It isn't but VCs have been known to avoid female founders for fear of them not working as hard as men since 'they'll have a baby sometime' or some nonsense like that [1]. Also women face gender discrimination in the bay area. [2]

source: [1] http://www.businessinsider.com/pregnant-founders-response-to...

[2] http://valleywag.gawker.com/this-is-why-there-arent-enough-w...