Chicago rocks for Brooke

She says her daughter doesn't really understand what she does for a living but she often comes to the matinées and tries to copy the dances. "She's so clever," she says. "She does all these little twists and turns and ... I worry about her all the time," she says. "I mean, she's so bright! I never thought I'd feel this way, but I totally adore her."

She could talk about Rowan forever which is surprising since, less than two years ago, she was diagnosed with debilitating postnatal depression (PND).

She has written a searingly honest book about it, Down Came the Rain, in which she charts the painstaking process it took her to get pregnant with her husband, screenwriter Chris Henchy, and how difficult Rowan's birth was. With great candour, she tells how she cried all the time, dreamt that the baby would meet with an accident and how she felt like driving into a wall and killing herself, she felt so desperate.

Her book, and her solution to her problems - therapy and antidepressants - have caused a bit of a stir. Recently, Tom Cruise said: "Here is a woman - and I care about Brooke Shields because she is an incredibly talented woman - where has her career gone? These drugs [antidepressants] are dangerous. I have actually helped people come off them. What you do is you use vitamins."

What does Brooke Shields think about his comments? "In many ways, I think he's just validated the book. Has he had a baby? Anything that highlights the debate and the help available to the thousands of women who suffer from it is fine by me. As I've said before, Tom should stick to saving the world from aliens [a reference to his new film, War of the Worlds] and let women decide what treatment is best for them."

She says she wrote it because she didn't want to forget how bad it all was. "I think it's important people know that this happens to women - to any woman." Up until she tried to conceive, she says, everyone thought she had the perfect life. She was a successful child actress who'd attained a degree in French Literature from Princeton and who went on to have a flourishing adult career in films, television and musicals. "Oh yes, I thought I'd get pregnant in a nanosecond," she says.

"I was 37 at the time. I was fit and healthy and happy and in love." It turned out that, due to previous surgery on her cervix to erase pre-cancerous cells, she could not conceive naturally so she and her husband chose to have IVF. The first implant took, but she had a painful miscarriage. Five more attempts failed. She got pregnant on the sixth but ended up, after 20 hours of labour, having a C-section and nearly dying when her uterus ruptured.

But the therapy and the drugs she eventually relented to take have, she says, sorted her head out.

She spends a lot of her life in therapy, yet there's still something quite brittle about her. Sometimes, when she's talking about Rowan, she looks as if she is about to cry. She says this may be because she's weaning herself off the drugs because she wants to get pregnant again.

After all that? "Oh yes," she says, laughing. "I know it sounds absurd because people have just read the book and they're all like, 'You felt like that and you want to do it again?' but PND is something that may happen with one child and not with the next."

But what about having to go through IVF again? "My doctor knows my physical history inside out so there are about a year's worth of steps we don't have to do. Hopefully, I can conceive quickly."

And yet ... she seems so wedded to her work. "I think part of my problem was that I found it almost impossible to think of myself as a mother. I was an actress! "

Even though Shields loves being a mother now and says her maternal side takes precedence over her professional side, she admits she needs validation in both spheres. She talks a lot about validation. How her mother - Teri, the showbiz-obsessed alcoholic who put her daughter in an advert for Ivory soap aged 11 months - "validates" herself, Brooke, as a mother.

"I think my mother was very worried about me," she says. "But I've always coped. Over the past five years I've been divorced [from tennis star André Agassi], had my best friend, David Strickland [her co-star in the US television series Suddenly Susan], commit suicide, my father died, I then couldn't conceive ... but I coped. I think all this stuff, coupled with the horrific birth and that surge and then massive drop in hormones, sparked everything off."

She says that Rowan has changed her life. "I feel an intensity of love for her that I've never felt before. Of course I love Chris. He's a saint! His friends who've read the book say to him, 'You just raised the bar, mate!'"

Just as she's telling me about how she hopes Rowan won't follow her into the acting profession, we hear a little voice. Brooke's face lights up. "It's my babee!" she says. I'm nice. I let her go. "Oh, thank you," she says, and she swoops off down the corridor to find the child that caused her so much pain and now, so much pleasure. Thank goodness for that.

Brooke Shields will be in Chicago at the Adelphi Theatre until 30 July. Down Came the Rain is published by Michael Joseph, £12.99.