong belief that the fact that many vaccinated people are avoiding harsh symptoms and hospital is largely because having the vaccine makes them less focused on symptoms, less fearful, less likely to test and pick up that they have covid, so less likely to be scared they will die, therefore less likely to go to hospital and therefore more likely to just look after themselves at home and get better.
Feels like common sense really. There is a strong link between the power of the mind and the severity of illness.
There will of course be no data around the differences in fear levels and hospital attendance between unvaccinated and vaccinated pregnant women (or people generally) but my guess would be that a big reason vaccinated pregnant women are less likely to end up in hospital is because they are less scared, less likely to test, therefore less likely to pick up that they have it and therefore less likely to go to hospital and more likely to just get better at home. Anecdotal evidence, e.g. conversations with friends, family, clients, supports this.
Side note, I do think if people have underlying health conditions, then they are more likely to get sick, vaccine or no vaccine. My post above applies to ordinarily healthy individuals. Really, if you're healthy, the risk is very low. Worrying all the time is probably more harmful overall.
All just my opinions.