Q. Are there any reliable methods of choosing your baby's sex before conception? I've heard that eating a certain diet can affect whether you have a boy or girl, and that there are other techniques for accomplishing the same thing. Do any of these work? A. There aren't any techniques doctors generally agree on for selecting a baby's sex, but I'll mention one theory you might consider discussing with your husband and doctor.

It's actually the father's sperm rather than the mother's egg that determines whether a baby will be a boy or a girl. If a sperm containing the X chromosome fertilizes an egg, the result is a girl. A sperm containing a Y chromosome produces a boy.

Because there are subtle differences in the activity of X and Y chromosome sperm, some scientists have looked for ways to give one type of sperm an edge over the other in the process of fertilization.

One theory suggests that intercourse taking place at or very near the time of ovulation (release of the egg from the ovary) favors conception of a boy. Intercourse taking place two or three days before ovulation favors fertilization with X chromosome sperm, resulting in a girl.

I suggest asking your doctor for advice about planning conception around the time of ovulation if you've been trying to get pregnant and haven't been able to, even if you're not trying specifically for a boy or girl. In addition to the techniques of checking what's known as your basal body temperature and your cervical mucus, there are now over-the-counter kits on the market used to tell when ovulation occurs.

Another factor that may play a role favoring one sex or the other is the acidity of the vagina. A more acidic environment supposedly favors girl-producing sperm. A less acidic, or alkaline, state favors a boy. I wouldn't advise readers to try altering vaginal acidity, such as by using baking soda or vinegar douches, without professional advice.

There is a book describing a diet method of sex selection, but the diet itself is complicated, potentially risky and lacks a good rationale for why it should work. I don't recommend it.

For further reading, try "How to Choose the Sex of Your Baby" by Landrum Shettles and David Rorvik (Doubleday, 1984). Though I can't endorse this book as a proven method of sex selection (it claims a 75 percent success rate), it does give a fairly sensible discussion of what's known about the subject. Q. What is the correct diet for a 2 1/2-year-old girl? Does she need a lot of milk? What if a child refuses to drink milk but likes yogurt and craves some sweets? She seems to eat so little but, still, she is very healthy. A. There's really not much special about the diet of a 2-year-old that differs from that of most children from age 1 up to adolescence.

Studies show that children will tend to follow a balanced diet if they're offered a variety of foods. By variety, I'm not only talking about types of foods, but also variety in taste, color, consistency and temperature.

Nutritionists recommend eating foods from four groups: 1) meat, fish, poultry and eggs; 2) dairy products -- milk, cheese, and milk products; 3) fruits and vegetables; and 4) cereal grains, potatoes and rice. Being mostly milk, yogurt is a good milk substitute, especially kinds low in sugar and fat. After the first year of life, children shouldn't depend on any single food, including milk, to provide for their nutritional needs.

In the "Pediatric Nutrition Handbook," the American Academy of Pediatrics recommends giving children three meals and two snacks each day. Because eating patterns develop at an early age, I think it's wise for children to follow the U.S. Department of Agriculture's recommendations for a healthful diet, which include avoiding large amounts of saturated fat, cholesterol, sugar and salt.

Bear in mind that a child's eating habits will vary, and that children can be very fickle eaters at times. They may eat a lot during some periods, and seem to thrive on next to nothing during others. Their favorite foods may change with the weather, or they may want to eat only one particular item for days on end.

The only special advice about foods for younger children is to avoid those that are known for causing choking. These "choke foods" include hot dogs, hard candies, nuts, peanuts, grapes, carrots, popcorn and apple pieces. Q. Why is it possible to buy and smoke as many cigarettes as one chooses and impossible to buy as much nicotine gum (Nicorette) as one would like without a prescription? Certainly the known side effects from the gum aren't as serious as those of longterm cigarette smoking. The gum works, and its use should be encouraged. A. You're absolutely right -- the side effects of the gum aren't as serious as smoking cigarettes.

The reason you can't buy the gum without a prescription is that nicotine is considered a drug with potentially dangerous side effects. As such, its use is regulated by the Food and Drug Administration. Tobacco, on the other hand, is not considered a drug, although it contains nicotine. Millions of smokers are probably addicted to nicotine and continue to smoke to treat their nicotine withdrawal symptoms.

I am astonished that despite our concern for the safety of products such as food additives and artificial sweeteners, and despite our strong control of drugs we consider dangerous, such as narcotics, we do so little to combat our nation's greatest preventable health threat, tobacco, which kills 350,000 people each year.

Nicotine gum works by satisfying your craving for nicotine, while avoiding the other harmful ingredients of tobacco smoke. It doesn't work for everyone, however, and some people have replaced their addiction to cigarettes with a dependence on the gum. But by far, I consider nicotine gum the lesser of two evils.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center in Northeast Washington.