A study conducted at the Westmead Institute for Medical Research has found 40 per cent of women aged 16 to 45 are deficient in iodine.

There are concerns many Australian women still suffer iodine deficiency before pregnancy, especially those who don't eat much bread.

A study of 94 women aged 16 to 45 from outpatient clinics across Sydney found 40 per cent were deficient in iodine, an element essential for healthy brain development in foetuses.

Lead researcher Professor Jenny Gunton, from the Westmead Institute for Medical Research at Westmead Hospital, says iodine deficiency the single most preventable cause of mental development delays.

In 2009, it became mandatory in Australia to add iodine and folate to bread to reduce the prevalence of neural tube defects such as spina bifida and reduce iodine deficiency.

A federal government report released last year found the measure had resulted in a significant 14.4 per cent overall decrease in the rate of neural tube defects.

But Dr Gunton believes it's not enough.

"Our study shows that adding iodine to bread has not been enough to meet the additional needs of women who are planning pregnancy," Dr Gunton said.

"Women in certain cultural groups who tend not to eat much bread are at even higher risk of iodine deficiency."

According to the study findings, the median level of iodine from the sample group was 117 micrograms per litre, well below the 250ug/L recommended by the National Health and Medical Research Council.

Dr Gunton, an endocrinologist, says women planning a pregnancy must be more aware of their iodine levels.

She also says women should be taking pregnancy multivitamins before they start trying to conceive, because a baby's brain starts developing "before you even know you're pregnant."

The research is published in the Asia Pacific Journal of Clinical Nutrition.