In fiction it's as common as dirt to hear "you're glowing" as a response to news that someone is pregnant or an indication of it without it being said.
This doesn't seem to be limited to just reactions upon hearing the news. It seems fairly common for people to believe that pregnant women exhibit some visual cues about their pregnancy, sometimes before the woman even knows she's pregnant.
Explanations include things such as hormonal changes to skin, increased blood flow, or even just general happiness.
Is there any evidence that this is true or is it just one of those clichés (or compliments) which has gone into common usage without any basis to it?
Are there actual visual cues related to coloration that are noticeable even before the woman is showing more obvious signs?