The tennis star was interviewed this week and discussed her good friend's pending birth.
Talking on E! News she referenced her 'friend' who's pregnant, and said the friend has shared her ideas for all the things 'her baby' is going to do.
The tennis player did not confirm that the pregnant friend is in fact Meghan Markle, 37, but her comments led to fans speculating.
The 37-year-old tennis champion is the mother of one-year-old daughter Alexis Olympia.
Talking about her babay experiences she said: "Accept mistakes and don't expect to be perfect. We put so much pressure on ourselves.
"My friend is pregnant and she's like, "Oh, my kid's gonna do this," and I just looked at her and I was like, "No, she's not." I'm like, "No, you're not,"'.
The Duchess of Sussex is certainly her most well known pregnant friend while Serena did just throw her a baby shower too.
It's unclear if Serena realized she might have made the slip by referring to the baby as 'she' rather than 'he' or 'it.'
She also quickly moved on from the potential gaffe, saying that she, too, 'had all those high expectations' for motherhood that she had to let go.
"Everything is not gonna go as planned,' she said. 'You just gotta go with the flow."
In line with Royal tradition Meghan Markle and Prince Harry have remained tight-lipped about the sex of their child.
Meghan Markle is expected to give birth to their first child later this month.