A slim profile around 17.5 inches wide at maximum is the key. Modern Nursery carries space-saving car seat designs built for three-across configurations from Britax, Nuna, Maxi-Cosi, and UPPAbaby. Look for slim convertible, all-in-one, or infant car seats marketed specifically as fitting three across, often with widths in the 17-inch range. Fit varies by vehicle, so check your back-seat width and the seat's dimensions before buying. Trying the seats in your actual car remains the most reliable way to confirm a three-across fit.

There isn't one single safest brand. The safest car seat combines strong crash protection with non-toxic materials, fits your child's size, and is installed correctly in your vehicle. Modern Nursery only carries brands vetted for both crash safety and chemical safety, including Nuna, Clek, UPPAbaby, Britax, Peg Perego, Babyark, and WAYB. Every seat in the collection is free of added fire retardant chemicals. When comparing options, look for features like rigid LATCH, a load leg or stabilizing support leg, an anti-rebound bar, energy-absorbing foam, and GREENGUARD Gold or GOTS certification.

Car seat expiration varies by brand and model. The expiration date is printed directly on the seat shell or label, and most seats are built to last several years before retirement. Some all-in-one designs are made to grow with a child for up to 10 years, and certain infant car seats feature expirations as long as 9 years on the high end. Always check the expiration stamp on your specific seat before using it, especially if it's been passed down or stored long-term. Registering your seat with the manufacturer also helps you stay informed of any recalls during its useful life.

A convertible car seat comes next. Convertible seats stay in the car and ride rear-facing first, then flip forward-facing as your child grows. Most kids move from an infant seat to a convertible seat once they outgrow the infant seat's height or weight limits, often between ages 1 and 2. Convertible seats hold rear-facing until at least age 2, then forward-facing with a harness up to around 65 pounds. After that, the next step is a booster. (Of course, you can always start infants out in a convertible car seat, too! Watch: Infant Car Seat vs. Convertible Car Seat)

Babies transition out when they outgrow the infant seat's height or weight limit, whichever comes first. Most infant car seats fit babies from 4 to 30 pounds and up to 30 inches tall. The exact timing varies from baby to baby, since some hit the height limit well before the weight limit. Once a baby outgrows the infant seat, the next step is a convertible car seat, which keeps your child rear-facing until at least age 2.