It was the surprise of Emma Williams life – giving birth to her baby girl on the back seat of an Uber.

Baby Lexi was born on Friday afternoon and delivered by her own mother.

Uber has offered to pay the cleaning fees. (9news)

Both mum and baby are doing well. (9news)

Since then, several people have asked her the same question.

“The first thing people say is ‘why didn't you get an ambulance?’ which I say ‘well I didn't expect to have a baby in under an hour,’” Ms Williams said.

The birth of her son Blake took seven hours so she thought she had enough time to get to Fiona Stanley Hospital, which is 20 minutes away.

She started feeling cramps while cooling off in the pool on the 37 degree day.

Emma Williams said she didn't expect her baby to be born so quickly. (9news)

The Uber driver was "shocked". (9news)

“I got out of the pool and started timing and basically messaged my husband to say maybe don't go for beers tonight potentially I might be in labour,” she said, before calling for a ride.

Ms Williams was picked up from home at 4.25pm and within 24 minutes she had given birth, however she didn’t spill the beans to her Uber driver until the last minute.

“I didn't say anything to the guy, I just said can you just switch the air-con up (because) I was in a lot of pain and needed to breathe through it,” she explained.

“Then I just shouted out to him I’m really sorry but I’m in labour and I’m about to have a baby in your car and I think I freaked him out a little bit.”

Lexi was born in the back of an Uber. (9news)

She said her Uber driver was shocked during the drive, but now can’t wait to meet the newborn.

Uber has offered to pick up the clean-up bill saying they are delighted to hear their newest and youngest rider made a surprise entrance into the world.