The average weight gain in pregnancy is 22lb to 33lb and women should increase their diet by around 300 calories a day according to the NHS direct.

However, the nationwide survey of 850 pregnant women and new mums found one in five were getting mixed nutritional messages from their midwives on exactly what they should eat. One in 10 blamed friends for their confusion, according to the Organix poll.

Lizzie Vann, founder of Organix, said: "While three-quarters of women think more about nutrition when they have children, they are clearly confused about what to eat when they are pregnant."

COMMON CONFUSING FOODS

Alcohol: The Department of Health advises that pregnant women should avoid drinking alcohol. Claire Friars, midwife for Tommy's, the baby charity said: "If women do choose to drink, they should have no more than 1 or 2 units of alcohol, once or twice a week and definitely shouldn't get drunk."

Caffeine: Drinking a lot of caffeine may also have a negative effect on the baby's health. The Food Standards Agency (FSA) recommends having no more than 300mg per day, which is equivalent to three cups of instant coffee or six cups of tea.

Cheese: Pregnant women are advised to avoid eating soft cheeses such as Brie, Camembert and blue-veined types, as they increase the risk of listeriosis, according to the FSA. You can enjoy hard cheeses such as Cheddar and Cheshire. Cottage cheese, processed cheese and cheese spreads can all be safely eaten during pregnancy.

Shellfish: You should avoid oysters and other shellfish while you're pregnant, unless they have been thoroughly cooked. "When raw, these types of seafood might be contaminated with harmful bacteria and viruses, such as salmonella, campylobacter and occasionally listeria," Ms Friars said.

Tuna: You can eat most types of fish when you're pregnant. But you should avoid shark, swordfish or marlin. You should also limit the amount of tuna you eat to no more than two tuna steaks or four medium-size cans of tuna a week. This is because they contain higher levels of mercury which could affect the baby's nervous system. You should also limit oily fish to a maximum of twice a week.

Nuts: Pregnant mothers may choose to avoid peanuts if they, the baby's father, brothers or sisters have certain allergic conditions such as hayfever or eczema. This is because the baby may have a higher risk of developing a nut allergy.

Eggs: Undercooked or raw eggs should be avoided. "When cooking eggs, they need to be cooked enough for both the white and yolk to be solid. This is to avoid the risk of salmonella," Ms Friars said.