I have more than one friend or client who tells me that she starts to blush, turning beet red in the face, when she begins a speech, presentation or talk. If this has happened to you, it can be among the most mortifying of public speaking experiences.
In part, that's because you can sense there's nothing you can do about it, and you're right: Blushing is an uncontrollable physiological response to stress, a physical manifestation of fight-or-flight syndrome. It's also a social way that your body demonstrates an apology, typically for some kind of bad social behavior, and that works with the audience--it's a credible way to say, "I know I just did something wrong," as you'll see in the smart video below that summarizes the science behind blushing.
Trouble is, you shouldn't be apologizing for speaking, so take some time to think about whether that's what is prompting the red-faced reaction. Are you feeling unsure of your authority to speak? Worried about insulting a prominent member of the audience? Anticipating that your biggest critic, also sitting there, will rip apart your logic? All that might bring on the blushing, or you might just have a typical case of the public speaking nerves, perfectly normal.
While you can't stop blushing while it is happening, you can prepare better. That might mean preparation to banish your nerves and find your comfort zone before your presentation, or preparation for the tough Q-and-A that will follow your talk, perhaps by getting ready with questions for your questioners. You can think through challenges to your logic and come up with answers that respond, rather than react, to your critics (just as you might do for a tough media interview). Just the act of preparing will make you feel better when the time comes to speak.
Watch the video so you'll understand just how natural (and uniquely human blushing is. A hat tip to the wonderful Brain Pickings blog, which pointed me to this video: