This is the underwear I use with my 14th-16th C kirtles.
I wore this size smock all through my pregnancy and extended the slit in the front by only 5 cm so that I could access my breasts for feeding my baby.
Normally, I fold the front opening to make a v shape but when it’s cold I pull it up and fasten it with an anular brooch.
Here’s a picture of a woman wearing something you could interpret as the garment I’m talking about
Here’s the pattern I’ve made so that I don’t have to keep measuring an old one.
and here’s the layout of the pattern on 110cm wide fabric (the linen I have at the moment)