Way back in 2014 I made my red bliaut and gold underdress ensemble for the coronation of TRM Konrad and Kortland. Here is the original post about that, and I will say that those hand-felled seams have lasted incredibly well. The garb still looks good, and has only needed a few minor repairs. Should last a good many years more, though I would like to line the bliaut sleeves and add trim at some point. Anyway, that red dress had a bug that turned out to be a feature. I cut the v-neck way deeper than I meant to, and it reaches almost to the bottom of my ribcage. Wearing the dress without an underdress proved that I could comfortably shift the neckline and get a breast free for nursing, but I still need an underdress for the sake of decency and historical accuracy, of course, so I decided to get creative.
I have looked at a lot of period nursing images to figure out how women did it back in the day. You can see a selection on my Maternity, Nursing, and Kids' Garb Pinterest board if you're interested. Nursing garb in period seemed to either involve vertical slits over each nipple, pulling a breast out of a low neckline, diagonal slits from center front over the nipples, or front-opening dresses, but as a bustier lady I like a little bit more coverage, so I used a different technique copied from my favorite nursing top. I tried on the good old gold underdress, marked the line below my bust line across the front (about where my bra band sits), then girded my loins and cut a horizontal slit from one side seam to the other along that line. I double turned the edges under and sewed them down with my machine. Here's a visual.