Pippa attended Princess Eugenie's Royal Wedding just days before.
Spencer has always declined to speak about his brother and Pippa's pregnancy in public, pointing out they are a very private couple.
Pippa and her husband James were married in May previous year in a ceremony attended by royals and celebrities.
Those who choose the prestigious location to give birth need spare no expense on their bub's arrival - with rooms at the Lindo Wing starting at a whopping, $11,000 a night.
The boy was reportedly born yesterday at 1.58pm, weighing 8lb and 9oz at the Lindo Wing in west London.
The news comes on the tail of Meghan Markle and Prince Harry's announcement that the Duchess of Sussex is pregnant. "Reality is finally kicking in". A suite, where Kate is believed to have given birth, is thought to cost closer to £7,500.
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Pippa's royal sibling welcomed all three of her children there, and Princess Diana famously posed on the steps of Lindo Wing with Prince William in 1982 and Prince Harry in 1984 after giving birth. Prince George and Princess Charlotte served in the bridal party, much like they did last week during Princess Eugenie's wedding in Windsor.