Safety is the most important reason to start your newborn in a dedicated rear-facing infant seat. These seats also offer the convenience of a detachable carrier that clicks into a base. You leave the base in the car, making it easy to pick up the baby in the seat and go. (See our infant car seat video.)
Rear-facing seats are a must for babies until their second birthday. But we recommend using a rear-facing seat until your child is 2 years old or at the upper weight or height limits for the seat. The American Academy of Pediatrics also says children should ride in seats facing rearward until they're 2 years old or reach the maximum height and weight for the seat. While some preemies might require special seats, there are infant seats for babies weighing as little as 4 pounds. Many infant seats have maximum weight capacities of 22 pounds, but some have weight limits up to 30 or 35 pounds. See our infant car seat Ratings, available to subscribers, for a range of tested models.
Infant seats come with a built-in five-point harness that secures the baby. They can only be used rear-facaing, and should be installed at an angle that provides optimum protection without interfering with baby's breathing. (Infants have little to no head control, and their airway can be cut off if their head slumps forward. The baby's seat should be reclined at an angle typically between 30 and 45 degrees.
Most infant seats come with a base that's installed in the car. You can then remove the seat using the built-in handle and snap it into a compatible stroller or universal frame. You can buy an extra base for most models--useful when one caregiver drops baby off and another picks him up in a different car. The Maxi-Cosi Mico Infant Car Seat ($180), for example, comes with a base; you can buy a second base for $60. Most infant seats also allow you to install the seat/carrier in a vehicle without a base, but a secure installation could be more difficult without a base, and some seats have lower maximum weight limits when installed as a carrier only.
Many car-seat manufacturers offer a "travel system" including an infant car seat, a base, and a stroller or frame that you snap the infant carrier into. Many stand-alone strollers can accommodate infant car seats from various manufacturers (see stroller types). The upside of a travel system is that they're a good value. But it's worth noting that these systems can be bulky. If you have to negotiate subway stairs, or your trunk is small, a separate car seat and seat-carrier frame or compact stroller might be a better choice.