Pregnant women are more likely to suffer problems giving birth if they are low in a hormone made by the thyroid gland in the neck, according to research.
A team from the University of Tilburg found mothers with "low to normal" amounts of the hormone thyroxine can complicate labour, as babies are more often in the wrong position.
They were usually still head down, but faced towards their mother's back instead of her stomach, which makes labour longer and more difficult, and increases the likelihood of an assisted delivery with a Caesarean, forceps or ventouse.
It was already known that very low levels of thyroxine increase the risk of pre-eclampsia, premature birth and miscarriage.
Research leader, Professor Victor Pop, and his team suggest a lack of thyroxine could stop unborn children moving properly, and believes there should be a blood test for it in the antenatal check as the problem affects one in 10 pregnancies.
Professor Pop said in Clinical Endocrinology: "Recent findings have shown that motor development in children at the age of two is related to low levels of thyroid hormone in pregnancy.
"It follows that impaired maternal thyroid function could also influence foetal movement."