Yep, I've seen it argued both ways over the past couple of decades. I remember a Navy pilot who sued to be allowed to fly while she was pregnant, and I've seen an airline pilot who sued to stay home with sick leave payed by the union when she found out she was pregnant.
PMS is a similar gender biased issue. For years some women claimed it had no effect, now some claim their schedules and sick banks should be adjusted to compensate.