The reason PC keeps having miscarriages is due to her necklace. It’s costume jewelry that was made in a time where lead paint was common. A couple results of lead exposure in women are miscarriages and infertility, and she’s worn the necklace every day for decades.

Not a lot of evidence for this one other than the time the necklace was made, but the idea that the one thing she wore as a symbol of hope and strength to her was actually causing her strife is definitely something the writers would do.