Q. My doctor says I have an aneurysm of the aorta in my abdomen and he recommends surgery. What exactly is an aneurysm, and what are the dangers of it and the surgery? Are there any warning signs of an impending rupture of an aneurysm?

A. An aneurysm is an abnormal bulging in an artery, usually caused by atherosclerosis or "hardening of the arteries." Atherosclerosis damages and weakens the blood vessel walls, which gradually expand under the force of the blood pressure. Common sites for aneurysms are the aorta, the large arteries in the pelvis and the arteries behind the knees.

The danger is that the aneurysm could rupture or pop open without warning, leading to heavy internal bleeding. If this happens to an aneurysm in the aorta -- the largest artery of the body carrying blood from the heart -- it is usually fatal.

The symptoms of a ruptured abdominal aneurysm include sudden, intense abdominal pain, often felt in the back, and a drop in blood pressure leading to shock. Emergency surgery is necessary, but up to half of victims don't survive. Most people don't have any warning signs of impending rupture.

The risk of an aneurysm bursting increases in proportion to its size, which can be accurately measured by ultrasound. Similar to radar, ultrasound uses painless sound waves to make images of internal organs. An aorta normally measures about two centimeters in diameter, and many physicians advise surgery for aneurysms larger than five centimeters. The operation entails cutting out the aneurysm and replacing that portion of the artery with an artificial substitute.

Aneurysms measuring six centimeters have about a 25 percent chance of rupture within five years. Aneurysms eight centimeters in diameter have about a 75 percent chance of rupture within five years. In general, surgery is called for if the aneurysm is larger than five to six centimeters, associated with symptoms, or enlarging.

On the average, people who have surgery before rupture have only a 5 percent surgical mortality rate, so the risk of routine surgery is less than the risk of no surgery and much less than that of emergency surgery.

Q. I am 36 years old and recently had an allergic reaction to shrimp (red blotches, swelling and itching, about two hours after eating) which responded well to Benadryl. About three years ago I had a much milder reaction to lobster. I have, however, eaten plenty of seafood over the past three years and in fact had a shrimp salad sandwich the week before the recent incident with no ill effects. What is known about such reactions to shellfish? Need I avoid all shellfish such as clams, oysters, crabs?

A. It's probably worth having an allergist confirm your sensitivity to shellfish by either skin testing or a blood test known as RAST. The reason is that you may have reacted to something else, such as sodium metabisulfite, a substance sprayed on foods to enhance their appearance.

Even if you are sensitive to certain foods, you may not get a reaction every time you eat them. Factors that may modify your reaction include:

Whether the food was raw, cooked or otherwise processed;

* how it was prepared;

* the amount of food consumed;

* the presence of other foods that may delay digestion;

* and the use of medicines, such as antihistamines like Benadryl.

People with sensitivity to shrimp and lobster are often sensitive to other shellfish. Your pattern of reaction in the past will tell you whether you will need to avoid shellfish altogether. If you've had no problems with oysters, clams or crabs, you probably don't need to avoid them.

Also important is the nature and severity of your symptoms. A reaction like hives may portend a potentially more serious reaction, like difficulty breathing, swelling of the throat or even shock. To protect against dangerous reactions, your doctor may recommend carrying an allergy kit (such as used for bee sting sensitivity) that contains antihistamine pills and injectable adrenalin.

Q. Do birth control pills cause birth defects if taken during early pregnancy?

A. It's controversial whether birth control pills cause birth defects if taken during early pregnancy, but if they do, the risk is very small.

Previous studies blamed birth control pills for heart defects, limb shortening and other abnormalities in offspring exposed during early development. More recent studies show little or no increased risk of malformations. The best estimate is that birth control pills result in a .07 percent increase in birth defects (less than 1 in 1,000) above what is expected. Because the risk of birth defects in normal, unexposed pregnancies runs as high as 3 to 5 percent, a .07 percent increase does not add appreciable risk.

Nonetheless, birth control pills should not be taken during pregnancy. But if a woman who has been taking birth control pills discovers that she is pregnant, she should know that the vast majority of exposed fetuses will not develop birth defects.

The risk of birth defects from any drug exposure depends on the dose, the duration of exposure and the time of fetal development at which the exposure took place. If your doctor isn't sure about the possible effects of a drug exposure during pregnancy, I recommend consultation with a genetic counselor.