St. Louis Zoo officials are unsure why Ellie, a pregnant Asian elephant who has successfully given birth to two calves in the past, suffered a miscarriage Thursday morning.

"Our veterinary pathologists will see if there is any problem, but we don't expect to detect anything," said Martha Fischer, curator of mammals for the zoo. "Miscarriages unfortunately happen among mammals, usually in the first trimester."

Ellie was 36 weeks pregnant. The duration of an elephant's pregnancy is 22 months.

One of Ellie's progeny, Rani, is in her 45th week of pregnancy and expected to give birth next summer. Rani was born in 1996. Ellie is also the mother of Maliha, born in 2006.

Fischer said veterinarians and staff had followed zoo procedure by closely monitoring Ellie's health since her impregnation by Raja, the zoo's bull elephant.

The protocol included physical exams, blood tests, ultrasounds and even a prenatal diet and exercise program.

Officials stepped up the monitoring when tests earlier this week revealed changes in the 38-year-old elephant's blood progesterone levels.

The staff discovered the miscarriage Thursday morning.

Fischer said problem pregnancies were no more unusual among elephants than any other mammals.

Catherine Doyle, a spokeswoman with In Defense of Animals, disputed that contention.

An opponent of captive breeding, Doyle tracks elephant births in U.S. and Canadian zoos.

Although 28 Asian elephants have been born in North America since 1998, another five pregnancies (15 percent) have ended in still birth, according to Doyle.

In Defense of Animals also has a complaint on file with the U.S. Department of Agriculture over a herpes virus that has infected at least two Asian elephants at the St. Louis Zoo.

There is a medical connection between herpes and miscarriage, Doyle said.