While it’s reasonably easy to eat in for a little while, and to stay off the lake for a year or two, biking can be harder to cut out of your life once Baby comes along, especially if it’s one of your main methods of transportation. Fortunately, Baby is growing into a little bike passenger significantly sooner than she will be ready to balance on waterskis.
While there’s no real research and no definite rule about when Baby can join you on your biking adventures, most guidelines suggest waiting until she is 9 months or a year old, since at that point, her neck and core should be strong enough to hold her head up and steady, and for the weight of the helmet not to be too much for her. These guidelines go for both bike seats and bike trailers, since both are subject to the same amount of bouncing and vibration, and both should always be accompanied by a helmet.
You can check out the manufacturer’s guidelines for the appropriate age to start Baby in any given seat, and ask her doctor about their opinion.