From Wikimedia.
There are only minor differences between this dress and the one seen in this image.
Keywords: Pourbus - Frans the Younger, Margherita Gonzaga, Margherita Gonzaga di Lorena, Marguerite de Gonzague, Marguerite de Mantoue, duchesse de Lorraine, Gonzaga family, Lorraine family, Duchess, straight coiffure, bangs, tiara, feathered headdress, earrings, necklace, under-bodice, high vee neckline, jeweled lace-edged wide deep neckline ruff, tabbed over-bodice, vee neckline, brooch, tabbed hanging sleeves, order bows, false sleeves, ruffled cuffs, basque waistline, farthingale, saya