I'm thinking my roommates should lower my share of the rent because they're having a baby
That can be troublesome. Do your roommates know about your thoughts on rent?
No, I'll just start to lower it bit by bit.
If you think that you have a good reason to adjust your share of the rent, you should talk to your roommates about it and reach a compromise. Having a baby in a shared household will be very stressful and it's permissible to ask for your rent to be adjusted on account of the the noise that will be present at all hours.
I'm going to go for the hit and run approach, put it out there and hope that they'll adjust my share.
Yeah, it never hurts to try asking about it! You don't know if they'd actually cooperate, given their circumstances of having a new baby that would disrupt what you'd want to do.