Ellis said plans are under way to raise money to expand the Sumatran Rhino Sanctuary, built with foundation funds in 1996 and managed in cooperation with the Indonesian government and the Rhino Foundation of Indonesia.
That way, "other animals that are found out in the forest that are reproductively viable can be brought there to be part of the managed breeding program," Ellis said.
Births of Sumatran rhinos in captivity are rare. Just four Sumatran rhinos have been born at breeding facilities, including the Sumatran Rhino Sanctuary and Cincinnati Zoo.
Ratu's first baby, Andatu, was born at the Sumatran Rhino Sanctuary in 2012, marking "the first Sumatran rhino born in an Asian breeding facility in more than 140 years," said a statement by Siti Urbana Bakar, the Indonesian Minister of Environment and Forestry.
The current pregnancy "represents nearly two decades of international collaboration to save the species," he added.
Andalas, the father of both Andatu and the expected calf, was born at Cincinnati Zoo. He was moved to Indonesia from Los Angeles Zoo in 2007.
The only remaining Sumatran rhino in the United States is Harapan, Andalas' younger brother.
He will be moved from Cincinnati Zoo to Indonesia later this year in an effort to boost the breeding population at the rhino sanctuary, which is currently home to five Sumatran rhinos.
Terri Roth, vice president of conservation at Cincinnati Zoo, said it is difficult to say goodbye, but it is the right thing to do for the rhino population.
"It is hard on the staff and there are a lot of people here in Cincinnati who have fallen in love" with Harapan and the other rhinos that used to live at the zoo, she told AFP.
Harapan's sister, Suci, died last year of an inherited disease, leaving him alone at the zoo.
As for Ratu, she has months to wait before giving birth to a calf that will likely weigh 50-60 pounds (23-27 kilograms).