Maternity and Nursing Gowns: a few examples from the Fourteenth and Fifteenth centuries
    Piero della Francesca Madonna 1467, note how her gown laces in front and under the arm in the side seam.
      Both lacings have been loosened, showing a way to make room for her pregnant belly.
Madonna del parto (detached fresco), Chapel of the Cemetary of Monterchi (Arezzo)
In this Visitation picture by Giotto di Bondone in 1306 it is easy to see that the loose clothing of  the beginning of the fourteenth century accomodates a pregnancy with very little effort.  The cotta style worn by the Virgin and by St. Elizabeth differs from their attendants only by eliminating the belt to allow the cotta to hang free.  Note also the use of a sopravesto (surcot, surcotte, cyclas) by Virgin.  It will continue to appear as maternity wear into the sixteenth century.
Giotto di Bondoni, Scenes from the life of the Virgin, Cappella Scrovegni (Arena Chapel), Padua