Breastfeeding has been linked to many benefits for babies
Mothers who find breastfeeding so hard that they give up should not blame themselves, researchers say.
A Norwegian study concludes that difficulty feeding a newborn may be down to higher levels of the male hormone testosterone during pregnancy.
Having reviewed all of the available evidence, researchers also cast doubt on the health benefits of breast milk over formula.
The work features in Acta Obstetricia and Gynacologica Scandinavica.
The team from the Norwegian University of Science and Technology followed 180 pregnant women.
These included those at risk of delivering a small baby - births which are known to be influenced by higher levels of the male sex hormone testosterone.
After taking into account other factors such as age, education and smoking, they still found a clear relationship between low rates of breastfeeding at three and six months and higher levels of testosterone.
There are many reasons why a mother might have high levels of this male hormone in her womb during pregnancy as the placenta - an engine of hormone production - kicks into action.
The team suggest that the hormone may impact negatively on the development of glandular tissue in the breast, in turn affecting her ability to feed her baby.
Lead researcher Professor Sven Carlsen said: "Basically a mother who finds she has difficulty shouldn't feel guilty - it probably is just the way it is, and her baby will not suffer for being fed formula milk.
"A mother should do what makes her happy."
He argues that it is the hormone balance in the womb which explains both a mother's ability to breastfeed and any apparent health benefits of a baby who is breastfed - rather than the breast milk itself.
Last year Professor Carlsen's team reviewed 50 international studies on the relationship between breastfeeding and health.
Based on this work he concludes the benefits of breast over formula milk may have been exaggerated.
"These health differences are really not so significant in any event.
"When you look at the epidemiological studies and try to strip away the other factors, it is really hard to find any substantial benefits among children who were breastfed as babies."
Professor Ashley Grossman of the Centre for Endocrinology at Barts in London said the suggestion that male hormones may influence the ability to feed was an interesting one.
"There may be all sorts of biological factors which affect a woman's ability to breastfeed, and when women are always being told to go away and try harder it's important to stress some simply cannot.
"We are learning more and more about how the environment of the womb may influence a child's future development - this is really where it's all happening, and it has a much greater impact than whether or not a child is breastfed or not."
The Department of Health recommends that all babies be exclusively breastfeed for the first six months.
A spokesman said: "The government recognises that breastfeeding is the best form of nutrition for infants.
"It gives health benefits for both the baby and the mother - even after they are no longer breastfeeding.
"It protects against stomach bugs and chest infections, provides perfect nutrition for the first six months and reduces the likelihood of becoming obese in later childhood."