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Is Kate too thin to bear William an heir?

Ok, Kate, the pressure's on. The woman whose husband is second in line to the British throne has been given an easy time about producing an heir in the year since her marriage.

Diana had produced William by her first wedding anniversary and the 'spare' -- Harry -- followed two years later.

But while there's been the odd bout of speculation over the past year, the Duchess of Cambridge has largely been left to settle into her life as a newly-wed.

And the main reason? The Diamond Jubilee. But from today -- the celebrations ended yesterday -- Kate's main duty will be to get pregnant.

And from today, expect the question that's been bandied around the gossip sites for months to become a respectable talking point: is she too thin to have a baby?

Oh yes, the discussions on disappearing Kate are about to take on a new tone.

She has been visibly losing weight for at least 18 months. It was expected in the run-up to her wedding and excused in the aftermath as she got to grips with her new role.

But now the athletic young woman, who just five years ago was a slim size 10-12, has dipped to a British size 6.

Yes, that's the US size zero, to which skinny Hollywood types aspire.

She is reportedly horrified that pro-anorexia websites are holding her up as an inspiration.

Because she is photographed so much, we've almost become used to her finely sculpted features, her tiny waste and ultra-narrow hips.

But pictures taken when she was dating William show a toned, healthy-looking, but much heavier woman.

The pressure on women such as Kate to be thin means that much comment on her changing shape has been positive, and many very thin women do have babies: think Victoria Beckham.

But not many are as duty-bound to do so as Kate. The young duchess could be about to find out that Wallis Simpson, the woman who changed the history of her husband's family, was wrong when she said a woman could never be too thin.