US athlete Lindsay Flach competes in trials at 18 weeks pregnant

Lindsay Flach ran 100 metres of the 800-metre run. Photo / Supplied

A US athlete has competed in national Olympic trials while 18 weeks pregnant.

Lindsay Flach took on her rivals in one of the toughest events, the heptathalon, while sporting a prominent baby bump.

She announced over the weekend that she had been keeping a "secret".

"The secret is no secret anymore," she wrote on Instagram.

"Every story has an end but in life every end is a new beginning."

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A post shared by Lindsay Schwartz Flach (@lcschwartz22)

Flach, 31, has previously tried out for the 2012 and 2016 Olympics.

The heptathalon sees athletes compete is seven different disciplines: the 800-metre run, 100-metre hurdles, high jump, shot put, 200-metre sprint, javelin throw and long jump.

Flach took part in all seven events, intentionally finishing 100 metres of the 800-metre run and eventually placing 15th out of 18 competitors.

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The trials took place in the middle of a record-breaking heatwave currently hitting the Pacific Northwest of the United States, seeing temperatures rocket and forcing officials to delay competition until the evening.

Flach's supporters praised her for being a "great role model," a "badass" and a "hero".

"I got emotional and cried watching you compete," wrote one fan on Instagram.

"So happy to see an expecting mother compete at such an elite level! What a story to tell your baby."