Healthy pregnant women should think twice before taking iron supplements, say researchers who have linked high doses to blood pressure problems.
Iron is often given to combat anaemia in pregnancy, but many women take extra iron, on its own or in a multivitamin.
The Iranian university study, published in the British Journal of Obstetrics and Gynaecology, gave some women a 30mg daily dose as part of its research.
UK experts say that this amount should only be taken on GP advice.
The UK Food Standards Agency says that lower doses in UK multivitamins are unlikely to harm women.
Anaemia is a common condition in pregnant women, which, if left unchecked, can contribute to premature births and low birthweight babies.
It is caused by a lack of red blood cells, and taking iron on prescription helps the body produce more.
The researchers from Tarbiat Modarres University wanted to check the effect of iron supplements on women whose red blood cell level was normal.
They gave 370 women a 150mg dose of ferrous sulphate, which equates to approximately 30mg of actual iron, every day throughout their pregnancy.
A similar number of women were given a "placebo" dummy pill containing no iron.
The number of women with diagnosed high blood pressure - a disorder which can also cause problems for mother and baby - was higher in the women given the iron pills than in the other group.
Many countries still offer iron pills as a routine measure for all pregnant women, and lead researcher Professor Saedeh Ziaei said: "Our trial suggests that administering it even may have some disadvantages in non-anaemic women."
Professor Philip Steer, the editor of the BJOG: An International Journal of Obstetrics and Gynaecology, said: "Anaemia is often associated with low birth weight and preterm births, but this does not mean that women should be popping iron pills, or any vitamin pills indiscriminately, to prevent poor pregnancy outcomes."
A spokesman for the Royal College of Obstetricians and Gynaecologists said that women should avoid taking extra iron unless recommended by their doctor or midwife.
Routine iron supplementation is not practised in the UK, but blood is tested for signs of anaemia more than once during pregnancy as a matter of course.
However, many healthy pregnant women take once-a-day multivitamin pills that contain smaller doses of iron - normally around 100% of the recommended dose of 15mg.
Official bodies such as the Food Standards Agency recommend that pregnant women take extra vitamins such as folic acid and vitamin D, but don't encourage any other vitamin supplementation, saying that a balanced healthy diet provides all the vitamins needed. However, they said that taking 17mg or less a day - the amount UK multivitamin brands contain - was 'unlikely to be harmful'.
A spokesman for the baby charity Tommy's also stressed the importance of a healthy diet, but added that women were free to take supplements if they chose.
"We consider that the supplements available in this country are safe, and it is up to the individual woman to choose."