Having an epidural to relieve labour pains is associated with problems breastfeeding, a study suggests.
Breastfeeding may be affected by how a woman gives birth
Researchers said those who have the anaesthetic are more likely to have problems in the first week after birth and to stop breastfeeding early.
University of Sydney researchers looked at 1,300 women who gave birth in 1997.
A chemical in epidurals may affect babies or it may be that women who do not have pain relief are more likely to persist with breastfeeding, they said.
However, a UK expert said that while the International Breastfeeding Journal study was interesting, women should not worry.
Around 20% of UK women have epidurals - inserted into a space near the spinal cord - to ease the pain of labour.
The researchers looked at the women's childbirth and breastfeeding history.
Of the 416 who had an epidural, 172 also had a Caesarean section.
Although 93% of the women studied breastfed their baby in the first week, those who were given an epidural were much more likely to have problems.
They were also likely to completely stop breastfeeding before six months compared with women who did not have any pain relief.
Three-quarters of those who had no analgesia were breastfeeding at 24 weeks, compared with 53% who received pethidine or epidurals.
The researchers, led by Dr Siranda Torvaldsen, say: "There is a growing body of evidence that the fentanyl component of epidurals may be associated with sleepy infants and difficult establishing breastfeeding."
They add: "Whatever the underlying mechanism, it is important that women who are at higher risk of breastfeeding cessation are provided with adequate breastfeeding assistance and support, both in the initial postpartum period [just after birth] and the following few months."
Pat O'Brien, a spokesman for the Royal College of Obstetricians and Gynaecologists, said it was possible that fentanyl had an effect on the baby.
But he added: "There are other factors which may explain this link, including that if a woman chooses not to have an epidural, she may also be more motivated to persevere with breastfeeding.
"Also, a lot of those women who had epidurals also went on to have Caesarean sections which - unless you have a lot of support - make it difficult to breastfeed because it's harder for women to pick their babies up."
He added: "For a woman who wants an epidural, there are a wide range of obvious benefits, and although the findings of this study are interesting and warrant further investigation, it is so far a theoretical concern."