So as of Game of Thrones season 7 Daenerys is in Dragonstone now and there hasn't been a mention of Stannis's preserved fetuses as of yet that I can tell.
Were they taken north or are they just forgotten?
There might be something I missed.
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TL; DR: We don't currently know.
In Season 4, after Joffrey's death, Stannis gathers his forces and heads north toward the wall. We never see these fetuses being toted along, so we can only assume they were either left at Dragon Stone or taken with them. After the scenes we've seen from Dragon Stone in Season 7, it looks like they took everything they could with them, save unimportant things. Stannis is, mostly, not an idiot and would know that taking them would be pointless in the fight.
Chances are that the fetuses were left at Dragon Stone, but they aren't an important part of the story at this point so they haven't been included since Danny took over.