Q: After reading your answer about the relative risks of vasectomy versus having your tubes tied, I wondered what the chances are of successfully reversing a tubal ligation I had five years ago. I am 30 and reconsidering my decision not to have children.

A: Tubal ligation -- cutting, clipping or electrically burning the fallopian tubes -- is the most common form of contraception for women over 30. But about 10 percent of women who have been sterilized later regret their decision, and about 1 percent try to have the procedure reversed.

Under ideal conditions, and using new microsurgical techniques, gynecologists can successfully reverse a tubal ligation 50 to 80 percent of the time. The success rate depends on the type of operation you had.

Reversal is more often successful in tubes that have been clipped instead of electrically burned, or cauterized. In addition, the longer the segment of tube removed, the less chance of successful reversal.

Your chances of having a pregnancy in a tube (ectopic pregnancy) increases after a reversal procedure and also depends on what type of tubal ligation you had. Ectopic pregnancy after a reversal also seems to occur more often in women who had their tubes tied right after delivery rather than at a later time.

Vasectomies, by the way, can successfully be reversed over 90 percent of the time, with the chance of fathering a child somewhere around 50 to 70 percent. Follow-Up:

Cholesterol

Several readers wrote in response to my answer about lowering your blood cholesterol level by dietary changes March 12 . They asked about the difference between saturated and unsaturated fats and why alcohol intake should be limited if it doesn't contain cholesterol.

One reader asked for more information about the six different types of high cholesterol levels I mentioned, and another wanted me to be more specific about sardines as a high-cholesterol food, pointing out that sardines are not a type of fish, but a way of preparing herring, pilchard or other kinds of fish.

Let me briefly answer these questions, and encourage those of you who must follow a low-cholesterol diet to get professional advice.

Saturated fats mainly come from animal sources, and include whole milk, cream, butter, cheese and meat, but they also include hydrogenated vegetable shortenings and hydrogenated margarines, and coconut and palm oil. Most other vegetable oils are high in unsaturated fat.

The term saturated refers to how many hydrogen atoms take up available places on fat molecules. If all the spaces are filled, the molecule is "saturated"; if many places are empty, the molecule is "poly" (many) "unsaturated"; and if one space is empty, "mono-unsaturated." Saturating or hydrogenating margarine makes it more solid; polyunsaturated margarine is more liquid, and better for you if you need to watch your fat intake.

Even though alcohol doesn't contain any fat, it works to raise the blood cholesterol level in many people. How bad it is for you depends somewhat on which of the six types of high blood cholesterol you have, although it's generally best to cut down or eliminate your drinking if your blood cholesterol is high.

Traditionally, scientists separated high cholesterol levels into six types, depending on your level of cholesterol and triglycerides (another kind of fat measured in the blood). This classification is not exact, however, and some people change from one type to another. This system is most useful for identifying individuals and family members with hereditary types of high blood cholesterol.

Sardines are one of several related fish with a high fat content. In lists of the cholesterol content of foods, sardines are near the top. Because the species of fish is not usually given, I can't tell you which fish -- herring, pilchard, or other -- is meant. But because sardines, along with eggs, organ meats and shrimp, are the foods highest in cholesterol, I recommend limiting your consumption of them if you're on a low-cholesterol diet.