Baby bump or blubber? When you're looking at a 221-pound female sea lion, it's hard to tell.

But the Pittsburgh Zoo & PPG Aquarium announced yesterday that Zoey, one of its two 14-year-old female sea lions, is pregnant and they're rushing to "pup-proof" the Kids Kingdom sea lion exhibit for a birth that could occur as early as next month but probably not until June.

To make the sea lion area more accommodating, zoo keepers are removing some of the clutter, including removing some vegetation, to free up more denning area, and building lower swim platforms to make it easier for the pup to get out of the big exhibit tank. If Zoey gives birth in the indoor holding area, she and her pup may be isolated for a week or two.

The prep time is short because even though zookeepers noticed "breeding activity" at the sea lion exhibit last May and June, they weren't able to confirm the pregnancy until January, when they started doing ultrasounds on Zoey and her sister, Maggie.

"We first came upon it accidentally," said Judy Obeldobel, the head sea lion trainer. "Because they're of breeding age and we have a young male, we started doing the ultrasounds. Now we're saying, teasingly, that we can see the baby bump, but it's not easy."

Mating season for sea lions is in late spring and early summer and gestation is nine months. But sea lions have the ability to delay implantation of embryos for two or three months, keeping the embryo in a dormant state before it begins to develop, so pups can be born in the summer when food is more plentiful.

Ms. Obeldobel said they first suspected Maggie might be pregnant, too, because she was exhibiting the same behavior and symptoms that Zoey was. That includes increased appetites -- each has increased her fish consumption from 16 pounds to 20 pounds a day. But all of Maggie's ultrasounds have been negative.

Whenever Zoey gives birth, her pup will be the first sea lion birth at the zoo in Highland Park. Zoey and Maggie were born at the Indianapolis Zoo. They spent time there with other pups until they were about 2 years old, an experience that may help Zoey mother her new pup, Ms. Obeldobel said.

"There have been some zoos that have bred sea lions in the past but not that many," she said. "At this time there is a push to breed more and to get genetic diversity."

Maggie came to Pittsburgh in 1997 and Zoey in 1998. Seahawk, the 4-year-old father, was born and orphaned in the wild in California and rescued when he beached himself on the sand at Santa Cruz and Bolinas Beach. He was brought to Pittsburgh in 2006.

Pups are born on land, weigh about 13 pounds and are 21/2 feet long. They are dark brown to black, but their fur fades to light brown within a couple of weeks. They are well developed at birth, opening their eyes and vocalizing to and with their mother.

"The mother will be vocal and loud," Ms. Obeldobel said. "The pups usually come out head first and answer to mom right away. That's how they identify each other, by sound and smell."

Within a half-hour after birth a sea lion pup is able to shake, groom, scratch and walk. Swimming first occurs a few days after birth and is awkward at first and sometimes assisted by the mother.

The mortality rate for sea lion pups is 10 percent to 15 percent within the first month, and the pup is dependent on development of a bond with the mother, said Dr. Barbara Baker, zoo president and chief executive officer.

"This is the first sea lion baby to be born at the Pittsburgh Zoo, and we are very excited, but like expectant parents, a little nervous, too," she said. "A milestone after the birth is Zoey's acceptance of her pup and her ability to take care of it and nurse it."

California sea lion populations, once decimated by hunting and fishermen, are increasing, but they remain protected under the Marine Mammal Protection Act.