This person is said to be Elizabeth Sidney who lived between about 1585 to about 1612. She married Roger Manners, Earl of Rutland. If she was actually born in 1585, then something is amiss with this portrait.
Elizabethan-portraits.com had a version of this and shows the artist as being William Segar. Detail-enhanced by gogm.
Elizabethan maternity wear is shown in this 1595 portrait. The dress of this expecting woman appears to be of dark material under a layer of gorgeous white lace or embroidered satin, possibly decorated with pearls. Her floating ruff also appears to be decorated with pearls. Detail-enhancement shows hanging sleeves not visible in many Web images.
Keywords: 1595, Gheeraerts the Younger, Elizabeth Sidney, Elizabeth Manners, Countess of Rutland, English, jeweled framing headdress, hair jewelry, earrings, necklace, lace floating ruff, Empire bodice, square neckline, surcote, hanging sleeves, draped jeweled bodice ornament, jeweled long puffed sleeves, bracelets, jeweled skirt, maternity dress