Nicole Kidman loves being maternal.

The 49-year-old actress - who shares an adopted son and daughter with ex-husband Tom Cruise, and two biological daughters with her current husband Keith Urban - has admitted she relishes the challenges of motherhood, but has come to realise she can't ''take care of everybody''.

Nicole explained: ''There is just the joy of being a mother; whether it's being an adoptive, biological, or foster mother, or a mother figure.

''Keith's always saying I'm so maternal. I like that. I am the eldest child in my family and so a lot of times the eldest child wants to take care of everybody.

''But I have had to learn that I can't take care of everybody. But a strong pull in my life is my maternal force and I love it. I think when you reach a certain age you go, 'I wish I had more children'.''

Nicole revealed she is especially keen to stay fit and healthy for the benefit of her two youngest children, eight-year-old daughter Sunday and six-year-old daughter Faith.

The Oscar-winning actress said: ''My two youngest children I have had much later in life and I would just like to be around for a lot longer so I can protect them and love them.

''That is probably my driving force right now, to stay healthy and vibrant so I can be around.''

Nicole said she is determined to protect her daughters until they are old enough to fight their own battles in life.

The 'Lion' star told the Express newspaper: ''Any mother will tell you this: I just want to protect them from the world right now. They are little, so you are always thinking about them.

''One of the big things in life as a woman is not getting yourself lost in that, being able to say, 'I will do my things', while at the same time the driving force is, 'How do I protect them?' It's bigger than the fear of disease, it's a primal instinct.''