Palace confirms what we already knew:

The Princess of Monaco is expecting twins at the end of the year!

The Palace's press office release a statement earlier today largely confirming what was already known.

"Le Prince et la Princesse de Monaco sont heureux de confirmer l'attente de jumeaux pour la fin de l'année."

On May 30, Derek Watts, a South African reporter, and a friend of Mike Wittstock, father of Princess Charlene, sent out this tweet:" My old bulawayo schoolmate mike wittstock has phoned to say his daughter is pregnant with twins. Congrats princess charlene and albert!"

This comment was never refuted by the palace.

Succession to the throne is male primogeniture.  If Charlene has a girl, then a boy, the boy is the Hereditary Prince.  If she has two girls, the elder will be the Hereditary Princess, but if Charlene has another child, and it is a boy, the boy will move ahead of his two older sisters.
http://royalmusingsblogspotcom.blogspot.com/2014/05/twins-for-albert-charlene.html