Monaco's Princess Charlene arrived at a maternal ward in the principality on Wednesday, where she is expected to give birth to twins, witnesses said.

Father-to-be Prince Albert II, who last month said the babies were due around mid-December, was also seen arriving at the hospital.

The birth of the royal twins, whose gender has been kept a secret even from their father, will be announced with 42 cannon shots, 21 for each child, if all goes well.

The first children of the glamorous South African princess will replace Albert's sister Caroline as the next in line to the Monaco throne.

Albert — whose late mother was the Hollywood superstar Grace Kelly — said that as they were twins, it would be the first born who would succeed him.

"In the case that one is a girl and the other a boy, it will be the boy."

Albert succeeded his father, Prince Rainer, in 2005.

He had a daughter, Jazmin, 22, after a fling with former waitress Tamara Rotolo. He denied being her father for years before DNA tests proved otherwise when she was a teenager.

The prince also has a younger son, Alexandre Coste, 11, from an affair with Nicole Coste, a former Air France hostess.

Under Monaco's inheritance laws, neither of the two children have any claim to royal titles or to be considered as heirs to Albert because they were born outside of marriage.

They do however have legal rights to a share of his huge personal fortune, estimated by Forbes magazine to exceed one billion dollars.

Charlene, 36, a Zimbabwe-born former Olympic swimmer, married 56-year-old Albert in 2011 despite rumors that their relationship was on the verge of collapse.

Media reports suggested she had attempted to flee Monaco just days before their wedding.

It was also widely reported that the couple spent at least part of their honeymoon in separate hotels. A steady stream of pictures of the princess looking gloomy continued to fuel reports she was depressed.