The 35-year-old tennis pro revealed the exciting (and surprising!) news in a selfie posted to her Snapchat early Wednesday morning.
The pregnant athlete showed off her adorable burgeoning belly in a lemonade yellow swimsuit, which featured silver buttons on the side.
“20 weeks,” the text over the photo, which she has since removed from her story, read.
Williams and Ohanian announced they were engaged last December, and for weeks she teased fanswho had been asking her to share a pic of her engagement ring.