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' Centre 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.
He said: "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.
"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.
He said: "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."
The experts from Canada's National Research Council said the painting was in fragile condition but should not suffer too much damage if taken care of properly.
The NRC said in a statement: "The wood panel on which the Mona Lisa is painted is sensitive to temperature and climate variations. However, if its current storage conditions are maintained, there is no risk of degradation.
"The 12cm split on the top half of the painting, which was probably due to the removal of the original frame and repaired between the middle of the 18th and beginning of the 19th century, appears to be stable and has not worsened over time."
The council had hoped to discover more details about Leonardo's "sfumato" technique of subtly blending one tone into another, which the artist used to create a hazy effect. But scientist John Taylor said the team had been frustrated by the lack of brush stroke detail on the painting.
"It's extremely thinly painted and extremely flat, and yet the details of the curls of hair, for example are extremely distinct. So the technique is unlike anything we've ever seen before. Leonardo was in a league of his own."
Close examination of the craquelure - the fine pattern of cracks formed on old paintings - showed the paint layers were still firmly attached to the poplar wood panel on which Leonardo created his masterpiece.
Taylor said: "We didn't see any sign of paint lifting. So for a 500-year-old painting it's very good news. And if they continue to keep it the way they have in an environment-controlled chamber, it could remain like that for a very long time."