HOUSTON -

The Houston Zoo says a pregnant elephant miscarried twin calves overnight.

Tess the elephant was nearly halfway through her pregnancy, which was developing normally according to veterinarians, but the zoo says its team did not discover she was pregnant with twins until the miscarriage.

According to the zoo, the instance of twin pregnancies in Asian elephants is extremely rare and the survival rate after birth of the calves is exceedingly unlikely.

Just two weeks ago, the zoo said it was putting Tess on a pre-natal weight management program because she was 6 percent overweight.

Tess is now being monitored closely by the entire elephant care team and veterinarians for indications of any possible signs of physical and psychological effects from the miscarriage, the zoo says.