Pregnant women over 35 years old are more likely to have complications at birth due to physiological as well as cellular changes that take place in the body of women as they age, a study has found.

These changes cause contraction in the structure of the uterus which further delays and causes longer labour stages, fuelling birth complications, the researchers said.

“Our research highlights that there are key physiological and cellular changes associated with a mother’s age that result in labour dysfunction. Timing of delivery and progress of labour is directly related to maternal age and this can cause complications during birth,” said lead researcher Rachel M. Tribe from King’s College, London.

Using mouse models, the researchers found that muscle contraction properties in the uterus of older mice were impaired, less sensitive to oxytocin — an important drug to speed up labour — and had reduced numbers of mitochondria — energy supplier for cells — indicating that the uterus muscles are less able to contract.

Signalling of the pregnancy-related hormone progesterone was also found altered, triggering a delay in labour, the researchers noted.

The average age for women to have their first child is increasing, with more women giving birth to their first child over 35 years of age.

This has been paralleled by an increase in pregnancy complications such as induction for women who have passed their due date, failure to progress in labour and bleeding after delivery.

The study was published in The Journal of Physiology.