Taking a lead role in Pirates of the Caribbean: On Stranger Tides opposite Johnny Depp's Captain Jack Sparrow was in itself a challenge for Penélope Cruz, then she found out she was pregnant only a few weeks into filming.

The actress said she loved being "pregnant and a pirate", telling Vogue magazine it was "a beautiful experience".

Cruz has taken on a wide variety of roles throughout her career but said she was not prepared for the amazing emotional reaction she experienced when she met her son Leonardo for the first time in January.

"From the first second, you feel so much love. It is a revolutionary experience. That's the best way I can describe it. It transforms you completely, in a second," Cruz said.

"Nature is very wise and gives you nine months to prepare, but in that moment — when you see that face, you are transformed forever. Even if you have heard from all your friends and family, 'This is what's going to happen,' until it happens to you, it's hard to understand in your soul."

Cruz said she purposefully stepped out of the media spotlight in the later stages of her pregnancy and in the first few months of her son's life in an attempt to maintain some privacy for her first child with husband Javier Bardem.

"I want my son — and my kids if I have more — to grow up in a way that is as anonymous as possible," Cruz said.

"The fact that his father and I have chosen to do the work that we do, doesn't give anybody the right to invade our privacy."

Despite the challenge of life — especially getting sleep — with a three-and-a-half-month-old baby, Cruz said becoming a mother has made her even more determined to maintain her charity work.

"Even if you were aware of children and felt compassion, when you have your own, it multiplies," Cruz said.

"It breaks your heart to know that there are so many children in the world suffering so much."