A third of pregnant women have iron deficiency, according to new research.

French scientists say the widespread low iron levels puts women at increased risk of having a thyroid disorder and suffering complications such as miscarriages and preterm births.

Pregnant women need to produce thyroid hormone for the development of their babies’ brains, especially during the first semester when the foetus does not have a thyroid gland of its own.

Thyroid autoimmunity is a disease where the immune system mistakenly destroys healthy thyroid cells; causing thyroid hormone levels to fall. It can be particularly dangerous for pregnant women.

Iron helps the normal functioning of thyroid peroxidase (TPO); a protein essential for the correct functioning of the thyroid.

While iron deficiency affects one in five of the world’s population, it is more prevalent in pregnant women, who need triple the daily requirements to make more red blood cells and help the foetus and placenta grow.

Researchers from the Saint-Pierre University Hospital (ULB) in Brussels followed 1900 pregnant women in their first trimester, measuring their blood ferritin (an indicator of iron deficiency), antibodies against the thyroid peroxidase (indicating thyroid autoimmunity), the thyroid hormone free thyroxine (FT4) and thyroid stimulating hormone (TSH).

Ferritin levels showed that 35 per cent of the pregnant women were iron deficient. Measurements of thyroid peroxidase antibodies then showed that 10 per cent of the women in the iron deficient group suffered from thyroid autoimmunity, compared to 6 per cent in the non-deficient group.

Levels of TSH indicated that 20 per cent of women in the iron deficient group had subclinical hypothyroidism, compared to 16 per cent in the non-deficient group.

“Considering that our study took place in a relatively wealthy country, our results show that even in 2016 iron deficiency remains an important problem,” said Dr Kris Poppe, lead author of the study.

“Women should be checked for iron deficiency during pregnancy, and ideally before too. Iron supplements should be given out to decrease a number of established pregnancy complications, but it needs to be proven whether they can decrease thyroid problems too.”

The next step is to find out whether pregnancy outcomes were affected by the higher rates of thyroid autoimmunity, iron deficiency or both.

The same team is looking to investigate whether it is iron deficiency that causes thyroid autoimmunity or the other way around, though other studies support the first of these two hypotheses.