The Duchess of Cambridge's third trip to the delivery ward is expected to be fast - if not faster - than before.
In 2013 Kate gave birth to Prince George in 10 and a half hours after being admitted to hospital, while Princess Charlotte made her appearance in just two hours and 34 minutes.
While it varies from woman to woman, many mothers report a much speedier birth when it comes to baby number three.
Kate and the Duke of Cambridge made their way by to London's St Mary's Hospital on Monday morning with Kensington Palace issuing a statement that the Duchess was in the early stages of labour.
Babycentre.co.uk says for first-time mothers, active labour may take around eight hours, dropping to five hours for baby number two.
"It will vary from one woman to another. Some women report a quicker labour with their third baby but not everyone," Clare Livingstone, professional policy adviser at the Royal College of Midwives, said.
"It seems to be though that they do go more quickly. It's a very individual thing."
It is not known how long the Duchess was in labour with George and Charlotte before she was admitted.
But quick labours can be more painful, Mrs Livingstone said.
"The body has natural painkillers - endorphins - which build up over a period of time. So a quick labour doesn't allow the body the opportunity to do that," she said.
Kate had a straightforward, natural birth with George and Charlotte with no complications.
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