Giant pandas Mei Xiang (search) and Tian Tian, star residents of the National Zoo, are gaining a little more privacy this year as they approach their oh-so-brief mating season.

Unlike in past years, when visitors could be on hand as the pandas mated, the zoo is closing the panda exhibit Thursday for a few days.

Making panda babies is a tricky business with no room for error. Females are in heat for all of a day or two a year.

Zoo officials tracking the hormones in Mei Xiang said Wednesday it appears she will be ready to start mating with Tian Tian (search) within the next three days. She is usually not ready until April or early May. Mei Xiang's estrogen is rising rapidly, however, and was expected to peak within 24 hours.

"Once the estrogen begins to drop, that's when prime time starts for panda mating," National Zoo (search) spokesman John Gibbons said. "It's once a year and that's one thing that makes panda breeding so challenging."

Mei Xiang, 6, and Tian Tian, 7, are the National Zoo's second pair of giant pandas. Both went on exhibit in December 2000 and are on a 10-year loan from China. The zoo said any baby would belong to China and probably would be sent there.

Three attempts to get Mei Xiang pregnant have failed. Taking tips from panda experts, the National Zoo plans to "create expectation between the two," as Gibbons put it. They will be put together for short periods, then separated, a process to be repeated at different times.