Dictionary:

pregnable

  (prĕg'nə-bəl)

Being such that attack, seizure, or capture is possible; vulnerable or assailable: a pregnable fortress.

[Middle English preignable, pregnabul, from Old French prenable, pregnauble, from prendre, to grasp, from Latin prehendere, prēndere.]

adjective

Open to attack and capture because of a lack of protection: assailable, attackable, vincible, vulnerable. See strong/weak.

Capable of becoming pregnant.