Women who did not plan to get pregnant stop breastfeeding sooner than women who did, researchers said today.
They are also 10 times as likely to stop breastfeeding exclusively by 12 weeks, according to new research by a Durham University expert.
The study suggests that women whose pregnancies were not deliberate often experience more emotional and physical discomfort with breastfeeding compared to women who had fully intended to have a baby.
Dr Alanna Rudzik, from Durham's Wolfson Research Institute, found that more than 40% of the women in the study had stopped exclusively breastfeeding within three months despite all of them originally intending to keep it up.
The research focused on mothers from low-income neighbourhoods in Sao Paulo, Brazil.
Dr Rudzik said: "Although my research focused on Brazil, there is much evidence that women with unplanned pregnancies have similar experiences and responses and that this ambivalence to breastfeeding is experienced by women elsewhere, including the UK.
"My research revealed that the negative feelings these women have about their unplanned pregnancy incline them to be strongly ambivalent towards breastfeeding."
Dr Rudzik conducted up to seven in-depth interviews with each woman.
These interviews demonstrated that women who had unplanned pregnancies had difficulty accepting their new role as a mother and this was expressed in part through strong resistance to the extremely close physical connection required by breastfeeding, which they were more likely to feel was unpleasant or difficult.
"I also found that satisfaction with a partner and family was conducive to longer breastfeeding," she said.
"This was more quickly curtailed when women were less satisfied with their interpersonal situation and home life."
Dr Rudzik hopes her insight into women's experiences might offer encouragement to organisations promoting breastfeeding.