Body Changes: Toddler, Britax

Seeking Advice on What to Watch Out for When Newborn Is in Car Seat

I will be a new mom in August and want to know what things I should watch out for when my newborn is seated in his/her rear facing car seat. I'm aware that I should have a mirror mounted facing the baby. Are there other signs/sounds that I should be watching/listening for and any gear that I should buy to ensure my baby will be safe in his/her rear facing car seat? I appreciate any input for this first time mom. Thanks!