Researchers at the Montana State University in the US found that women who breastfeed are seen as incompetent and are even thought to be poor at maths, the Daily Mail reported.
For the study, the researchers used a series of experiments to test people`s perceptions of women who breastfeed.
In one, participants were shown fictional resumes, including ones that described a female applicant as having written a book describing her experiences breastfeeding.
In another, college students were paired with one of the female researchers, who pretended to be just another participant.
During a break in the session, the confederate would listen to a voice-mail message that was loud enough for the partner to overhear.
The researchers alternated between four messages: one that implied she was breastfeeding, a second that suggested only that she was a mother, another that she had a boyfriend and owned a strapless bra, and one that was neutral.
The participants were then asked whether they would hire their partner if they saw her profile on an employment website.
Those who received the breastfeeding message were rated lowest in maths and work competence and were less likely to be "hired" by their partners than those who were thought to be mothers who did not breastfeed.
This prejudice, the researchers suggested, might exist because women are viewed as "objects" when they breastfeed.
Study author Jessi Smith said: "It`s possible women who breast-feed are viewed as less competent because observers see the women as objects, as is sometimes the case when women are sexualised.
"Incompetence comes because when you think about an object, you don`t think of an object as something that`s intelligent or smart or thinking."
Dr Smith said the prejudice towards breastfeeding mothers could be even worse than the study`s findings - because the participants had not actually seen the mother breastfeeding.
"I can only imagine if, in fact, you came upon a person, a mother who did have an infant with her or had a breast pump with her. That the bias would be exaggerated even more."
Although they are surprised at their findings, the researchers insisted that women should not stop breastfeeding because of this findings.
However, they should try to be more wary of prejudice towards them, they added.