Pregnant women have long been advised to take prenatal vitamins to ensure that they receive the nutrients needed by their unborn child.

But a recent study has casted some doubt on the medical advice that all pregnant women should be taking these daily vitamins.

The study, which was published in the British Medical Journal's Drug and Therapeutics Bulletin, showed that while some nutrients are important to the expecting mother, there are many more that could be unnecessary.

"I think we were surprised," Dr. James Cave, editor-in-chief of the bulletin and a medical practitioner at the Downland Practice in Newbury, Berkshire, said on a report in CNN.

"Vitamin deficiencies are common in women of child-bearing age and you would therefore expect there to be plenty of good evidence that taking a multivitamin helps in some way," Cave added.

After reviewing the effects of various nutrients on pregnancy outcomes, Cave and his colleagues found that only folic acid and vitamin D have evidence that support their use in pregnancy dietary supplements.

Folic acid, which helps prevent neural tube defects in the fetus, was found to lower the risk of birth defects by 70 percent. Data to support vitamin D was less conclusive, and results about whether it can actually prevent rickets among newborns are still unclear, researchers said.

According to Cave, there has been not enough support to recommend that pregnant women should spend money on vitamins, and that the belief that pregnant women need prenatal vitamins for a healthy pregnancy is nothing more than a marketing stint.

In the study, the researchers looked into previous studies, reviews and individual trials in examining the evidence available to support the use of folic acid, iron and vitamins A, C, E and D in prenatal supplements. Many of the studies were conducted in countries where expectant mothers have poor nutrition.

However, Dr. Scott Sullivan, director of maternal and fetal medicine at the Medical University of South Carolina and a spokesperson for the American College of Obstetrics and Gynecology (ACOG), does not see the harm in expectant mothers taking multivitamins, Time.com reports.

According to Sullivan, while there might be no solid scientific proof that they can benefit the fetus, there is also no strong evidence that prenatal vitamins are harmful.