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WASHINGTON (Reuters) - Zoo officials in Washington hope they can get lucky again with female giant panda Mei Xiang.

The Smithsonian’s National Zoo is trying to get the 18-year-old giant panda pregnant in hopes she will deliver her fourth living cub in a move to boost the species, officials said on Friday.

There are about 1,800 giant pandas in the wild, according to the International Union for Conservation of Nature, which classifies them as “vulnerable.”

Mei Xiang had artificial insemination performed on her twice on Thursday after she had shown signs of being in estrus, or capable of becoming pregnant, the zoo said in a statement.

The pregnancy window is very narrow for pandas, which are native to China and star attractions at the National Zoo. Estrus lasts only 24 to 72 hours per year, and Mei Xiang’s estrogen levels had peaked on Wednesday, it said.

If Mei Xiang becomes pregnant, the cub likely would be born around the end of September, the zoo said. It turned to artificial insemination using semen from the zoo’s 19-year-old male, Tian Tian, when he and Mei Xiang were unable to mate.

Giant pandas have low reproductive rates and Mei Xiang has had several false pregnancies. She has three surviving cubs - Tai Shan, born in 2005; Bao Bao, born in 2013, and Bei Bei, born in 2015 and still at the zoo.

Tai Shan, the first panda born at the National Zoo to survive past birth, is at a research center in China. Bao Bao left for China in February as part of a breeding and research program.

The National Zoo received its first pair of giant pandas in 1972 as a gift from China.