Beth rushed to the hospital with suspected kidney stones. She left with a newborn baby.

When 45-year-old California mum Beth Clay rushed to the hospital last month, she had no idea she would soon be meeting her third child.

Suffering from excruciating abdominal pain, Beth believed she had kidney stones but was soon told by her doctors that little Liam Ryder Clay was set to enter the world.

Already proud parents to two adult children – 27-year-old Maggie and Will, 20 – Beth and her husband, 51-year-old Scott, had no idea that a third baby was on the way.

“The shock is just amazing,” Beth told KTLA News.

"I was like, 'That just can't be real. It's all contrived, never could happen. You would know if there was a baby in your belly'," Scott said.

"But we're living proof."

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Beth said she was in "tiptop" shape before she went into surprise labour, and had been maintaining a regular exercise regime and even keeping to a strict diet that saw her lose almost 7kg during her pregnancy.

For nine months, Beth said she never even felt her baby move or kick.

"The doctor feels sure that he was behind everything, behind my kidneys, behind my liver," Beth said.

"I was in tiptop shape, and then... here's this little rascal."

The couple revealed they were so shocked, they waited a whole day before telling friends and family of Liam's surprise arrival.

"Just so we could absorb what was actually happening. It's huge," Beth said.

Friends have left happy comments and congratulations on Facebook after hearing the news.

"Holy moly! What a wonderful surprise!" one friend wrote, while another commented, "This is the craziest, best news I've heard in a while!"

Others joked that the newborn baby was "way better than a kidney stone".

Baby Liam spent a few days in the intensive care unit before heading home to be with his admittedly bewildered parents, who are now preparing to re-enter the nappy-changing and sleepless nights of raising a newborn.