Mainly I am referring to clothing for the expectant or breastfeeding mother.
There have always been young mothers in society, but what they wore has not always been passed from one generation to another. In some cases their normal clothes were adjusted to their needs, in other cases specialty clothing was made.
Fashions of Motherhood by Colonial Williamsburg discuses the maternity fashions of the 18th century.
Much has been written about how convinent a high empire waisted dress is for the expectant mother, to this day it is the staple of Maternity wear. The fashions of the Regency time were far more forgiving to the growing figgure than those that preceded it for centuries.
As a mother myself I have looked into what was worn and even tried recreating some items.
My advice to Guests who will be, or may become pregnant is that the Regency fashion is likely a better bet for you as it is easier to adjust to a changing shape and can be easier for nursing too.
For Nursing mother, I would suggest you look for the "Drop Front" or Apron Front" style of gown.
Jennie Chancey of Sense and Sensibility patterns gave a way to modify her patterns to be a drop front.
The Hungarian Chick has one of the best blog posts on this type of gown that I have seen, check out the pattern list on the bottom!