> containment vessel to protect the mother (host) from the fetus (roughly, xenomorph) that would otherwise explode like an aggressive parasite (killing them both).

You can also flip the perspective the fetus is trying to survive in a hostile environment designed to strangle it. If it isn't clawing for every ounce of food and air it will become a miscarriage. It must interface with a system built for millenia designed to kill anything that doesn't have its code.

In truth, it is the equilibrium that evolution has achieved. Placenta must account for the most vicious fetus, and fetus must account for most vicious placenta.

I think in this metaphor the placenta is actually on the fetus' side and also had the baby's DNA.

Not to mention when multiple fetuses are involved. It's a miracle there are as many twins+ as there are.