Pregnant mothers who have low levels of a hormone produced by thyroid gland are more likely to have longer and harder labour.
A team of Dutch researchers have found that even "low to normal" levels of thyroxine may cause problems.
They said babies were more often positioned wrongly, making labour more difficult.
Although still head down, the babies tended to face the wrong way - towards their mother's back rather than stomach, which leads to harder labour.
They are also more likely to end in an assisted delivery with forceps, ventouse or a Caesarean.
In the study involving 1000 apparently healthy mums-to-be, researchers found that lower levels of thyroxine at 36 weeks of pregnancy was strongly linked to abnormal positioning of the baby's head and risk of assisted delivery.
Lead researcher Professor Victor Pop believe the relative lack of hormone might stop the unborn child moving as well as it should.
This means that instead of getting into the optimal position for labour, the baby is stuck in a more awkward one.
"Recent findings have shown that motor development in children at the age of two is related to low levels of thyroid hormone in pregnancy," BBC News quoted Professor Pop as saying.
"It follows that impaired maternal thyroid function could also influence foetal movement," he added.
"However it does highlight the importance of checking thyroid hormone levels in pregnancy," Professor John Lazarus, an expert in endocrinology at Cardiff University School of Medicine.
The study appears in Clinical Endocrinology.