When can I bring my baby in the shower?

36+ months. I'm not convinced that sharing a shower with a baby is ever a good idea. If you are sitting on a bench in an oversized enclosure with a hand held spray wand for baby i might see the process as less risky. Many adults are injured yearly slipping in a shower and i don't see exposing any baby to the risk. A sure footed 3 to 4 year old might be at less risk.
4-6 months. ...And mommy is sure she won't lip and fall in the shower while holding the baby. Also, test your shower to make sure it can't change temperature abruptly, such as when someone flushes. This is, at best, a riskier way to clean the baby than the standard baby bath.