Pregnant women who have low levels of the hormone thyroxine are more likely to have a difficult birth, experts say.
Low levels of the hormone have already been linked to miscarriage, premature birth and pre-eclampsia.
But now scientists say even "low to normal" levels of the hormone, which is made by the thyroid gland in the neck, can cause problems.
Babies are more likely to be positioned wrongly, facing the wrong way, back to back with the mother.
These labours are usually longer and more difficult and more likely to need forceps, ventouse or a Caesarean.
Researchers say there should be a routine test for the hormone problem as it is so common, affecting about one in 10 pregnancies.
Lower levels of the hormone were strongly linked to abnormal positioning of the baby's head and risk of assisted delivery.
Professor Victor Pop told the BBC that the lack of hormone might prevent the unborn child from moving so easily, leaving it stuck in an awkward position.
He said: "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."
Professor Pop told the BBC that more work was needed to find out whether giving pregnant women extra thyroxine would correct the problem even if they did not have full-blown thyroid disease.
Professor John Lazarus, an expert in endocrinology at Cardiff University School of Medicine, told the BBC that the link was not necessarily causal.
"However it does highlight the importance of checking thyroid hormone levels in pregnancy," he said.
Anything that can make giving birth easier has got to be welcomed... Did you have a "back to back" baby? What was your labour like?
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