Brain aneurysms/cerebral aneurysms form when the walls of the arteries in the brain become thin and weaken.  Aneurysms typically form at branch points in arteries because these sections are the weakest.  Occasionally, cerebral aneurysms may be present from birth, usually resulting from an abnormality in an artery wall. Reference          

Risk factors for developing an aneurysm

Sometimes cerebral aneurysms are the result of inherited risk factors, including:

• genetic connective tissue disorders that weaken artery walls
• polycystic kidney disease (in which numerous cysts form in the kidneys)
• arteriovenous malformations (snarled tangles of arteries and veins in the brain that disrupt blood flow.  Some AVMs develop sporadically, or on their own.)
• history of aneurysm in a first-degree family member (child, sibling, or parent).

Other risk factors develop over time and include:

• untreated high blood pressure
• cigarette smoking
• drug abuse, especially cocaine or amphetamines, which raise blood pressure to dangerous levels. Intravenous drug abuse is a cause of infectious mycotic aneurysms.
• age over 40.

Less common risk factors include:

• head trauma
• brain tumor
• infection in the arterial wall (mycotic aneurysm).

Additionally, high blood pressure, cigarette smoking, diabetes, and high cholesterol puts one at risk of atherosclerosis (a blood vessel disease in which fats build up on the inside of artery walls), which can increase the risk of developing a fusiform aneurysm.

Risk factors for an aneurysm to rupture

Not all aneurysms will rupture.  Aneurysm characteristics such as size, location, and growth during follow-up evaluation may affect the risk that an aneurysm will rupture. In addition, medical conditions may influence aneurysm rupture.

Risk factors include:

• Smoking.  Smoking is linked to both the development and rupture of cerebral aneurysms. Smoking may even cause multiple aneurysms to form in the brain.
• High blood pressure.  High blood pressure damages and weakens arteries, making them more likely to form and to rupture. 
• Size.  The largest aneurysms are the ones most likely to rupture in a person who previously did not show symptoms.
• Location.  Aneurysms located on the posterior communicating arteries (a pair of arteries in the back part of the brain) and possibly those on the anterior communicating artery (a single artery in the front of the brain) have a higher risk of rupturing than those at other locations in the brain.
• Growth.  Aneurysms that grow, even if they are small, are at increased risk of rupture.
• Family history.  A family history of aneurysm rupture suggests a higher risk of rupture for aneurysms detected in family members.
• The greatest risk occurs in individuals with multiple aneurysms who have already suffered a previous rupture or sentinel bleed.

For my case, I had been diagnosed with high blood pressure in the first trimester of my second pregnancy and the condition was being managed when I suffered the brain aneurysm. No family history of brain aneurysms that I am aware of. No smoking, no alcohol abuse, and absolutely no drug use except for the ones for BP and vitamins. My take from this is that we are all pretty much at risk. I have heard stories of completely healthy people, very athletic, and with no family history get them. The more information we know about brain aneurysms, the more lives we can help save.