Serena Williams is 5 months pregnant, and even though she just announced it to the world ... she's barely showing.

She just posted a profile pic of herself in a one-piece swimsuit and captioned it, "20 weeks."

Sources connected to Serena tell us it's exactly what it looks like ... she IS pregnant, even though her reps aren't confirming yet.

Serena got engaged to Reddit co-founder Alexis Ohanian back in December. If ya do the math, it kinda adds up. Looks like they might be celebrating with a vacay.

Congrats to Serena and Alexis -- and also to every other female tennis player! Your chances of winning a tournament this year just went up.