Brookfield Zoo’s bottlenose dolphin is making healthy progress toward giving birth to the zoo’s first bottlenose dolphin calf in a decade.

Allie, the zoo’s 37-year-old bottlenose dolphin, has entered her third trimester of a 12-month pregnancy, the zoo said Friday, while sharing some of the first glimpses of the growing marine mammal from recent ultrasounds of the mother dolphin.

“The calf is currently about two to three feet long or about the length of two footballs laid end-to-end,” said Dr. Jen Langan, senior veterinarian at Brookfield Zoo Chicago. “During the third trimester, dolphin calves are in a folded ‘u’ or ‘j’-like position as they continue to grow up to four feet long before birth.”

Zoo officials also are monitoring the calf’s organ development, fetal heart rate, movement and other health conditions through medical check-ups and ultrasounds on Allie.

The zoo announced Allie’s pregnancy in January, when she entered her second trimester.

The calf is expected to be born sometime in June, though the zoo said changes in Allie’s behavior that would indicate imminent labor could begin in May. It’s still too early to tell the calf’s gender.

Zoo officials said they are optimistic about a smooth delivery later this year because Allie is an “experienced mother” that has already given birth to five previous calves.

The pregnancy has provided an exclusive first-hand look at the bottlenose dolphin mating process that is otherwise unfeasible in the wild.

“This pregnancy is more than a joyous event for the Zoo – every dolphin pregnancy is an opportunity for us to advance our scientific knowledge and understanding of the biology and life sciences of marine mammals,” Brookfield Zoo Chicago President and CEO Dr. Michael Adkesson said in a previous statement.

“It underscores our Zoo’s commitment to marine mammal conservation and enhances our field-based scientific research programs that benefit wild dolphin populations in the Gulf of Mexico and around the globe.”

The calf will join seven other dolphins at Brookfield Zoo and spend its early years closely following Allie and learning from other dolphins in the exhibit.

Before zoo guests can see the new calf, staff will monitor it closely to make sure it’s progressing properly when learning how to nurse and swim alongside its mother, using her wake to propel the calf along.

Bottlenose dolphins grow to 7 to 12 feet in length and can weigh 400 to 1,000 pounds, according to the zoo. They can live to be 40 to 60 years old.