On top of this, Meghan Markle, 37, and Prince Harry, 34, announced yesterday they were expecting their first child.
Pippa was seen arriving at the private London hospital the Lindo Wing with her husband James Matthews, 43, yesterday, leading to speculation the birth is imminent.
So where is Kate today as another addition to her family arrives? Is she with her sister?
Mum-of-three Kate has not been seen arriving at the hospital, and is most likely spending the day with her own brood.
She has no official engagements in the diary today, although her husband, Prince William, 36, will attend the World Premiere of ‘They Shall Not Grow Old’, Peter Jackson’s First World War film at the British Film Institute tonight.
When Kate was in the Lindo Wing giving birth to George, Charlotte and Louis, she was not visited by her siblings.
So it is likely Kate will not be seen today, and will instead meet her new relative in the privacy of her home.
Heavily pregnant Pippa was last seen just this weekend, attending the wedding of Princess Eugenie and Jack Brooksbank.
Pippa wore a forest green dress by Royal favourite, Emilia Wickstead, and chose a matching green hat to go with her calf-length dress, completing her look with black accessories.
She donned a pair of black velvet court shoes, which had an impressive heel height for a woman so close to giving birth.
Unlike her sister, Pippa admired to a relatively stress-free pregnancy, writing about her experience in her Waitrose Weekend column.
She said: “As the final month nears and the bump grows prouder each day, movement is certainly getting more awkward.
“As a result, one of the biggest changes is that my lower body has tightened up.
“The knock-on effect is making my back and sacrum both uncomfortable and achy.
“The transformations to my body that are taking place as it prepares for childbirth have meant that the sound sleeps and the baby bubble effect have gone.Reality is finally kicking in.
“Perhaps this is the body’s way of making sure that you get in tune with what lies ahead.”
Pippa’s visit to the Lindo Wing could be a simple checkup visit, as she was already seen visiting the hospital on October 2.