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What is Placental insufficiency and explain in detail?
Placental insufficiency or utero-placental insufficiency is the failure of the placenta to deliver sufficient nutrients to the fetus during pregnancy, and is often a result of insufficient blood flow to the placenta. The term is also sometimes used to designate late decelerations of fetal heart rate as measured by cardiotocography or an NST, even if there is no other evidence of reduced blood flow to the placenta, normal uterine blood flow rate being 600mL/min.

Causes
The following characteristics of placentas have been said to be associated with placental insufficiency, however all of them occur in normal healthy placentas and full term healthy births, so none of them can be used to accurately diagnose placental insufficiency:
Abnormally thin placenta (less than 1 cm)
Circumvallate placenta (1% of normal placentas)
Amnion cell metaplasia, (amnion nodosum) (present in 65% of normal placentas)
Increased syncytial knots
Calcifications
Infarcts due to focal or diffuse thickening of blood vessels
Villi capillaries occupying about 50% of the villi volume or when <40% of capillaries are on the villous peripheryPlacental insufficiency should not be confused with complete placental abruption, in which the placenta separates off the uterine wall, which immediately results in no blood flow to the placenta, which leads to immediate fetal demise. In the case of a marginal, incomplete placental abruption of less than 50%, usually weeks of hospitalization precedes delivery and outcomes are not necessarily affected by the partial abruption.

Pathophysiology
Maternal effects
Several aspects of maternal adaptation to pregnancy are affected by dysfunction of placenta. Maternal arteries fail to transform into low-resistance vessels (expected by 22–24 weeks of gestation).  This increases vascular resistance in fetoplacental vascular bed eventually leading to reduction in metabolically active mass of placenta like a vicious cycle.

Fetal effects
Placental insufficiency can affect the fetus, causing fetal distress. Placental insufficiency may cause oligohydramnios, preeclampsia, miscarriage or stillbirth. Placental insufficiency is most frequent cause of asymmetric IUGR.

Fetal metabolic changes
Metabolic changes occurring in uteroplacental insufficiency:

Fetal hormonal changes
Decrease in overall thyroid function is correlated with fetal hypoxemia. Serum glucagon, adrenaline, noradrenaline levels increase, eventually causing peripheral glycogenolysis and mobilization of fetal hepatic glycogen stores.

Fetal hematologic changes
Fetal hypoxemia triggers erythropoietin release. This stimulates RBC production from medullary and extramedullary sites and eventually results in polycythemia. Oxygen carrying capacity of blood is thus increased. Prolonged tissue hypoxemia may cause early release of erythrocytes from maturation sites and thus count of nucleated RBCs in blood increases. These factors, increase in blood viscosity, decrease in cell membrane fluidity and platelet aggregation are important precursors in accelerating placental vascular occlusion.

Fetal immunological changes
There is decrease in immunoglobulin, absolute B-cell counts  and total WBC count.  T-helper and cytotoxic T-cells are suppressed in proportion of degree of acidemia. These conditions lead to higher infection susceptibility of infant after delivery.

Fetal cardiovascular changes
There is decrease in magnitude of umbilical venous volume flow. In response to this, the proportion of umbilical venous blood diverted to fetal heart increases.  This eventually leads to elevation of pulmonary vascular resistance and increased right ventricular afterload. This fetal cerebral redistribution
of blood flow is an early response to placental insufficiency