Pregnancy multivitamins are a waste of money, researchers have concluded.
A review found "no evidence" that multivitamins result in better health for a mother or her baby and were an "unnecessary expense".
Instead, experts said women should focus on taking the single vitamins - which are available for a few pence per day - recommended by the NHS. These are folic acid in the first three months of pregnancy, and vitamin D. They should also follow a healthy diet.
Pregnancy multivitamins can cost more than £15 a month. On Boots.com, Pregnacare tablets plus Omega 3 cost £15.29 for a 28-day supply. Pregnacare Max - described as the "ultimate formula" in the range - costs £19.69 for a 28-day supply of tablets.
Regular Pregnacare costs £4.45 for a 30-day supply, while Boots' own brand Pregnancy Support Plus with fish oil costs £12.99 for a 30-day supply. Boots Pregnancy Support vitamins cost £3.49 for 30 tablets and Seven Seas Pregnancy tablets cost £5.25 for 28 tablets.
Seven Seas Pregnancy Plus with Omega 3 tablets cost £14.99 for a 28-day supply.
The vitamins come with various marketing literature, including saying they provide all the "nutrients vital for both mother and baby", and "all the essential nutrients for pregnancy".
In the new research, published in the Drug and Therapeutics Bulletin, a large panel of experts in the field reviewed available evidence on folic acid, vitamin D, iron, vitamins C, E, A, and multivitamins in pregnancy.
The found good evidence for the use of folic acid to reduce the risk of neural tube defects such as spina bifida, and some evidence - although "less clear cut" - for the use of vitamin D, which is important for bone and tooth formation and the ability to absorb calcium.
But the experts found "no evidence" that women should take multivitamins, which often contain 20 or more vitamins and minerals.
They added: "For most women who are planning to become pregnant or who are pregnant, complex multivitamin and mineral preparations promoted for use during pregnancy are unlikely to be needed and are an unnecessary expense.
"Pregnant women may be vulnerable to messages about giving their baby the best start in life, regardless of cost, and be unaware that the only supplements recommended for all women during pregnancy are folic acid and vitamin D, which are available at relatively low cost."
The authors said that women are subject to "heavy marketing" of multivitamins in pregnancy but "much of the evidence for vitamin supplementation in pregnancy comes from studies carried out in low-income countries, where women are more likely to be undernourished or malnourished than within the UK population".
In contrast to expensive multivitamins, Boots' vitamin D tablets at the correct dose of 10µg cost £1.09 for a 90-day supply. Boots' folic acid at the correct dose of 400µg per day costs £1.99 for a 60-day supply.
The researchers criticised the fact that some pregnancy vitamins can cost £15 per month.
Janet Fyle, professional policy at the Royal College of Midwives (RCM), said women should take the recommended daily dose of folic acid and vitamin D.
She added: "This is an interesting study and adds to a growing body of evidence that the benefits of eating a well balanced nutritious diet cannot be underestimated in improving outcomes for both mother and baby."
The Health Food Manufacturers' Association, which represents the food supplements industry, said: "This article refers to clinical intervention studies, rather than addressing the true picture of dietary intake in the UK.
"The National Diet and Nutrition Survey analysis highlights that a substantial proportion of women of child bearing age are consuming inadequate amounts of micronutrients - such as iodine, calcium and iron - so it is misleading to suggest that low-income countries are the only populations which show evidence of the risks of deficiency."
It added: "Ideally, we would all get sufficient nutrients from a healthy diet, but for a large proportion of the population, and for certain at-risk groups, such as pregnant women, this is simply not the case.
"For these groups, the belief they can obtain all the nutrients necessary from the average UK diet poses a very real risk to the health of both mothers and their unborn children."
Dr Carrie Ruxton, from the industry-funded Health Supplements Information Service, said: "The authors of this study wrongly claim that vitamin and mineral supplements must produce clinical effects before pregnant women are encouraged to take them.
"This is absolute nonsense. Except for folic acid which does have a therapeutic role by actively preventing neural tube disorders, the role of food supplements is simply to combat dietary gaps.
"This ensures that more people achieve recommended intakes of vitamins, minerals and fatty acids, such as long chain Omega 3s.
"They are not drugs so would not be expected to improve birth outcomes or treat conditions, such as pre-eclampsia, in non-malnourished populations."
She said the useful role of supplements, including fish oils, alongside dietary improvements "should be recognised".