She had a miserable time with morning sickness during her first pregnancy two years ago. And now Britain's Princess Diana, due in September, is suffering a reprise. "I haven't felt very well since Day One this time," she has revealed, and countless moms everywhere empathize. Yet as common as the malady is, modern medicine knows surprisingly little about it, concedes Dr. Luella Klein, 59, who next month will become the first woman president in the 33-year history of the American College of Obstetricians and Gynecologists. Dr. Klein, deputy chief of obstetrics at Atlanta's Grady Memorial Hospital and a medical professor at Emory University, is married to Dr. Alfred Colquitt, a practicing obstetrician in Marietta, Ga. A stepmom and grandmother, Dr. Klein spoke with correspondent Giovanna Breu and offered queasy mothers-to-be some consoling advice. Discomfort aside, she said, morning sickness isn't all bad.

Do most pregnant women suffer from morning sickness?

A large percentage of mothers-to-be feel at least some nausea during the early part of their pregnancy. I would estimate that half to three-quarters of all pregnant women have some morning sickness. But it is unpredictable, and it bothers some women a great deal more than others.

What causes it?

No one really knows, but I think it is the level of chorionic gonadotropin, a hormone put out by the placenta. It is the hormone you check for in a pregnancy test, and people who have a high level of this hormone and of estrogen tend to have morning sickness.

Is this sickness the first noticeable sign of pregnancy?

Not usually, though it can be one of the earlier symptoms for some women. Most, however, complain of breast soreness first. As for morning sickness, some patients experience it within three weeks of conceiving, lots of patients within five weeks.

Can morning sickness be regarded as a healthy sign?

Yes, indeed. People seem to think that if they are nauseated, then something bad is happening, but in fact the prognosis probably is better for a woman who has morning sickness. It generally means that levels of placental hormones are very high, which ensures a healthy fetus. The patient tends to gain weight, which is now considered good. As a group, these patients tend not to have as many problems and complications in their pregnancies.

Does morning sickness occur only in the morning?

For most women—though not for all—it tends to be worse in the morning. Characteristically, a woman becomes nauseated when she first wakes up.

Is it partly a psychological problem?

Doctors have pretty much set aside the idea that the problem is just in your head. People who are under stress situations and who are neurotic do have more trouble with morning sickness, but I don't think it is psychological.

What can a woman do about it?

A variety of things. Pregnant women learn that they won't get as nauseated if they don't allow themselves to get really hungry. They can eat small meals and take crunchy foods like crackers. Some women drink Coke or ginger ale; the carbonation may be helpful. Dietitians say a pregnant woman should stick with fruits and sandwiches and avoid fatty foods. She should get plenty of fluids, substituting skim milk for whole milk. By lunchtime most women can begin to take food and fluid without too much trouble.

Anything else?

You are much less likely to become nauseated in a light, airy place. Every woman learns very quickly not to get on the bus or walk along a road where she can smell a lot of exhaust. She wouldn't want to sit next to someone who is smoking a cigar, maybe not even a cigarette. Lots of patients quit smoking for this reason, and it's a fine opportunity for them to stop habits like that one and drinking.

Are there drugs for morning sickness?

Bendectin was once the standard medicine, but it has been withdrawn from the market because of lawsuits claiming that it caused congenital malformations. There really aren't any alternatives except some antinausea medications, such as sedatives and tranquilizers. The problem is that many of these have side effects and their risk-benefit ratios aren't well known.

Is it usual, as in the case of Princess Di, to have morning sickness in the second pregnancy as well as the first?

Not necessarily. Some women say, "I'll never get pregnant again because I was so sick." Well, you have to say to those people that not every pregnancy is the same. Because they had morning sickness one time doesn't necessarily mean they'll have it again. Women who have several babies know that morning sickness wears off and is not usually a terrible problem.

When does it end?

In most cases by the fourth month of pregnancy. After 16 weeks the baby starts to move around. The mother may have occasional bouts of queasiness after that, but most of the discomforts disappear. Some women, poor souls, are nauseated for their entire pregnancy. A very few, who can't stop vomiting, have to be hospitalized.

Is there any truth in the old wives' tale that morning sickness indicates the baby is a girl?

No. That's an old man's tale.