Your placenta acts as heart, lungs and kidneys for your developing baby, so very much a transfer station for baby’s waste products to be dealt with by your organs, rather than it absorbing and retaining anything nasty. That said, the placenta struggles with heavy metals, but in a recent study potentially toxic elements such as arsenic, lead, cadmium and mercury were found in raw placenta, but at levels well below the EU toxicity threshold for foodstuffs. As long as you are a non-cigarette smoker (cigarettes contain high levels of cadmium), your placenta will be perfectly safe to consume.
 Johnson SK, Groten T, Pastuschek J. Human Placentophagy: Effects of dehydration and steaming on hormones, metals and bacteria in placental tissue. 2018. [PubMed: 29941176]