It depends. If it is something that causes you continual stress and anxiety, it might be worth removing, even if it is nothing and would never turn into anything at all.
But, if you can manage your worries a bit, then I'd do what is suggested by the dermatologist and "keep an eye on it." Take a couple of pictures of it (from different angles, close up, and with a ruler or something to be able to measure it in the photo). Then compare it once a month or whatever to your photos. Print and Date your photos, don't just rely on your phone or whatever to keep them in memory.
It is fairly normal for pregnancy and all the hormonal changes that happen during pregnancy and the post-partum period to cause some skin changes. For some people it is just dark spots. I have one on my left cheek that I never noticed until several years after my kids were born. As I aged, it became more apparent. But it isn't anything... just a darker patch of skin, also commonly referred to I believe as a "pregnancy patch"....
If you've had it for 13 years, it isn't doing much if it only changed colors during pregnancy. I have a few small ones that are flat and have all the "classic" signs that online sources tell you to be careful of: irregular edges, varying colors within the same spot, not being a regular shape (asymmetrical... I have one that is an asymmetrical almost heart shape), etc. But they have been examined and are nothing. I did have ONE biopsied as a precaution, and not one I would have thought was concerning... it was nothing.
But, for the most part, I was oblivious to most of my moles until I actually went into the dermatologist about a rough spot on my nose. They did a whole body check while I was there. I was a little freaked by how many moles I actually have! I didn't realize. But then, none of them seemed new, either. I've just had them for years and never paid any attention to them. They do seem to have become more prominent maybe as I've aged. Darker.
So, if you can follow instructions and just keep an eye on it, do that. If you see any changes (and document it right now so you will KNOW if they are actual changes and not your imagination at work) call/go in. If you cannot stop the worry or anxiety over it... then see if they'll just remove it, AND what the instructions will be once they have removed it. If they still will want you to monitor something, it doesn't seem like it will solve your anxiety problem..