I did it while breastfeeding DD, but she was about 9 months at the time.

As Em said, basically they just give you extra points to make sure you're getting enough calories etc to support your milk supply.

I lost weight just as fast as I have previously and it had no impact on my supply. I went to the meetings, found more motivation that way.

I've already decided that if after 12 weeks or so I'm still carrying too much weight, I'm going to do WWs again this time.