Marijuana will show up in your breast milk so your baby will consume it. Your baby will test positive for marijuana for 2-3 weeks afterward. Marijuana may affect brain growth. Marijuana tends to stay in fat, and breast milk is loaded with fat because babies need the calories. That's why marijuana shows up in your body for 30 days after, because it gets stored in fat.
Smoking marijuan is also a bad idea because marijuana reduces your initiative. That's why people smoking marijuana want to sit around. That's not a great idea when you have an active child who wants to run around and get attention.
Further, smoking marijuana, even once a week, is a bad idea because if anyone decides to mess with your life and report you, then you'll fail a drug test and the authorities might decide to take your baby for a few months until you can prove you're a good mother. So not worth the risk.