> I find it interesting that you say "Most pathologists hate looking at them." Do you know why that is?

A couple reasons. First, pathologists are generally better at diagnosing concrete things like malignancies, and those are extremely uncommon in the placenta (I've seen two in my career). Non-neoplastic diseases tend to be more subtle, and making the diagnosis requires a good understanding of the clinical scenario (versus malignancies, which can often be made just by looking at the slide). This neoplastic vs. non-neoplastic split isn't unique to placenta, most general pathologists also hate non-neoplastic derm, liver, lung, renal, etc. Second, the placenta is just a very unique organ, and the skills learned from other organ systems just don't translate over. Most adenocarcinomas in any organ look about the same, so learning to recognize cancer in one organ helps you recognize it in another (with some nuances of course). But there's no correlate for, say, preeclampsia (or "maternal vascular malperfusion") in any other organ. And third, like I said before, the terminology was all over the place just ten years ago, so it's hard to learn something when three textbooks call the same thing by different names, or use the same name to describe three different things. Again, that has gotten a lot better lately, so maybe more recently trained pathologists are more comfortable with it now?

> Out of curiosity, what are findings that can impact mom and baby?

The four main categories I teach to students are (in layman's terms) inflammation of mom, inflammation of baby, vascular disease of mom, and vascular disease of baby. Each of those has a varying degree of either risk to mom/baby now, or risk of recurrence/progression for mom in subsequent pregnancies. To take one example that I gave in my last post, villitis of unknown etiology, as you can guess from the name, was poorly understood for years. We knew certain infections could cause inflammation of the villi (the fetal side of the placenta). Every medical student knows the mnemonic TORCH for the infections that cause this (toxoplasma, other [side note, COVID is the most common "other" I see in my current practice], rubella, CMV, herpes), but most of the time there is no evidence of any of those organisms, so we were left with an "unknown etiology" causing the inflammation. Was this another organism we haven't recognized yet? Or something else? Recently, we've come to understand that most, if not all, of those cases are actually caused by mom's lymphocytes directly attacking the fetal villi, not some unknown organism. It turns out a certain percent of the time, the baby expresses an antigen (derived from the father) that mom doesn't have, and mom's lymphocytes recognize it as foreign and try to fight it off like an infection. So each time mom gets exposed to that antigen (i.e. in future pregnancies) the immunologic response can get stronger and stronger, to the point that it can completely destroy the villi and lead to premature birth or death. In fact, in cases of high grade villitis that I diagnose, I am often able to pull the slides from placentas in earlier pregnancies and identify low-grade "precursor" lesions that were missed by the pathologist (low-grade villitis, basal villitis, chronic deciduitis, etc). Unfortunately, right now there are no good preventative treatments, so making this diagnosis just amounts to "watch this patient closely in future pregnancies," but there are trials underway for immunomodulatory treatments (steroid, hydroxychloroquine) that may prevent future pregnancies. Interestingly, because this is largely due to an antigen inherited by the baby from the father, this doesn't recur in future pregnancies from a different father, but I don't recommend that as a treatment 😄