A Caesarean delivery may be required when labour fails to progress naturally
Expectant mums who are low in a hormone made by the thyroid gland in the neck are more likely to struggle in labour, findings suggest.
Too little of the hormone thyroxine is already known to complicate pregnancy, increasing the risk of miscarriage, premature birth and pre-eclampsia.
Now a Dutch team has found even "low to normal" levels of thyroxine may cause problems, Clinical Endocrinology says.
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.
Not only are these labours generally longer and harder, they are also more likely to end in an assisted delivery with forceps, ventouse or a Caesarean.
The researchers from the University of Tilburg believe the hormone problem is so common - affecting about one in 10 pregnancies - a blood test for it should become a routine part of the antenatal check.
In their study of nearly 1,000 apparently healthy mums-to-be, 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.
Professor Victor Pop and his team 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.
The thyroid gland in the neck makes hormones that regulate metabolism
Too much of these hormones speeds up metabolism causing symptoms like weight loss and anxiety
Too few of the hormones slows metabolism causing problems like fatigue and weight gain
Medication can correct the imbalance
Professor Pop 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."
The baby is unable to make its own thyroid hormones until 20 weeks into the pregnancy. Before this, it is entirely reliant on its mother's stores, he said.
Professor Pop said more work was needed to explain the link found and to see if giving pregnant women extra thyroxine, even if they do not have full-blown thyroid disease, would be beneficial.
Professor John Lazarus, an expert in endocrinology at Cardiff University School of Medicine, said the link found was not necessarily causal.
"However it does highlight the importance of checking thyroid hormone levels in pregnancy."