According to a Fox news article, Mona Lisa was either expectant or a newly delivered mother. My vote is for newly delivered because her face looks thin.
From the article:
PARIS — Mona Lisa, the woman depicted in Leonardo da Vinci's 16th century masterpiece, was either pregnant or had recently given birth when she sat for the painting, a French art expert said Wednesday.
"Thanks to laser scanning, we were able to uncover the very fine gauze veil Mona Lisa was wearing on her dress. This was something typical for either soon-to-be or new mothers at the time," Michel Menu, research director of the French Museums' Center for Research and Restoration, said on LCI television.
Menu said a number of art historians had suggested that she was pregnant or had just given birth.
Researchers have established that the picture was of Lisa Gherardini, wife of obscure Florentine merchant Francesco del Giocond, and that da Vinci started painting it in 1503.