Denmark's Crown Princess Mary is expecting twins.

The Australian-born royal - who already has two children, Christian, four, and three-year-old Isabella, with husband Crown Prince Frederik - has been at the centre of pregnancy rumours for months.

I'd like to have that experience again. That boundless freedom of childhood is so wonderful. The world is so different for a child, waking in the mornings, wide-eyed and ready to take it all in.

The royal is expected to give birth to the two babies in January in Rigshospitalet - the national hospital in Copenhagen - according to an official statement by the palace.

However, the pair have denied suggestions the princess fell pregnant after undergoing IVF fertility treatment, and said in a statement: "It's a natural pregnancy".

Speculation the couple were expecting a third child was sparked when numerous pictures showed the princess, 38, wearing loose-fitting clothing and she declined to go horseback riding during a recent visit to Graasten Palace.

Mary and Frederik have been trying to extend their family for over a year.

The princess admitted last March she wanted more children and looked forward to the joys of motherhood once again.

She said: "I'd like to have that experience again. That boundless freedom of childhood is so wonderful. The world is so different for a child, waking in the mornings, wide-eyed and ready to take it all in."