The Duchess of Sussex can expect to spend around 12 to 18 hours in active labour, according to an antenatal expert.
On average, first babies take between a day to a day-and-a-half to arrive, but some come much quicker or take even longer.
It is not known how long Meghan has already been in labour as she and the Duke of Sussex prepare to welcome their first child.
Although every woman’s labour is different, Meghan’s sister-in-law, the Duchess of Cambridge, went into hospital at around 6am on the day her first child, Prince George, was born and he arrived 10-and-a-half hours later at 4.24pm.
When Kate gave birth for a second time, she delivered Princess Charlotte just two hours and 34 minutes after being admitted to the Lindo Wing of St Mary’s Hospital in Paddington.
With Prince Louis, news came at 8.34am that Kate had gone to the Lindo Wing early in the morning, and her third child arrived less than three hours later at 11.01am.
It is not known how long the duchess was in labour with George, Charlotte and Louis before she was admitted.
Val Willcox, antenatal teacher at the National Childbirth Trust (NCT), said: “On average, the active stage of labour, once the woman has got to about 4cm or 5cm dilation, from there to the birth is around 12-18 hours.
“But that’s an average. You can have some births that are much much faster and some that unfold over two or three days.
“It’s a very, very individual thing. So long as the mum and baby are OK, it usually doesn’t matter how long it takes.
“It’s about making sure mum and baby are both OK, resting, staying fed and hydrated – if it’s a longer labour that’s really, really important.”
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Mrs Willcox added that Meghan, like many pregnant women, might have initially been in labour without knowing.
“Some women don’t realise when their labour starts because they might be getting aches and pains and niggly things that they’ve been having during their pregnancy,” she said.
“There might be quite a lot going on in their body before they realise these are regular contractions and this is definitely labour.”