Princess Märtha Louise and her husband, author Ari Behn, with their two daughters in December. PHOTO: HEIKO JUNGE / SCANPIX

The couple already has two daughters, Maud Angelica, born in 2003, and Leah Isadora, born in 2005.

An official statement from the palace said the new royal baby is due in mid-October. None of Princess Martha Louise’s and Ari Behn’s children, however, is granted a royal title.

The princess was said to be "in fine form" and that a normal pregnancy was expected. She and her family intended to spend the first half of next week's traditional Easter holiday with her parents, King Harald and Queen Sonja, at the royal mountain cabin known as Prinsehytta in Sikkilsdalen.

The new baby will be the sixth grandchild for Norway's king and queen. Crown Prince Haakon and Crown Princess Mette-Marit have two boys and a girl.

Palace officials, meanwhile, wouldn't reveal where the crown couple would spend the Easter holiday. "It's private," said a spokesman. Haakon and Mette-Marit have been in the San Francisco Bay Area this week, attending a conference on AIDS.