A growing number of women may be having serious childbirth complications due to postpartum bleeding, a study from Australia suggests.

In an analysis of hospital records for more than 500,000 women who gave birth between 1999 and 2004, researchers found that the rate of "adverse maternal outcomes" increased by nearly 4 percent per year.

In 1999, the rate of such problems — including kidney failure, shock or a need for blood transfusion — was 11.5 per 1,000 deliveries. In 2004, the figure was 13.8.

Nearly all of the increase was seen among women who suffered hemorrhaging after giving birth, the researchers report in the online journal BMC Pregnancy and Childbirth.

It's not clear why postpartum bleeding increased over time. Women in the study all gave birth at hospitals in New South Wales, but similar trends have been seen elsewhere.

"The rise in (postpartum hemorrhage) has been observed internationally and to date it cannot be explained," lead researcher Dr. Christine L. Roberts, of the University of Sydney, told Reuters Health.

In theory, factors such as increasing maternal age and growing rates of twin pregnancies and C-section deliveries could be to blame.

However, a previous study by Roberts and her colleagues found no evidence that those trends explain the increase in postpartum bleeding. A study from Canada came to similar conclusions, Roberts said.

Increasing weight among expectant mothers is another potential factor, since overweight women are a greater risk of postpartum bleeding. But there is no data on that so far, Roberts noted.

It's also possible, she speculated, that the problem is related to doctors' management of the third stage of labor, during which the placenta is delivered.

Some cases of postpartum bleeding might be prevented by more "active" management of this stage of labor — which, Roberts explained, involves giving medication to help the uterus contract and delivering the placenta in a "controlled way."

She noted that a recent review of four clinical trials found that such active management cut the risk of postpartum hemorrhage by 60 percent.