I am pregnant with my third baby. You'd think I'd know what to eat and drink by now. But I have already, it seems, transgressed the laws of pregnancy nutrition more than once. I have consumed alcohol and sushi in substantial quantities, after a doctor's pregnancy test was erroneously negative. I failed to take folic acid supplements (my pregnancy was unplanned). I forgot that peanuts can cause the baby to have allergies. I forgot that feta cheese can carry listeria, causing miscarriage. I forgot that Parma ham is uncooked and could do the same.

Even the things I thought I should be proud of proved potentially misguided. My midwife balked when I told her I'd taken a multivitamin a day before I knew I was pregnant (when I diligently switched to a prenatal brand). I could, she said, have overdosed on vitamin A - which can cause birth defects. Forgive me, then, if I feel it will be a miracle if my unborn child is not already riddled with life-threatening ailments.

Of course, it is almost certainly entirely healthy. But I am not, it seems, alone in my confusion, guilt and anxiety about all these rules. One study, reported yesterday, found that pregnant women are confused about which foods to avoid. Indeed, the survey of 500 women, commissioned by baby milk company SMA, suggested that we may even be putting our babies "at risk" because we are cutting out foods that are beneficial, in the mistaken belief that they are forbidden. Take cottage cheese - it's a good, low-fat source of calcium. But many women apparently avoid it in pregnancy thinking it a "soft" cheese, and therefore verboten.

Our paranoia is understandable. For a start, the goalposts keep moving. When I was pregnant for the first time (five years ago), I stuffed myself with tuna, which I'd been told was an excellent source of fatty acids, protein and vitamins. This time around, however, I'm only allowed a couple of medium-sized cans a week as it may contain too much mercury and harm my baby's developing nervous system. Swordfish, marlin and shark are now officially out for the same reasons.

Phil Baker, professor of foetal and maternal health at St Mary's hospital, Manchester, "fully understands why people get confused". There is, he says, "an increasing understanding that the foetal environment affects human health". Indeed, there is "a whole new area of science growing up around the foetal origins of adult disease". New studies are constantly emerging; some go as far as to link birth weight with future risk of heart disease, not to mention adult IQ. But there is still a lot of conflicting data, not least because much of the existing information comes from animal studies that have been extrapolated to humans.

There are some things, says Baker, that we do know for sure. "Too much vitamin A [found in foods like liver] can cause malformation of the foetus and an increased risk of birth defects. There is much evidence that folic acid can protect against neural tube defects [for instance, spina bifida], and there is weaker evidence that it may even protect against the failure of the placenta to work well, which can lead to pre-eclampsia, stillbirth or growth problems". There is also, says Baker, evidence that calcium and iron are important to the baby's growth. (Anaemic mums may have low birth-weight babies.)

However, other studies into nutrition and pregnancy are complicated by "confounding variables". A mother who has extremely poor nutrition may, for instance, also have poor access to healthcare, may smoke, or drink excessively. It is therefore hard to narrow down the factors to get a simple cause-and-effect from one food. Coffee is a good example. Many of us obsessively cut out the lattes entirely when pregnant, believing that even a cup could trigger miscarriage. The real evidence is far less straightforward. Even the Food Standards Agency does not say you should cut it out. (They advise limiting your intake to no more than about three cups of filter coffee a day.)

Then there's alcohol. I once ordered a glass of wine in a New York bar when visibly pregnant, and was advised against it by the (absurdly youthful) barman, who pointed to a sign saying: "Pregnant Women Should Not Drink Alcohol."

"There is no doubt that excessive alcohol intake is a problem," says Baker, who has treated many babies suffering foetal alcohol syndrome. "But on my own reading of the literature, I would not stop all women from having any alcohol." The FSA concurs, advising us to cut down to one to two units a week (1-2 small glasses of wine). French women, meanwhile, would probably fall off their bar stools at such an idea - many drink entirely as normal when pregnant (though this would be moderate by British women's standards).

There is, of course, sensible evidence that certain foods should indeed be avoided during pregnancy. Mostly these are ones that can cause food poisoning, which studies have found may damage your foetus or cause miscarriage (soft, mould-ripened or unpasteurised cheeses, uncooked or partially cooked meats and eggs, and ptés are the main culprits). In general, however, once you have taken on board this relatively short list of forbidden foods, most doctors say we should eat as normal during pregnancy. The bad news is that you only need an extra 300 calories a day.

Most of us, however, get our pregnancy nutrition advice from astonishingly fascistic pregnancy books. "Before you close your mouth on a forkful of food," says the one I hate most, "consider, 'is this the best bite I can give my baby?' ... if it'll only benefit your sweet tooth or appease your appetite, put your fork down."

We need to get this sort of thing into perspective, says Baker. "I see so many women who are blaming themselves for a miscarriage, or stillbirth," he says, "when usually they could have done absolutely nothing to avoid it." It is, he emphasises, "important to have a basically balanced diet, which includes things like vegetables, protein, carbohydrate, fats and vitamins." But there are many complicating factors with diet and pregnancy outcomes, and much will depend on exactly what the imbalance is. Indeed, your body may have really good stores of certain nutrients, which will get you through, even if you are not eating enough of them at a given time in pregnancy. (Which could explain why women with severe morning sickness, who can keep nothing down, can still have big healthy babies.)

Tommy's Campaign, which funds research into problems in pregnancy (it also funds the maternal and foetal health centre which Baker directs), has published a list of "pregnancy superfoods". And the FSA has more detailed and clear guidelines about what to eat in pregnancy. But in general, says Baker, most grown-up women who "use moderation in all aspects - who don't have particular food fanaticisms, or drink excessively" are doing all that is necessary, nutritionally speaking, to produce perfectly healthy babies.

Don't eat

· Soft mould-ripened cheese, such as camembert, brie and blue cheese like stilton because of risk of listeria

· Pté, including vegetable, as it can contain salmonella

· Raw or partially cooked eggs, for the same reason

· Uncooked, or partially cooked meat, or ready-prepared meals for the same reason. (Take particular care with sausages and minced meat)

· Swordfish, shark and marlin, as high mercury levels can damage the foetus's nervous system

Do eat

· Bananas, oranges and other fresh fruit, as well as dried apricots and prunes

· Salmon and other oily fish

· Wholemeal bread and brown rice

· Lean red meat,chicken and turkey

· Yoghurt

· Broccoli and green vegetables

· Pulses and lentils

· Fortified breakfast cereals