The tennis champ posted a selfie on Snapchat Wednesday with the caption "20 weeks." She posed for the picture in a yellow one-piece bathing suit that appeared to show a baby bump.
"I'm happy to confirm Serena is expecting a baby this fall," a representative for Williams confirmed.
Williams and Reddit co-founder Alexis Ohanian were engaged in December.
This is the first child for the 35-year-old athlete.