Felix gave birth to daughter Camryn by emergency caesarean section, 32 weeks into her pregnancy in November.

The 33-year-old ran 52.20 seconds to qualify for Friday's 400m semi-finals.

"Things that were once very easy for me are now pretty challenging," said the 11-time world champion.

"I'm a regular mom. I'm in the hotel cleaning bottles, changing diapers and getting ready for races."

Felix was fourth in her heat, nearly three seconds slower than her personal best, at the four-day meet in Des Moines.

The US Championships serve as trials for the World Championships in Doha, which start on 27 September.

"It was rusty, but kind of expected since I haven't raced in a long time," she added. "It was very humbling. I'm just happy to be running because I didn't know if that was going to be the case."

Hours before her race, Felix posted an emotional statement on Instagram, with a picture of herself in a neonatal intensive care unit.

She wrote: "Today I'll step on the starting line for the first time in over a year. It might sound cliche, but making it there for me is a huge victory.

"Almost eight months ago this was my entire world. staying in the NICU all day and night watching my baby girl fight. I can still hear the beeping and alarms of the machines. the uncertainty, the fear.

"There were a lot of days I wasn't sure this was going to be possible. I worked harder than I even knew I could. there were tears, frustration and doubt. At times it felt like everything was against me.

"So today I'm far from my best, but I'm grateful for this opportunity and to experience the joy of competing again."

Felix posted this throwback picture when her daughter was in a neonatal unit hours before her race

Elsewhere, reigning world champion Justin Gatlin ran 10.16 seconds in his 100m heat, but was second behind training partner Isiah Young, who crossed the line in 10.14.

Gatlin needed to run just one round to collect his bye to the World Championships, but is expected to race in Friday's semi-finals.