No sooner does a woman break the news that she is pregnant than she is bombarded with a list of things she must eat or avoid, from alcohol to sushi, blue cheese to Parma ham.
The advice is ever changing and still controversial. But what's becoming clear is just how crucial a woman's diet is to her child's future health.
Last month, scientists from King's College London and the University of Bristol reported that a high-fat, high-sugar diet during pregnancy could alter a growing foetus's DNA, leading to brain changes that raise the risk of attention deficit hyperactivity disorder (ADHD). Here, experts explain the latest thinking on what to eat in pregnancy.
How children's tastes are set in the womb
At five months, a foetus begins to recognise flavours in the amniotic fluid from the food and drink their mother is consuming. Scientists now believe that the tastes that babies are exposed to in the womb can determine the foods they prefer in later life. In one study, women who drank carrot juice during the last trimester had babies who responded more positively to carrot flavours during weaning. Conversely, animal studies suggest that mothers who eat junk food - fatty, sugary, processed foods and drinks - during pregnancy and breastfeeding have children who prefer these foods.
"Following a healthy, balanced diet isn't just good for you and your baby's health - early research suggests maternal diet in pregnancy could have both a positive and negative influence on your child's taste preferences," says Sian Porter, spokeswoman for the British Dietetic Association.
And while all pregnant women should take vitamin D throughout pregnancy, and folic acid during the first 12 weeks, Porter says it's better to get vitamins and nutrients from food. In July, a review in the journal Drug and Therapeutics Bulletin said multivitamin supplements marketed to pregnant women were "an unnecessary expense". Researchers found no evidence to recommend supplementation during pregnancy beyond folic acid and vitamin D.
The myth of eating for two
"You shouldn't really try to lose weight while you're pregnant, but it's important not to eat too much, too," says Porter. "Normal weight gain for women is about 12kg during pregnancy, but most of that should come from the baby.
"Gaining excessive weight during pregnancy raises the risk of complications such as pre-eclampsia and gestational diabetes, and the need for a caesarean section. Government advice states that pregnant women need to eat only an extra 200 calories per day in the last trimester.
Runny eggs are back on the menu
Pregnant women must avoid anything that carries a risk of food poisoning such as rare or undercooked meat, including cured meats, unpasteurised cheeses, unwashed salad or vegetables and raw fish and shellfish, as the infection, and ensuing dehydration and fever, can harm a growing baby.
"Cook things well and wash them well," says Dr Patrick O'Brien, spokesman for the Royal College of Obstetricians and Gynaecologists.Pregnant women have also long been told to eat only eggs that are thoroughly cooked, because of the risk of salmonella. But this summer, the Food Standards Agency said British eggs with a "Red Lion" stamp were now free from the bacteria, and could be safely eaten runny or raw during pregnancy.
"It now seems salmonella is far less common, so it's possible the official advice for pregnant women may change, but I would maybe hold off and keep cooking them through until it does," says Dr O'Brien.
Salmon is in, but sushi is out
A series of studies has shown that oily fish, such as salmon and mackerel, is beneficial in pregnancy: one recent trial by the University of Southampton found that children born to women who regularly ate salmon during pregnancy were less likely to develop asthma.
However, mothers-to-be should limit the amount of oily fish they eat to two portions a week, because it contains pollutants that can harm the development of a foetus. Some fish, including shark, swordfish and marlin, should be avoided completely because they contain high levels of mercury.
Confusingly, though, sushi is to be avoided during pregnancy, because raw fish, including salmon, can carry small parasitic worms that can cause infection. "We advise avoiding sushi completely. If somewhere along the line the fish has been frozen for at least 24 hours, then the risk is minimal," says Dr O'Brien.
"I suspect most high-street places will freeze it, but they may not declare it, and if you're not sure, it's not a difficult thing to cut out while pregnant to be on the safe side."
Smoked salmon, on the other hand, is generally considered safe.
A last word on alcohol
Alcohol during pregnancy, particularly in the first 12 weeks, has been linked to miscarriage and an increased risk of premature birth and low birth weight. Government advice is to avoid alcohol - which concurs with advice in many other countries, including the United States and France - and if you do decide to drink, to consume no more than one to two units once or twice a week.
Part of the problem is that research on the effects of light drinking in pregnancy is limited and often of poor quality, says Dr O'Brien. "Because we cannot be certain what the safe low threshold is, we advise it's best to avoid it completely."
However, research published last week by Dundee University showed that almost half of pregnant women are still drinking, despite the new guidelines. Of the 40 per cent of women who said they continued to drink, many said they knew the advice had changed but had researched the issue and decided it was safe to drink. Julia Brown, of the Foetal Alcohol Syndrome Disorder Trust, said the findings were a concern.
Not only is exercise safe in pregnancy, it's also excellent for the health of mother and baby, protecting against depression, weight gain and diabetes, and improving strength and stability, which helps to prevent common problems such as back pain. Yet women are often left confused about what they should do, says Professor Greg Whyte, an expert in sports science and author of a new book on the subject, Bump It Up.
"There is so much misinformation. I've even spoken to women who were told by their midwife to do no exercise in the first trimester. Invariably that's going to be more damaging." He says two of the biggest misconceptions are that exercise can trigger a premature birth, or that it reduces oxygen supply to the baby. "There's no evidence for this, and in fact we know that women who are physically active are less likely to require a caesarean."
Professor Whyte says that in our modern, sedentary culture, pregnant women should be given recommendations on exercise by health professionals in the same way as they are about diet and issues such as avoiding alcohol.
"If you're someone who already exercises regularly, in the vast majority of pregnancies there should be no reason why you can't carry on doing it right through the pregnancy."
Just remember that, unlike a normal exercise plan, whereyou work harder and harder over time, in pregnancy you need to gradually reduce the intensity and go easier as things progress.
"If you're pregnant and new to exercise", he adds, "start very easily and progress slowly". Whatever your exercise, whether it's running, swimming or weightlifting, it's important to work at, or below, a moderate intensity to avoid overheating, dehydration and injury.
Professor Whyte says that women should measure their exertion on a scale of one to ten, with one being how you feel when sitting down watching television, and 10 being the hardest that you have ever exercised, and stick to the magic number - seven - or lower.
Another method is to exercise only to a level where you can still manage to hold a conversation.
He says most exercises are considered safe, although impact sports and anything where there's a risk of injury or falling - such as trampolining, aerobics, boxercise or team sports - are probably best avoided. Sit ups and crunches are also out after the second trimester because of the strain on the linea alba abdominal muscle, which tends to stretch during pregnancy.
A world of difference
Here, raw fish is seen as healthy, and the idea that pregnant women should give up sushi is laughable.
India: Some Indian families believe that eating saffron helps to produce a fair-complexion child, encouraging women to drink saffron milk. Although there is no scientific evidence to support this, the spice is good for digestion and has anti-inflammatory properties. Eating extremely spicy foodis also encouraged.
China: Mothers-to-be are told to avoid cold foods such as ice-cream, while watermelon, bananas and mung beans are thought to containa negative yin, or energy, that can result in a miscarriage.
France: Historically, French women have a more relaxed attitude to drinking alcohol and eating soft cheese while pregnant, even if the country's medical professionals disagree.