Almost one in 10 women who give birth via caesarean section suffer an infection, statistics show.
The Health Protection Agency (HPA) found that of 4,107 women who had C-sections, 394 suffered infections.
Infection was more common in overweight and obese women, according to the findings published in the British Journal of Obstetrics and Gynaecology.
The study looked at data from 4,107 caesarean operations from 14 hospitals across England in 2009.
While the majority (88%) of the infections were minor, 25 of the women suffered from infection of the lining of the womb and two had reproductive tract infections.
Those who were overweight were 1.6 times more likely to develop an infection while obese women were between 2.4 and 3.7 times more likely.
Dr Elizabeth Sheridan, head of healthcare associated infections at the HPA, said: "Reducing rates of surgical site infections following a caesarean should be made a priority.
"Given that one in four women deliver their baby by caesarean section, these infections represent a substantial burden.
"They will impact not only directly on the mother and her family but also are a significant cost in terms of antibiotic use, GP time and midwife care, and every effort should be made to avoid them.
"Women choosing to have caesarean section for non-medical reasons should be aware of the risk of infection, particularly if they are overweight."
Researchers also found that women under the age of 20 were almost twice as likely to develop an infection compared with those aged 25 to 30, but they suggested that more research is needed in this area.
Gail Johnson, education and professional development adviser at the Royal College of Midwives, said: "This further supports the need to ensure that any caesarean section is performed only where clinically indicated, following full and detailed discussion with the woman, the midwife and the obstetrician in accordance with Nice guidelines.
"Women who develop an infection postnatally are likely to feel less able to provide care to their baby and will take longer to recover from the birth.
"The research identifies a number of factors which increase the risk of infection post-caesarean section, one of which is obesity.
"Obesity is recognised as a serious public health problem that needs addressing. There is increasing evidence about the potential risks of obesity during pregnancy and birth.
"Midwives are in a position to advise and support women with weight management in pregnancy, with the aim of early detection or preventing the problems that can develop as a result of obesity."
A Department of Health spokeswoman said: "We keep all emerging findings under review. Whether to have a caesarean or not remains a decision that a woman must reach with the health professionals providing her care. Decisions should take Nice (National Institute for Health and Clinical Excellence) guidance into account.
"Annually the numbers of MRSA and C.difficile (Clostridium difficile) infections are now at their lowest-ever level since mandatory reporting for each was introduced. Hospitals should extend measures that have reduced the number of these infections across all areas."