Clare Livingstone, professional policy advisor at the Royal College of Midwives, said third-time mothers like Kate - who can be distracted as they care for older children - need to be aware of any changes in the run up to delivery.
Will the royal baby be a boy or a girl?Mrs Livingstone, who was a midwife for nine years, said: "Third-time mums are more relaxed.
"They're busier than before, especially when they're looking after toddlers.
"They're probably more tired so it's the importance of rest, nutrition, hydration, monitoring baby's movements and reporting any noticeable difference with that."
She advised Kate and other women expecting their third "to be very aware".
"When they're preoccupied with a busy family... they may not be as aware as they have been with first pregnancies when you're very watchful."
When the Duchess gave birth for a second time, she delivered Princess Charlotte just two hours and 34 minutes after being admitted to hospital.
All you need to know about the royal babyHer third labour will be expected to be as fast - if not faster.
Mrs Livingstone said: "It will vary from one woman to another. Some women report a quicker labour with their third baby but not everyone.
"It seems to be though that they do go more quickly. It's a very individual thing."
In 2013, Kate went into hospital at around 6am on the day George was born and he arrived 10-and-a-half hours later at 4.24pm.
It is not known how long the Duchess was in labour with George and Charlotte before she was admitted.
Subsequent labours tend to be much faster, with active labour lasting, on average, five hours, according to parenting site BabyCentre.co.uk.
Mrs Livingstone added that quick labours can appear more painful.
"I would say that very rapid labour can feel more painful because of the intensity and how quickly it can feel quite overwhelming," she 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."
Advice site Madeformums.com said 2% of women experience a precipitate labour - where labour is extremely rapid and lasts less than two hours from first contraction twinge to giving birth.
"They're more common among women who've already given birth at least once," the site said.
"If you've already given birth, your cervix has been stretched, so the uterus may simply know what it's doing and work a bit too efficiently."
It adds: "If labour starts and there's any doubt whether you'll make it to hospital in time, don't even try."
If this were to happen to the Duchess, her medical team would decamp to Kensington Palace to assist with the birth.
She could even have a consultant or midwife on standby at the palace ready to help, if she wanted to be prepared for all eventualities.
Mrs Livingstone said: "Some women have what are known as precipitate labours. They can be extremely fast. It tends to be the case that if you've had one before, you're more likely to have another one."
Kate had a straightforward, natural birth with George and Charlotte with no complications.
As well as having a hospital bag packed with nappies and babygros, the Duchess will be making preparations for appearing in front of the world's media when she takes her baby home.
Her hairdresser Amanda Cook Turner will be on call ready to come to St Mary's, as will Kate's stylist Natasha Archer, armed with Kate's carefully selected outfit.