Lots of plot holes here.

Rating: 5/10 (26 November 2018)

Other reviews cover the main problems with the plot, I agree mostly with them.
Three things though, I believe they decided to have a baby to help overcome the loss/grief of their Son.
The reason a single shotgun blast killed the monster is due to the fact that the high frequency noise made the monster retract/reveal its vulnerable spot (on its head).
The nail on the stairs had its head pulled off by the bag Emily Blunt was carrying, this is why it looked like it had been hammered 'up'.
Beyond that other reviews cover it... I read this could have been part of the Cloverfield universe, don't get me started on that. Purchased for $10 sight unseen, so I guess I saved a bunch not going to the theater.