When Maxxandra Ford found out she was 35 weeks pregnant, she was very surprised.
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However the US woman got an even bigger shock when her third child was born weighing 6.35kg (14 pounds) just a few weeks later.
Ms Ford initially went to the doctor to find out why she had been putting on excessive weight in recent months.
But the mum, who already has a one-year-old son and five-year-old daughter, did not realise she was pregnant, and thought the weight gain was being caused by some other medical issue.
After doctors informed Ms Ford she was 35 weeks pregnant with her third child, the thought she might be expecting twins crossed her mind due to the amount of weight she had gained.
Instead, she gave birth to son Avery Denton, who was about the same weight as two average babies, at St Joseph's Women's Hospital in Tampa, Florida on January 29.
Ms Ford said she realised her son was a very big baby wile she was giving birth.
"I was cussing up a storm," Ms Ford told local news outlet WFLA. "When I felt his head come out, I knew he was bigger than 10 pounds."
The boy is the biggest baby ever born at St Joseph's and one of the largest born in the state.
His older brother and sister weighed 4.08kg (9 pounds) and 4.5kg (10 pounds).
Doctors say that not-so-little Avery is healthy at the moment, but his parents will need to watch his diet to ensure he doesn't become obese as he grows up.