Women who have an epidural to ease the pain of childbirth are more likely to need medical help to have their baby, scientists say.
Pregnant women 'should be aware of the pros and cons of epidurals'
A review of 21 studies comparing epidurals to other forms of pain relief showed women who chose them were 40% more likely to need intervention.
The Cochrane Review found this could mean instruments such as forceps being used to deliver the baby.
Around 20% of UK women have epidurals to ease the pain of labour.
The Cochrane Review, an independent health database, reviewed studies involving 6,664 women.
The review found women opting for an epidural were more likely to experience a longer second stage of labour - when the baby is pushed out of the birth canal - and to have drugs to stimulate contractions.
They also faced a greater risk of being unable to move for a short time after birth and to experience low blood pressure.
But there were no significant differences in the risk of having a Caesarean, long-term backache or immediate adverse effects on the baby between the women having epidurals and those who did not.
'Better pain relief'
However, there are concerns over using instruments to help deliver babies.
Risks associated with forceps use include possible damage to the woman's bladder and bruising on the infant's head.
A ventouse vacuum pump, which can also be used to help speed delivery, has been linked to the baby's head being misshapen for a few days afterwards.
The authors of the review, led by Dr Millicent Anim-Somuah from Liverpool Women's Hospital, said that more research was needed to evaluate the risks linked to epidural as pain relief during birth.
They also said that women should be made aware of the risks associated with epidurals.
They added: "Evidence from this review demonstrates that epidural analgesia offers better pain relief in labour.
"However, women who use this form of pain relief have an increased risk of instrumental delivery when compared with women who used non-epidural forms of analgesia or no analgesia at all.
"The evidence presented in this review should be made available to women considering pain relief in labour.
"The decision about whether to have an epidural should then be made in consultation between the woman and her carer."
Belinda Phipps, chief executive of the National Childbirth Trust, said: "This research shows forceps and ventouse births are more common after epidurals.
"Epidurals provide the most effective form of pain relief but it's important that women are told about the side effects and the downside of having this procedure."