True about coarctation of aorta –
Most common site of COA is distal to the origin of left subclavian artery.
Hemodynamics of COA during fetal life 

In fetal life, blood from right ventricle passes to pulmonary artery and then to descending aorta through ductus arteriosus.
Blood from left ventricle passes to Aorta and then to innominate, left carotid and left subclavian artery.
If constriction (coarctation) is proximal to ductus arteriosus (preductal or infantile coarctation), there is no obstruction to flow because constriction is not obstructing the flow of upper limb and lower limb --> Collaterals are not formed.
If the constriction is distal to ductus arteriosus (post ductal or adult type coarctation of aorta), it causes obstruction of flow of desending aorta, and parts of body that are supplied by descending aorta, have impaired circulation —> This stimulates formation of collaterals in fetal life.

Hemodynamics after birth 

After birth, ductus arteriosus closes, so that blood flow to descending aorta from right ventricle does not occur and descending aorta must receive its total supply from left ventricle via ascending aorta.
But there is obstruction due to coarctation.
In preductal coarctation, there are no collaterals Neonates become symptomatic immediatly --> Hypertension results in CHF.
In postductal coarctation, due to presence of collaterals neonates are spared from developing hypertension & CHF.
Due to obstruction, there is pressure overload to left ventricle -4 Left ventricular hypertrophy.
Most common site is distal to the origin of the left subclavian artery
Most common age of presentation is at 15–20 years
Upper rib notching is due to erosion by dilated collateral vessels
Right ventricular hypertrophy