If it is your wife or significant other (i.e. exclusive partner of many years), I would strongly recommend you let them know. This endeavor is an elective one that happens to potentially impact the
mutual intimacy of you and your partner, positively or negatively.
Furthermore,
hiding things they will inevitably find out are only good as Birthday Surprises or Spontaneous Getaways. Beyond that, this can actually trigger the opposite effect of an
"oh wow, you did this for me?" reaction. They may wonder why you felt compelled to do this
(especially if sex or intimacy has been good) and could feel threatened that your pursuit for a larger
Phallus may imply you wish to go on more phallic adventures -- in other words, getting a bigger penis while in an already happy relationship behind their back could suggest you wish to be more promiscuous, even if that isn't the intent.
Since this topic is about women specifically, I can tell you that the vast majority of positive reactions to these gains were with spouses/long-time partners who were a part of the journey, and that knowing you can trust them and not made felt ashamed for your desire to get bigger only strengthens your relationship. You may also learn they are adamantly against it, especially if they believe you are "large enough," or that the potential complications (even if uncommon) outweigh the impact on your overall sex with her.
In a nutshell, from my many years following the PE scene (well before the advent of the
PhalloBoards nearly 15 years ago), having your woman informed, and even better on board, is the way to go!