Last November, as I battled a shiny Cyberman on the set of Dr Who, I momentarily caught sight of my reflection in his chest and for the first time in my life thought, "Hmm, not bad, the yoga's paying off." Elle McPherson could still sleep soundly - the title of The Body wasn't about to be ripped from her - but having returned from a yoga retreat in Tuscany I was feeling OK. Don't get the idea that I had chosen this for myself. It was a surprise anniversary present from the fitness-obsessed husband. The Tuscany bit I didn't mind but four hours of intense yoga a day, no thank you. Something gorgeous from Agent Provocateur would have sufficed. But I loved it and returned to Blighty with a sense of wellbeing, tone and hitherto unknown determination and was managing two gym sessions, one ashtanga yoga and a bit of power walking every week, plus downing spirulina juice and wheat-free bread like there was no tomorrow. For someone who had spent most of her adult life looking not bad for two children (without having had the children) I was almost on nodding terms with my true figure.

So imagine my surprise, dear reader, when, returning to the Dr Who set post-Chrimbo hols in India, I was suffering from constant nausea. How was my new health regime letting me down? I went off to the nearest doctor (a real doctor, not Dr Who) for a blood test assuming I had picked up Delhi belly, only to be told that I had, in fact, picked up a baby. Amazing, miraculous and totally unexpected. At 37, I was going to be a mummy, a role I had only vaguely entertained without developing the whens and hows. Imagine my further surprise to discover that not only was I going to become a mummy but I was going to become a "celebrity" mummy.

This became apparent when, after an early viability scan (they do that for us older mums) - a Sunday tabloid was tipped off and announced that I was up the duff before I had told family or friends. Next, two well known celebrity periodicals started a minor bidding war for the post-birth photos and a host of other requests followed. How long was I intending to keep the baby weight on for? Would I keep a pregnancy diet journal? (No.) Would I allow myself to do a post-pregnancy fitness video? I figured that I lacked the necessary discipline for this, even for hard cash and the thought of failing at something that even Jade Goody has twice mastered filled me with enough horror to eat an entire packet of Jaffa Cakes. I also had a coterie of paparazzi following me. I think they hoped to catch me doing something illicit and newsworthy like having a fag (in the manner of a heavily pregnant Kate Garraway a few months back) or my waters breaking in public. The offers culminated in the request to attend Birth Night Live on Channel 5 complete with a live caesarean. For one horrible moment I thought they wanted me to have the live caesarean. I graciously declined. Watching somebody else's c-section might shatter the illusion of what I had been assured could be a beautiful experience. On the upside of this madness, Bugaboo gave me a gorgeous pram.

Losing control of one's body is a process that in the past I could pretty much regulate. I would crash diet for filming. If I had a blow- out one weekend then I would up the protein intake and exercise output the following week. I relied on a good cleavage and a small waist and dressing to enhance those assets. But after 18 weeks my shape was changing beyond belief, despite barely keeping down any solids. The waist thickened, then disappeared, the cleavage became less pert and more petrifying, the stomach just got bigger and bigger. I couldn't shimmy; I waddled. I had to ditch the Gina s and discover the Birkenstock. How miserable - I was losing my figure before I had even found it. Then something miraculous happened. Liberation! The sickness subsided and I embraced my pregnancy and relinquished control. I wore my baby belly proudly and with glowing skin and shiny hair I became comfortable in my own (not inconsiderable amount of) skin. I may have been the size of the Hindenburg but I got more compliments during the last 12 weeks of pregnancy than at any other time in my life. What a life lesson in the beauty of being relaxed and at ease with oneself.

Towards the end, however, I started to panic, looking at photos of Geri Halliwell looking super-slim two hours (or whatever it was) after having her daughter. Angelina Jolie: amazing. Katie Holmes (all right, she's about 12 but it didn't help): amazing. How did they do it? I also hated myself for buying into this crap. I have been on enough photo shoots and seen the unrecognisable results to know the power of Photoshop. I once thought that Mariah Carey was looking pretty foxy on a front cover only to realise on closer inspection that it was me. Still, how could Posh Spice walk out of the Portland hospital looking that svelte? It had to be down to the scalpel, the airbrush or a phantom pregnancy. The rumour once was that Posh had discreet tummy tucks at the same time as her caesareans. Never having considered cosmetic surgery, I found myself mid-birth, high as a kite on gas and air and the epidural asking, "How much for a Mend It Like Beckham?" Perhaps it was worth remortgaging the house to skip the months of dieting ahead. The medical team chuckled and told me that such an operation was definitely an urban myth, that the reason said celebrities were so trim is that they practically moved a Power Plate and personal trainer into the delivery suite and that a tomato juice and a celery stick was considered a post-birth treat. Boo!

I despise the culture that has turned pregnancy weight into an issue. I very much believe in nine months on, nine months off, or in an ideal world, nine months on, two years off. Mind you, as a "celebrity" mum you're damned if you lose it quickly and damned if you don't. A girl still has to work for a living. Double boo!

Six weeks on and as I stare down at my beautiful daughter all thought of my weight seems trivial in the face of this miracle. On the other hand a potential television project looms and I would like to do it. And I would like to be reacquainted with my pre-pregnancy size. There are no healthy short cuts, no crash diets, no Vacunoughts (wearing a rubber wetsuit and attaching yourself to a vacuum cleaner as you work out), no colonics.

My old friend Melinda Nicci is a personal trainer and nutritionist who specialises in pre- and post-pregnancy fitness. She is adamant that women must slog it out slowly and surely thus taking care of themselves and their babies. Mel will guide me through the journey ahead, which is good because I would never even make it out the front door alone. The good news is that she says breastfeeding mums should absolutely not diet as they have to keep the fat supply to produce milk. The bad news is that breastfeeding does not need the extra 1,500 calories a day I had been banking on, but a mere extra 300 which should be made up of proteins and complex carbohydrates. Exercise, she says, should start slowly and build up gradually, particularly post-caesarean, and most women should wait until their six-week check-up before even contemplating a sit-up. For the time being, I have been told to keep doing my kegels (pelvic floor exercises), to walk 30 minutes a day and to pull my stomach muscles in at all times. This sounds do-able, but it is going to be touch and go.

I have just got back from a 30-minute walk with the baby. I am about to do a feed and I am famished. I have stuck a picture of Jade Goody, post-second baby weight loss video, on the fridge as an incentive to stop myself reaching for the marble cake my mum dropped in yesterday. I'll keep you posted.