Serena didn't give away her friend's identity, but she is most likely talking about Meghan.
Bookies are also putting their money on a girl. In March, British bookmaker William Hill revealed that the odds of Prince Harry and Meghan having a girl are 8-11, according to Vanity Fair.
No matter the sex, Serena is confident that Meghan "will be the best mum for sure."
WATCH: Prince Harry Shares The Cutest Moment With A Baby!
This article originally appeared on marie claire Australia.