Every woman with a bump knows that the running commentary about your size by friends, family, work colleagues and strangers is endemic to the pregnancy experience. Slim or shapely, your body is under observation. Typically though, pregnancy has been a time when women can stop counting calories, revel in being "big" and eat what they want, or at least - if they are overweight - chalk up their size to pregnancy and cease fretting about it for nine months.

Yet the days of living large during pregnancy may be drawing to a close. The talk amongst pregnant women these days isn't about having a full-bodied pregnancy but a slimmed-down version of yourself thanks to that seeming oxymoron: the pregnancy diet.

There's no guru or best-selling book about it (yet) but it involves not only avoiding high-calorie pickles-and-ice-cream indulgences but also harnessing the less pleasant aspects of pregnancy - morning sickness and fatigue - to good effect.

During my first pregnancy, I piled on the pounds, happily tucking into the second helpings and puddings I’d deprived myself of for the best part of my late twenties. Why not scoff those teacakes? After all, I was blooming.

Now pregnant for the second time, I'm far more careful with my diet, having battled to lose the excess weight post-pregnancy. This time everyone has noticed, commenting positively on my slimmer bump, and the message is loud and clear: the skinny pregnancy is in.

When I mentioned to a friend that I felt nauseous and knackered for the first four months of my pregnancy, she brightly said, 'Well you must have lost loads weight.' Another friend boasted of losing 15lbs in the first few months due to morning sickness, before she rushed out to buy some skinny jeans. It was the first time in years she could fit into a size 10.

Then over supper with a heavily pregnant friend, about three spoons into our chicken and vegetable stir-fry she groaned. 'I'm so full!' and the food was whisked away. "I lose loads of weight when pregnant as the baby just takes everything," she said, rubbing her slim arms.

To be sure, some women lose weight early in pregnancy because of medical issues. "There is a condition Hyperemesis Gravidarum where women are excessively sick in early pregnancy and often lose a substantial amount of weight," says Anita O’Neill, Consultant Midwife at the Zita West Clinic. "This occurs during the first 12 to 16 weeks, but once the early pregnancy hormones settle down, women tend to re-gain the weight with no negative consequence on foetal development."

Yet the idea that the baby "helps" a pregnant woman to lose weight is just a fad, says O'Neill. For the nine months of pregnancy, "the baby is very good at getting all it needs," she says, "but a mother will only lose weight in pregnancy if she’s not eating enough calories," not because of the growing baby.

What a woman eats during pregnancy can have a profound effect on the health of the foetus. Vitamin C, calcium and iron are important vitamins for the developing baby, and folic acid helps prevent spina bifida. Too much vitamin A has been linked to birth defects. Even indulging in biscuits, sweets and crisps can "program" babies to crave junk food and set them up for health problems later in life, research shows.

In addition there's a lot of confusion about what constitutes healthy weight gain. Guidelines from the National Institute for Health and Clinical Excellence suggest that, taking into account women of different shapes and sizes, a total gain of almost 18kg (40lb) during pregnancy - including the weight of the foetus - is an upper limit for a healthy

pregnancy. However your weight gain depends on your starting weight. If you start pregnancy with a low BMI, a weight gain of 3 stone is normal, if you're teetering above a normal BMI of 20-25 then you'll gain less weight. An obese woman may only gain 3 - 5 lbs in the whole of her pregnancy, but that's not to say she'll come out of pregnancy slimmer than when she started. The very overweight and underweight are in a higher risk group when pregnant and a poor diet may inhibit the growth of the baby.

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