Don't buy a bicycle trailer or a bicycle-mounted seat until your baby is at least 1 year old. We don't recommend bicycle trailers and bicycle-mounted seats for children younger than that because they may not be physically equipped to withstand the forces they'll be exposed to when riding in a bicycle seat or trailer. And when they're younger than age 1, they can't support their head properly with a helmet on, which all riders should wear.
Choose based on your needs, riding ability, and where you are riding. Trailers are "off-road vehicles"; use them only in parks and on safe, smooth trails where there's no risk of encounters with cars.
Follow the manufacturer's recommendations regarding the maximum weight, which is usually up to 100 pounds.
The better bicycle trailers have sturdy construction, tinted windows, a comfortable interior, and a wide wheel base. But before you buy, ask yourself if you will use the trailer enough to justify the price. If you think you'll use it only occasionally, buy the most durable trailer you can find at the low-end price. Also, consider how much weight you'll tow. If the weight of the bicycle trailer plus the passenger or passengers exceeds 50 pounds, you may start to feel like a beast of burden. Pedaling uphill can be especially difficult. At that point, maybe it's time for riders to get their own bikes.
Take your cycling ability into consideration. If you opt for a bicycle-mounted seat, you might find a rear-mounted seat with a child in tow unnerving and exhausting to operate. If you're a novice or not in top shape, you'd probably be better off with a front-mounted seat. If you go with a bicycle trailer, ride with only one child at a time if you're an infrequent rider. Finally, have your child wear a lightweight, well-fitting bike helmet, and never leave a child in the seat with the bike on the kick stand, which isn't made to support the weight.