Esposito is due to give birth to her first child in August - the Tokyo Olympics start on July 24.

Esposito says she's not planning to retire from the sport in which she won gold at the 2016 Rio Olympics.

"A wonderful unexpected surprise has happened. My husband, Matt Cooper, and I are bringing a mini Cooper into this world in August," Esposito posted on social media on Thursday.

"True to form, nothing ever works out the way we plan it.

"Defending my title will have to wait another four years. Can't wait to be a mum.

"My priorities will change but the challenge of pregnancy to podium is a powerful dream."

Esposito became Australia's first Olympic modern pentathlon gold medallist in Rio, then took a year off from the multi-discipline sport.

She returned to international competition to win the World Cup final in 2018 and take over the No.1 ranking in the sport.

The 28-year-old endured frustration last year when hamstring injuries curtailed her output, but had scheduled a return to competition within months until pregnancy altered that plan.

Modern Pentathlon Australia president and former chef de mission of Australia's Olympic team, Kitty Chiller said the pregnancy wasn't an end to Esposito's career.

"I would like to express my congratulations to Chloe and Matt ... they will be fantastic parents," Chiller said in a statement.

"We would have of course loved to have seen Chloe on the start line in Tokyo to defend her Olympic title.

"However we know this is not the end of her remarkable career and look forward to supporting her in due course when she returns to the sport with an eye to the Paris Olympics in 2024."

Australian Olympic Committee president John Coates also congratulated Esposito on her announcement.

"While disappointed our Tokyo team will be missing one of its brightest stars, we are delighted with her news," he said in a statement.

"Chloe has been a wonderful ambassador for her sport, Australia and the Olympic movement.

"I am sure we will see her back in Australian colours to recapture the title she won in Rio."

Australian Associated Press