A woman has a baby by c-section (because even though she's only six months pregnant, the baby has supposedly doubled in size), she's out of it basically cause of the meds and all. There are two docs and two nurses in the room with her. They deliver her son, say he looks fine... then a little bit later another hopsital employee finds the operating room all bloody. The two docs and nurses are dead. The mother is out of it and the baby is lying on her belly. The baby's father has been in the waiting room through the entire operation. The cops are baffled as to how someone got in there and killed the docs and nurses without anyone having seen a thing.
The parents take the baby home and strange things start to occur, only the mother notices it at first. She finds several dead animals in the baby's crib and he'd been eating on them. She also finds some people (who have visited her) dead as well. She's heavy in denial about what her child is throughout the whole movie.
Eventually she breaks down and tells her husband that when she first found out she was pregnant, she was scared and worried about losing everything she'd worked for (she was in college) and her friend talked her into getting these pills off the internet that make you have a miscarriage. She took the pills and got really sick, but soon discovered she was still pregnant, so she decided to keep the baby after all. I guess the writers wanted the audience to believe that these mystery pills that the woman got from the internet are what caused the baby to become violent and basically monsterous (although the majority of the time it looked perfectly normal). The baby's father soon realizes what's going on and tries to put a stop to his son's killing spree.