It’s supposed to signify that Gabrielle d’Estrées is pregnant.
Sorry to disappoint.
The woman sewing baby clothes is also pretty symbolic. Notice that Gabrielle is holding a Ring? The King had given her his Coronation ring as a promise to marry.
The unknown artist of the Fontainebleau School is conveying a message to the viewers.
Actually there is another earlier painting at the Uffizi Gallery in Florence of the same scene minus the nipple grab. Notice the hand gesture towards the ring finger symbolic of the Kings intent to marry her. Why the nudity? It symbolized fertility at the time.
Here’s the short version. The back story of this painting is that Gabrielle d’Estrées was one of several Royal Mistresses of Henry IV of France. Somewhere around 1590 the King fell in love with her and even though he was married to Queen Margaret of Valois their relationship was quite public. She was even at the Kings side during several battles not only to ensure his needs and care, but also became one of his closest advisers on all matters. She had such an influence on the King that he renounced Protestantism and become a Catholic under her influence.
You can imagine that some in the royal court did not take kindly to the fact that she had the King’s ear. They were probably even more infuriated when she began working with the King’s sister Catherine to try and help ease the tensions between the Huguenots and the Catholics of the time. Catherine a Huguenot herself and Gabrielle a Catholic became a very powerful enforcers of the Kings edict of 1598 bestowing rights to the Huguenots.
Even with all the the political intrigue she bore the King three children from 1594 to 1598.
Divorce at the time wasn’t easy and The King was waiting for the Pope to annul his first marriage because Marguerite de Valois couldn’t have any children and also give him the right to remarry. Engaged to the King, Gabrielle was by then pregnant with their forth child, when suddenly suffered a seizure and died. There were rumors of poisoning at the time. Their forth child a son was still born.
The King was heart-broken and gave her a Queens funeral.
Never prejudge a work of art until you know the back story.