Pregnant women who believe in the "eating for two" myth risk harming the health of themselves and their babies, experts have warned.
A survey suggests that more than two thirds of UK mothers-to-be have no idea how many extra calories they need during pregnancy.
More than 63% of participants felt under pressure from others to eat larger meals than normal.
Alex Davis, from the National Charity Partnership which commissioned the poll, said: "The 'eating for two' myth has been around for years, but it's very unhelpful.
"Eating healthily and consuming healthy portion sizes are important before, during and after pregnancy to increase the chances of conceiving naturally, reduce the risk of pregnancy and birth-related complications and stave off health problems like Type 2 diabetes and heart and circulatory disease in the long-term."
Official guidelines from Nice (National Institute for health and Care Excellence) say women do not need any extra calories in the first six months of pregnancy.
During the last three months they only require about 200 extra calories - the equivalent of two pieces of wholegrain toast with olive oil spread or a small handful of nuts, seeds and dried fruit.
A total of 2,100 women from across the UK took part in the survey. More than four in five (85%) said they did not know how many extra calories to consume during pregnancy.
Of women who were currently pregnant, 69% lacked this knowledge.
More than a third of pregnant women thought they needed to eat 300 or more extra calories per day. Six in 10 believed the extra calories should be consumed in the first or second trimester.
Around a quarter of mothers-to-be admitted "eating for two" was an excuse for guzzling unhealthy snacks or meals.
Professor Janice Rymer, vice-president of education at the Royal College of Obstetricians and Gynaecologists, said: "Eating too much during pregnancy and putting on too much weight can be detrimental to both mother and baby.
"Women who are overweight during pregnancy are at an increased risk of having a miscarriage and developing conditions such as gestational diabetes, high blood pressure and pre-eclampsia.
"They are also more likely to have a premature baby, require a Caesarean section, experience a haemorrhage after birth or develop a clot which can be life-threatening.
"In addition, overweight women have bigger babies who are themselves more likely to become obese and have significant health problems as a result."
The National Charity Partnership is a corporate association between Diabetes UK, the British Heart Foundation and Tesco.
As part of its Let's Do This campaign, the Partnership has published a series of online articles aimed at raising awareness of the importance of a healthy lifestyle.
Ms Davis added: "We encourage all women to think about how they can eat well and our new information should make this easier for those who are currently pregnant or are trying to conceive."