It doesn't make sense when you think about it. Even if a midwife is right, she's got about a 50/50 chance (actually slightly better if she errs on the side of boy, since boys seem to be a teeny bit more common). It's like most urban legends, we tend to remember the incidents that fit in with the theory, and ignore the ones that don't. Which isn't to say that you shouldn't look at it for fun, but I'd hate to think anyone was counting on it.
Both my girls hovered right at 140, usually. Of course as MightyMoo said, I found out through occasional monitoring of an hour or two at a time that my babies' heartrates would vary quite a bit.