Women who give birth in hospitals during the day may be more likely to have a difficult labour, researchers suggest.

A study carried out in the US has found doctors are more likely to use forceps, vacuums or administer drugs to women if they go into labour during the day.

They are also more likely to carry out surgery to prevent vaginal tearing compared to women who give birth at night.

The researchers suggested pressure on hospitals during the day and "convenience factors" could be to blame.

Dr David Webb, from the Philadelphia Department of Public Health and colleagues, examined the medical records of 37,000 women who had given birth at more than 25 hospitals in the city between 1994 and 1997.

They found that one in 10 births involved the use of a vacuum or forceps. One in three women received drugs to help them through the labour while one in four underwent surgery to prevent vaginal tearing.

However, the likelihood of having these interventions increased sharply among women who went into labour between 10am and 10pm.

Women who gave birth between these hours were 86% more likely to be given drugs and were 43% more likely to have an instrumentally-assisted delivery.

They were also 10% more likely to undergo surgery to prevent vaginal tearing and 30% more likely to suffer severe tearing.

Writing in the Journal of Epidemiology and Community Health, Dr Webb suggested the pattern was linked to "increased pressures on doctors and hospital staff to 'clear' patients at times when they have other patients to see".

He added also suggested that doctors were less likely to allow women to have a natural birth if the hospital is busy.

"Busy doctors in busy hospitals may simply have less tolerance for the otherwise time consuming natural progression of labour and delivery during these times to perform procedures that hasten the labour and delivery process."

The researchers urged hospitals to carry out similar studies to see if they had the same patterns.

Recent figures from the Department of Health showed less than half of births in English hospitals are completely natural.

They suggested doctors are now more likely to use either forceps or suction devices to aid delivery, or to give epidural pain relief.