More new mothers dying from strep bug
While the overall numbers are low, there are concerns about Group A streptococcal disease (GAS), which can develop into the so-called "flesh-eating" bug.
Experts said women should be vigilant to symptoms of the bug and make sure they wash their hands regularly to stop it spreading.
Group A streptococcus is commonly found on the skin and inside the throat, and in many people does not cause any symptoms.
The most common types of GAS infection are a sore throat, sometimes called strep throat, and minor skin infections such as impetigo - a skin condition often carried by children.
GAS presents a more serious problem if the bacteria penetrate further into the body, such as into the blood or into a deeper layer of skin.
Pregnant women and those who have just given birth are particularly at risk from this more serious type.
Experts at the Centre for Maternal and Child Enquiries (CMACE) issued a warning to health workers to be alert to signs of GAS.
They said midwives in the community may be first to notice women who are in trouble.
In a three-year study period from 2006 to 2008, 29 women in total died as a direct result of infections. This is up on the 21 from 2003 to 2005 and 13 from 2000 to 2002.
Of the 29 women in the most recent study, 13 died from GAS, up from eight and three in the previous reports.
Dr Imogen Stephens, clinical director at CMACE, said: "It's shocking that something like infections - and infection from a reasonably well-known organism - is a major cause of maternal mortality."