My Google-fu is weak this morning and my sister’s coffee grinder just bit the dust. Le sigh. Bear with me.
There’s an image in Janet Arnold’s 16th/17th century Patterns of Fashion (try to get it through interlibrary loan; it’s absolutely wonderful for research), of a high-necked bodice/doublet that’s slashed from the shoulder to practically the waist. It’s held together in two points (I think), with brooches or buttons. She’s accompanied by a toddler-aged child. You can see the underbodice through the slashes, but I think it would look just as period with your chemise pulled through. You could slash the chemise in the appropriate place and hide the slashes in the gathers.
You could also do something simple like this. It would be easy enough to either slash your chemise strategically or to use the (shudder) elastic drawstring suggested by MrDibble. Or you could do a keyhole neckline and tie it shut. You could get away with wearing a nursing bra under this, I think. If anyone objects, use The Mom Look. You know, the one that lets grown men know they’re in timeout.
My next suggestion was going go be adhemar’s. shakes fist
If you want to go slightly fancier, you could go Landskenecht/German Ren. Here, here, and Hungarian version here and here. Most people pin or baste that highly decorated piece of fabric at the top of the bodice in place. You could cover slits in your chemise with it or take it off to pull your chemise down. Plus, this style calls for some seriously big hats, if that’s your thing.
My favorite option is this, Italian Ren, ca. 1520. It’s the same one that I pimped to FaerieBeth in the thread that brought us here. It ties shut, you could put boning/hemp cording in the bodice for support, and it’s a style that flatters big busts. And here’s the best part: the woman who made it has a rundown on it at her website. This would be my choice, definitely.
Just as a warning, I’ve never been pregnant or breastfed, so I’ve absolutely no experience with this sort of thing. This is all wild conjecture, that I hope helps you out. I’ve read the saga of WhyBaby, and it’s so amazing that she’s strong enough to take to RenFaire. I hope you both have a blast.