According to this week's Grazia magazine, everyone is talking about the actor Milla Jovovich's incredible pregnancy weight gain. I want to make a snide remark here about the trivial concerns of the modern media, but I'm not kidding around - five stone in seven months. Seeing the pictures, you will let out a spontaneous, "Oh, my good Christ." It's not so much the shape of the actor - which is pretty much the shape of any above averagely tall, heavily pregnant lady - more the effect of seeing a person who was famous for being thin, walking around not thin. It's just weird, like seeing Big Ben in a giant sock or a mouse in a handbag.

Of many priceless remarks from the model-cum-actor, my favourite is, "My friend and I went round Saint Sulpice cemetery, where French royalty are buried. On the way back, I said to her, 'Let's eat like kings!' I was craving bone marrow and we scoured the whole of Paris searching for the leg of a cow." And you thought it was impossible to put on weight in an affected, high-fashion way ...

Now, let's run through the normal conventions of a celebrity pregnancy - they do not whack on weight like ordinary women and they hide in high-security spas for those awkward is-she-isn't-she months, emerging with perfect seven-month bumps looking like the proverbial python who has swallowed a football. This has been the case for the past 14 years - I can place it exactly because I remember having a conversation at university, pondering the demise of the modest-pregnancy-smock (favoured by Lady Di and her contemporaries). The modern celeb eschews the modest smock for the simple reason that it is impossible to see what is going on under it, and so a waste of her efforts to stay entirely fat-free.

But there are exceptions. I shall run you through them: first, there is the person who is so tiny to begin with that any weight gain at all represents a 100% increase in her BMI, and changes her appearance so radically that she can no longer use her own passport. Kate Hudson put on 70lb during the gestation of her first born, but she only weighed about 90lb beforehand. She claimed at the time to love her "womanly curves", but I think it's fair to say that she didn't look beefy so much as possessed. With the rider that I know nothing about Hudson's eating habits, you do tend to find this sort of preggers-star dropping the weight very fast after delivery, because she was in the grip of a demon invasion rather than a habitual overeater.

Second, there is the star who has been ruling her appetite with Stalinist vigilance since she was 13, and suddenly lets go a bit. She isn't so much eating for two, as speed-eating every single second helping she has refused for two decades. I will hazard that Jovovich falls into this category, but the queen in this category was Elizabeth Hurley, whose pre-pregnancy diet tips were legendary for their nuttiness (eg, eat with a child's knife and fork so you don't put so much in your mouth. Yes, really. She would take junior cutlery to restaurants). She gained an unspecified amount of weight during her confinement and managed to dodge the paparazzi so effectively that no photographic trace remains. But her excess weight was such that she ended up moving in with Elton John for four months post-partum because her own household security was insufficient to keep the paps from getting a look-see at her "womanly curves".

Third, there is the star who is Welsh, and who finds that fat, which has always wanted her, wants her more in pregnancy only (eg, Catherine Zeta-Jones, Charlotte Church).

Fourth, there is the naturally slim star who has never given her shape a second thought, assuming that you just eat when hungry, and then get caught out by the giant baby-making appetite (Zoe Ball).

The thing is, they all lose it again, in the end - whether at Elton's or at the local branch of WeightWatchers (that's what Zeta-Jones did). So I think more celebrities should embrace the opportunity for heft, since the only other time they will get one is if someone remakes Bridget Jones and Renee Zellwegger is busy.

If you will permit me a brief excursion into the world of medicine, the word is that one should gain between 24lb and 35lb, to account for the baby's weight, placenta, amniotic fluid, extra blood supply, heavier breasts and maternal fat stores laid down for breastfeeding. Pressed on the matter, the official advice is that you should neither eat more in early pregnancy nor gain too much weight - the eating for two should all be in the last trimester, to account for your lumbering shape, and the weight gain should mostly be in the middle trimester, when the foetus has its growth spurt.

All this is really irritating and, I am almost certain, not true - you can spend the whole first trimester berating yourself because you are famished and eating 3,000 calories a day, only to find that in the last trimester you have such bad indigestion that you can barely eat at all. Breastfeeding can shift a lot of weight, fast, partly because you are producing food for someone else to eat, and partly because it kills your appetite while you are doing it, which I think is nature's way of stopping you from getting crumbs in your baby's ears. Don't worry, Milla. Eat as much marrow as you like.