You know how in the Alien movies those dudes walk around, supposedly for weeks, with an alien growing in their chest cavity, and have no idea until the thing erupts at the dinner table ready to suck face?
I'm here to tell you people that there is no way in hell you can have a fetus of any variety growing inside you and not know.
At the very least, when their 8 (or 2) legs twitch and stretch and move you are going to feel it. You are going to feel that foot kick your kidneys, shove your liver out of the way, stretch towards your diaphragm and lodge in your ribcage. I mean really, where else are they going to put it?
So this theory that they could ignore the movement of a multi-legged animal growing in their chest? I don't think so. The heartburn alone would kill them. Not to mention the other digestive issues that come with having your organs displaced and squished to one side, the heart palpitations, the sleep disruptions and the mood disturbances. Sure, the hosts are mostly guys in the movie (right?) but even guys aren't that oblivious to changes in their body. And it's funny that something that big could be inside them and they don't even notice some weight gain or even a pot belly? I mean where do all their displaced organs go? Are the aliens dissolving them as they go? In which case I have to protest again that they would not notice.
All I'm saying here is that when a 4 lb baby is inside you, anywhere? You know. You know pretty much 24/7. And as they take that left foot and lodge it in your pancreas, and then take that right foot and try to lever it agains your abdominal wall, while taking their left hand and jabbing into your hip while maintaining a handhold on your urethra? It just is not comfortable. Because, deciding whether to pee, eat or breathe first is a losing proposition. And God help you if you sneeze.