While an infant-only car seat is a good choice for a newborn, it is not the only choice. Convertible car seats are available that will accommodate a baby from 5 lbs to 35 lbs or more rear-facing, and then can be turned to use forward-facing to 40 lbs or more. Each car seat's weight limits are posted on the car seat side labels, so you can easily find out whether a convertible car seat would be suitable for your baby. Convertible car seats are larger, so it may be difficult to find a good fit if your baby is on the small side. However, many average to larger newborns can fit in a convertible car seat and be perfectly safe. If your baby fits a larger car seat, it is OK to use a convertible car seat instead of buying an infant-only car seat and then moving to a convertible car seat when the infant seat is outgrown.
One potential problem with a convertible car seat is that a newborn baby's shoulders may be below the lowest set of harness slots. It's important for baby's shoulders to be at least even with those harness slots, or above them, when rear-facing. If you go for a convertible car seat, look for one that includes infant padding to help a small baby fit the large car seat, and one with a fairly low set of harness slots for rear-facing. Unfortunately, you won't be able to know ahead of baby's arrival whether or not your little one will fit a convertible car seat. Infant car seats are usually sized appropriately for newborns, so you're less likely to encounter this problem with an infant car seat unless your baby is very tiny.
If your car seat budget allows the purchase of two car seats within a year or so, an infant-only car seat is likely to provide a better fit for newborns, and these car seats do offer a measure of convenience that some parents find invaluable. Some infant car seats provide additional side-impact protection and anti-rebound features, as well as a deeper shell to nestle baby inside the car seat, so infant-only car seats are not without their advantages.
Regardless of whether you start with an infant-only car seat or a convertible car seat, the most important thing is to choose a car seat that fits your baby and your vehicle. Check the car seat labels to be sure your baby fits within the weight guidelines for that seat. Learn to install the car seat tightly and properly, using the manufacturer's instructions and your vehicle owners manual. Finally, keep your infant in a rear-facing car seat, whether that is an infant-only car seat or a rear-facing convertible car seat, until your baby has reached an absolute minimum of 20 pounds and one year of age. Rear-facing until age 2 means your baby is 5 times safer in a crash, so finding a car seat that rear-faces to 30 or more pounds is a wise investment.