QUESTION: This is my first pregnancy, and I can't believe what's happening to my body, my thighs especially. They have increased in size unbelievably. I haven't changed my diet or anything. Maybe I need more exercise. What's happening? - J.Y.

ANSWER: It happens to some extent to every woman during the first three months of pregnancy. The female thighs are natural fat cell storage areas. In the first trimester of pregnancy, the fat-storing enzymes there are at peak activity, nature's way of insuring adequate calories for the developing fetus in the months ahead.The good news is that the fat melts away during the last three months. Afterward, you should return to normal, and if you breast feed, you may even enjoy slimmer thighs. You don't have to exercise for this, but your doctor will want you to have some degree of that anyway, so check that out.

QUESTION: This is not really a medical problem, unless the fact that I'm climbing the walls over it is makes it that. What do they recommend these days for dealing with youngsters at bedtime? My young son nearly goes into conniptions over it all. I'm ragged. - Mrs. C.G.

ANSWER: There are several approaches, from accepting any reasonable in-bed caterwauling, to adjustment of bedtimes, to more positive approaches, typically engaging in brief 15-20 minute pre-bedtime joint activities. Choose a favored activity, a brief reading session if that is it. Many youngsters, I'm told, will respond to the subsequent second urging after that. Some parents cheat, gradually shortening the activity incrementally until the previous desired bedtime hour has been reached. I'm sure we're going to hear from parents who have "been there." I've only given you suggestions some authorities have made, perhaps without benefit of real life situations.

QUESTION: I've started breaking out with bruises on my legs and knees. I'm not bumping into things. They are the size of quarters. Anything serious, do you think? I'm 45. - P.I.

ANSWER: Crops of bruises that emerge out of nowhere demand an explanation. The problem can lie with a deficiency of platelets or factors required for firm clotting.

There seems to be some confusion about what I mean by platelets and clotting factors. They are not the same. Platelets are the substances that stick together to form clots. Clotting factors are specific proteins that assist the process of forming sturdy clots. If either is deficient, then clotting fails and bruising occurs. Blood tests can pinpoint specific clotting problems.

There are less serious causes of bruising. With age, tissues that support blood vessels give way. Smaller vessels may break easily. There is not much to do about this fragility. Finally, younger women (younger than you usually) can develop a specific bruising syndrome. It's not likely that at your age you fit either of these two categories. At your age, one would suspect one of the clotting disturbances I mentioned earlier.

FOR M.P. - Yes, psoriasis affects the scalp. The common sites for it are the scalp, shins, arms, elbows, knees and lower back. It may itch and burn. Psoriasis can be controlled, but not totally eliminated. It comes and goes. If drugstore medicines aren't helping, it is time to try prescription drugs. You have to see a doctor for that, so why not also get an accurate diagnosis of just what you do have? Lots of conditions look like psoriasis and aren't that at all. Using psoriasis medicine on them is a waste of time and money.

FOR MRS. S.S. - Apnea means loss of breath. Treating this can do more than end the nocturnal awakenings and next-day drowsiness typical of the problem. It might reduce the chances of stroke. A number of persons who have strokes also had obstructive sleep apnea. That's the kind of breathless awakening brought on by airway obstruction. Overweight seems to go hand in hand with that kind of apnea.

Having trouble getting to sleep at night? Dr. Donohue examines the insomnia problem and offers some ways to help you get your needed rest. For a copy of the booklet No. 30, "Why Can't You Sleep?" send $2 and a stamped, self-addressed envelope to Dr. Donohue/No. 30, P.O. Box 830, Gibbstown, NJ 08027-9909.

Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.