I am estimating that this portrait is from about the time of her marriage in 1539.
From HEN-Magonza's photostream; paint cracks in background removed by blurring, paint cracks in neckline removed by clone stamping, and discoloration of her left shoulder removed by clone stamping and blurring with Photoshop by gogm. Spots in coiffure removed with clone stamping by gogm.
Beatrix is wearing what may be a fall of wavy hair to complement her straight coiffure and a beret hanging from the back of her head. The black material above her puffed sleeves suggest she may be wearing hanging sleeves or a cape. Her over-sleeves appear to be a lattice left over after the material was almost all cut away in slashing.
Keywords: Clouet - Jean, Beatrix Pacheco, Countess of Montbel and Entremonts, comtesse de Montbel et Entremonts, Countess, French, Lady-in-waiting, Queen Eleanore, side curl coiffure, cap, vest bodice, crescent neckline, modesty piece, rolled sleeves, puffed cloth ornaments, hanging sleeves, girdle, natural waistline, puffed virago sleeves, arm bands, shallow necklace, draped necklace, jeweled crucifix, jeweled pendant chain