Six-time Olympic champion Allyson Felix has revealed that she gave birth to her daughter eight weeks premature last month.
The American, 33, had not publicly announced that she was pregnant.
Her daughter Camryn is still in a neonatal intensive care unit (NICU) after she had to be delivered by an emergency caesarean section on 28 November but Felix said she is "OK".
"I'm so, so grateful," said sprinter Felix in a first-person piece for ESPN.
"Every day I sit with my daughter in the NICU and watch her fight. Every day she gets stronger and more beautiful."
Felix, whose Olympic honours include 200m gold at London 2012 and five relay gold medals, said she competed twice in June, running the 400m in 51 and 52 seconds while eight and nine weeks pregnant respectively.
She said she felt if she "kept the news to herself" and kept training hard then she could still win races, citing tennis' 23-time Grand Slam singles champion Serena Williams, who won the 2017 Australian Open in the early stages of her pregnancy.
Felix added the pressure of "trying to live up to" her "pristine nice girl image" contributed to her not announcing she was pregnant.
"Having a child felt like I'd be risking my career and disappointing everyone who expected me to always put running first," she said.
Subsequently Felix did make plans to reveal her pregnancy, until a check-up revealed the baby's heart-rate was decelerating, leading to the emergency C-section, with Camryn born at three pounds seven ounces.
"I didn't care if I ever ran track again, I was just praying that she would be OK," said Felix.
Felix has won nine Olympic medals in total - the most by a female track and field athlete alongside Merlene Ottey - and intends to race at her fifth Games at Tokyo 2020. She is also an 11-time world outdoor champion.
However, she said: "If I come back and I'm just not the same, if I can't make a fifth Olympic team, I'm gonna know that I fought, that I was determined, and that I gave it my absolute all."