The National Zoo’s giant female panda is in heat, officials said, and scientists are monitoring her closely.

Mei Xiang, who is 18 years old and has birthed three cubs at the zoo, has been showing signs over the past few weeks of being in estrus. Instead of eating bamboo or climbing trees, the giant panda has been more restless and pacing around her enclosure. She has also been playing more often in her pools and water features, according to Jen Zoon, a zoo spokeswoman.

All are signs, Zoon said, that Mei Xiang is “approaching estrus.”

Zookeepers and scientists are closely monitoring the levels of estrogen and progesterone in the panda mom’s urine to determine when she is in “peak estrus.” Then, officials said, there will be a “very small” window for when she will be able to breed and conceive, according to Zoon.

Giant pandas have a yearly peak estrus period that runs between 24 and 72 hours. “It has to happen within that window because that’s the only time her egg is released for a whole year,” Zoon said.

Once scientists determine that she is in peak estrus, they’ll watch her behavior to see how she is acting toward Tian Tian, the zoo’s giant male panda. “If they are both acting interested in one another, the keepers will give them an opportunity to breed naturally,” Zoon said. Giant pandas are solitary animals and are kept in their own respective inclosures.

Zoon said that if the pandas are interested they’ll vocalize by making a bleating sound like a sheep and also will probably make a chirplike sound to each other.

Enter the “mesh door,” also known as the “howdy door.” According to Zoon, the door allows the two giant pandas to “see and smell each other.” The zookeepers can open the door, and “depending on who moves first when the door is open is where the pandas will interact,” Zoon said.

“Mei Xiang and Tian Tian have never bred naturally successfully, but that doesn’t mean we don’t try,” Zoon said. If they don’t show signs of interacting, Zoon said, Mei Xiang will be artificially inseminated with semen from Tian Tian.

But it could be several months before officials can confirm whether Mei Xiang is pregnant. She is trained to allow keepers to perform ultrasounds, but they do them only if she is willing. Giant pandas can also act as if they are pregnant — what experts call “pseudo-pregnancy” — where they mimic all the behaviors of being pregnant, including going into their dens and sleeping more and cradling toys.

A panda’s gestation period ranges from 90 to 180 days. The average pregnancy lasts 135 days. The gestation period varies because the fertilized egg floats in the mother’s uterus before it is implanted and starts to develop. The embryo begins to develop once it attaches to the uterine wall, zoo officials said. Then, of course, the baby-panda watch begins.

“We are very excited of the prospect that Mei Xiang could have another cub,” Zoon said.

Mei Xiang has three surviving cubs — Tai Shan, Bao Bao and Bei Bei. All of them were fathered by Tian Tian. Under an agreement with China, the zoo’s pandas are under lease to U.S. zoos. Bao Bao and her brother Tai Shan are in China. Bei Bei is still at the zoo with his mother.