How can a placenta turn hostile to the mother or fetus ? What are the abnormalities found during placental delivery ?
The placenta does not turn "hostile" to the mother or fetus. Placentation abnormalities can result in harmful situations, but these are a secondary result.
Structural anomalies of placentation:
placenta accreta/percreta: means that the placenta took hold not only in the uterine mucosa, which is the normal state, but through it up to the myometrium (muscle layer, in placenta accreta), or even up to the serosa (outermost uterine layer, in placenta percreta). These anomalies will convey a much higher chance of massive haemorrage at delivery, and therefore are dangerous to the mother.
placenta praevia: placental insertion covers the entrance to the uterine cervical canal, partially or totally. Therefore, the way of the fetus for natural delivery is blocked, and the placenta will tear during labor, causing massive haemorrage.
marginal implantation of placental vessels: vessels linking the placenta to the fetus (the umbilical cord) are implanted on the periphery of the placenta. This causes structural fragility, and a greater risk of bleeding for both the mother and fetus.
retroplacental hematoma: in such cases, the placenta partially or totally loses its hold on the mucosa, and blood fills the space left behind. This is an immediate threat to both the fetus and mother because of the potential for massive haemorrhage and interruption of oxygen delivery to the fetus.
Deleterious side effects of pregnancy:
preeclampsia/eclampsia: misunderstood biological syndrome that causes the classical signs of arterial hypertension, peripheral edema, hepatic cytolysis, and thrombocytopenia in the mother. Eclampsia is all that plus added seizures (in which case maternal mortality is high). This is not due to placentation abnormalities, even if it is true that delivery is the only cure, and can justify a premature artificially induced delivery in extreme cases.
fetal/maternal rhesus factor incompatibility: During labor and delivery, a small amount of fetal blood will come into contact with the mother's organism. If the mother is Rh neg, and the baby Rh pos, the mother can develop antibodies against the rhesus factor. This can cause haemolysis in the newborn, and in grave cases, kernicterus (toxicity of degraded red blood cell metabolites to the baby). While this may threaten the baby to be born, problems are much more common for subsequent Rh pos pregnancies. We check for antibody presence in the mother with the Kleihauer test and give preventive injections of rhesus antibodies to Rh neg mother with a Rh pos pregnancy to decrease the chance that the mother develops her own antibodies.
Other special cases:
molar pregnancy/choriocarcinoma: A structural genetic accident happening on fecundation that can cause the embryo to behave as a type of cancer.
ingesting placental extracts: this is mentioned is another answer here. This has nothing to do with the placenta, but the deleterious secondary effects are due to direct toxicity of the hormones contained in those extracts ingested in an inadapted situation. This is not to say that one should take them while pregnant, because hormonal deregulation would also be toxic in a normal pregnancy.
An abnormal placenta position can lead to increased risk of heavy bleeding during placental delivery which further complicates the process by needing blood transfusion drugs. Anterior placental attachment is associated with increased risk of pregnancy induced hypertension , gestational diabetes mellitus due to dysfunction of lactogen hormone secreted by placenta, placental abruption, intra uterine retarded growth or fetal death.Rh factor incompatibility is a complication during the second delivery. Also, placenta encapsulation(conversion of placenta into capsules) has side effects on mother's health like pelvic girdle pain, constipation or diarrhea , PMS like emotional outbursts. Placenta is an intimate connection between the fetus and mother but locational abnormalities can make placenta hostile to both fetus and mother. Also, the Rh factor incompatibility stays a major factor in being harmful.