An aneurysm happens when part of a blood vessel (artery) swells either because it is injured, or if the wall of the blood vessel is weak. Aneurysms can happen in the aorta (aortic aneurysms), the brain (brain or cerebral aneurysms), or peripherally in other body parts (peripheral aneurysms). 

Cerebral or brain aneurysm

A brain aneurysm happens in the brain in a blood vessel, usually an artery. When a blood vessel weakens, it can initially develop a small sac or balloon. Smaller aneurysms, especially if they happen in front of the brain are less likely to rupture than larger ones. The most frequent brain aneurysm is a so-called "Berry aneurysm" named because it looks like a berry, like a sac or out pouching of the artery. The berry aneurysm is responsible for 90% of all intracranial aneurysms. The other 10% are fusiform aneurysms, which are aneurysms that are less likely to rupture.

When the brain aneurysm weakens further, the aneurysm can rupture and cause bleeding into the brain. The bleeding inside the brain, such as the subarachnoid space, results in a subarachnoid hemorrhage. Another general term of this is also a hemorrhagic stroke which is a stroke resulting from bleeding as compared to a thrombotic stroke which happens from a blood clot inside a blood vessel. 

Pregnancy and aneurysm

In a study reviewing ruptured aneurysms associated with pregnancy and delivery, the authors did not show an increased association between pregnancy or delivery and the risk of rupture of cerebral aneurysms. Patients with unruptured aneurysms had a higher rate of cesarean deliveries because some doctors believe that labor and delivery increases pressure in the brain. However, studies have not confirmed this with certainty and a cesarean delivery may not be always necessary.