Angiotensin needs a receptor- like a chemical "slot" to fit into or bind with- in order to constrict the blood vessel. These drugs block the effects of angiotensin, a chemical that causes the arteries to become narrow. They can cause low blood pressure, severe kidney failure, excess potassium (hyperkalemia) and even death of the newborn.
These drugs have been shown to be dangerous to both mother and baby during pregnancy.
If you're taking an ACE inhibitor or an ARB and think you might be pregnant, see your doctor immediately.