On Wed, 22 May 1996, Ken Arromdee wrote:

(By the way, I wonder what Nodoka is doing in this series.  She would have to
know that Ranma is female, and could not have accepted any promise about
raising Ranma to be manly.  Having Tsubasa or Konatsu show up might be
interesting, too, especially with Ukyou at the same time....)

Hmm maybe Genma *somehow* made Nodoka think Ranko was a boy? Like maybe 
he told Nodoka that Ranko was a boy when she was born and kept it from 
Nodoka all the time by doing all the diaper changing and stuff? Nah... 
just a thought. :)

Or how about this... Nodoka died giving birth to Ranko but Ranko survived.