Queen Elizabeth II will be the first person to receive word that the royal heir has been born.
Unless ... she's sleeping.
Prince William will call his grandmother on an encrypted phone to let her know that Duchess Kate has given birth.
But if the birth is in the middle of the night, he will not wake her up, British media are reporting.
Kate has been in labor since early Monday, when she was admitted to St. Mary's Hospital in London. Experts estimate that first-time mothers are generally in labor 12-24 hours.
The queen, who returned to Buckingham Palace today from Windsor Castle where she usually spends weekends, joked this week, in answer to a child's question, that she hoped the baby would arrive soon because she's going on her annual vacation next week to her Balmoral estate in Scotland.
So how will we mere mortals find out about the birth?
If it happens in the next few hours and it's still daylight, a royal courier will emerge from the hospital and, with a police escort and a news helicopter clattering overhead, will head to Buckingham Palace to post a notice of the baby's gender, size and weight on an easel at the palace front gates. (If the birth is in the middle of the night, this will wait until morning.)
Then the news will flood onto palace websites and social media and TV screens, and the celebrations will commence. Relief that it all went well will mix with joy, and pride that the British monarchy endures with another royal heir in hand.
This baby, whose name we will likely not learn for at least a few more days, will move to third-in-line to the throne, behind William at second and grandfather Prince Charles at first. Great-granny the queen is 87.