At the end of class...
"Could you please confirm you're pregnant?" everyone sort of looked at me funny; she was eight months pregnant. She said yes.
"Okay, so the reason why I asked is because as a teacher you never assume someone is pregnant. Wait until your student tells you. I say that because there was a time in my life I was REALLY out of shape; REALLY out of shape. I remember taking a class the teacher came up to me and asked, "Are you pregnant?" I just looked at her and said nope. I'm just out of shape.
The class laughed; some had similar stories that happened to them or someone they knew.
But, yes, never assume someone is pregnant; she might not be. You can always teach modifications without drawing attention to them.