The discovery was made by a team of Canadian scientists who used special infrared and three-dimensional technology to peer through hitherto impenetrable paint layers on the work, which now sits in the Louvre museum in Paris.

Bruno Mottin of the French Museums' Center for Research and Restoration said that on very close examination of the painting it became clear that the Mona Lisa's dress was covered in a thin transparent gauze veil.

"This type of gauze dress ... was typical of the kind worn in early 16th century Italy by women who were pregnant or who had just given birth. This is something that had never been seen up to now because the painting was always judged to be dark and difficult to examine," he told a news conference.

"We can now say that this painting by Leonardo da Vinci was painted to commemorate the birth of the second son of the Mona Lisa, which helps us to date it more precisely to around 1503."

The young woman with the ambiguous half smile has been identified as Lisa Gherardini, wife of Florentine merchant Francesco de Giocondo. She had five children.

Mottin also said that, contrary to popular belief, the subject had not let her hair hang freely but in fact wore a bonnet from which only a few curls managed to escape.

"People always wrote that the Mona Lisa had allowed her hair to hang freely over her shoulders. This greatly surprised historians because letting your hair hang freely during the Renaissance was typical of young girls and women of poor virtue," he said.

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