I'm a bit worried about something going on with my wife. She's 18 weeks pregnant, and for the past month, she's been experiencing these shivers in the middle of the night, like once a week. They only last a short time, maybe 30 seconds to a couple of minutes. She feels really cold when it happens, but then shes back to normal pretty quickly. Is this something we should be concerned about? Its only happened twice, but I just want to make sure everything's okay. What do you think?