Why did they get pregnant?

Maybe it was accidental.

Seems pretty obvious that it was because they lost one of their children...

Yeah they had access to it, until it ran out. They never said whether the pregnancy was deliberate or simply caught too late. Not many abortion clinics still running I would guess. So if you manage to get pregnant, having the quietest sex ever so the aliens don't kill you, you're pretty much carrying that baby to term. I thought at first that the enclosed crib with the gas cylinder was their humane way of putting the child down once it was born. But clearly they had one soundproof room prepared and were probably planning to do the whole basement over the next year or two. Then the child would have a place to live (and learn to talk) until they were old enough to be reliably quiet outside.

Only one way to stop the human race from going extinct... People in this scenario should be having as many babies as humanly possible, very carefully of course.

so we can have a stupid climax to the movie.