Q. I have a 5-month-old baby who must be nursed and rocked or bounced to sleep and then often wakes up when I lay him in bed, so we must start the whole process over again.

I have tried letting him cry himself to sleep for up to 1 1/2 hours and it only results in his getting hysterical and sometimes vomiting. He won't have a thing to do with a pacifier.

I realize a pattern has been established but during his first 3 months he had colic in the evenings. Helping him get to sleep was more of a need than a habit.

I was still rocking my older son when he was 2 and don't want to get in the same rut, but I don't know where to draw the line. I also can't afford to spend 1 1/2 hours getting a baby to sleep because my 2 1/2-year-old has needs also.

At what point and age can one put a baby to bed and expect him to go to sleep on his own? Is this something only our society expects? Am I ignoring his needs by letting him cry, and how long should I let him cry? I don't want him to feel abandoned. Please help.

A. Of course you don't want your child to feel abandoned, nor should he be--and you shouldn't have such a tough act to juggle.

A healthy, contented baby may grumble for 10 minutes or so before he falls alseep, but when he takes so long to settle down, something is the matter. A child doesn't get hysterical and vomit because he isn't rocked to sleep. He just doesn't feel well.

Since this behavior follows the nightmare of colic, it's probably a lesser version of the same thing.

Nobody knows where colic comes from, but according to the new book, Infant Colic by Christopher Farran (Scribners, $10.95), it might be caused by an immature central nervous system; a shortage of hormones or digestive enzymes; a kink or a spasm in the intestine, or by allergies. If the crying still goes on at 5 months, however, it's probably a reaction to something he's eating--like vitamins, fruit juice, eggs--or to something you're eating. Anything you eat or drink enters your milk supply as it once entered the placenta.

As our foremothers knew so well, certain foods like cabbage, onions and chocolate may bother a breast-fed baby more than others. Doctors are finding that other foods also can cause the problem. Milk is a big offender and so are corn, beef, wheat and eggs.

You might get your pediatrician's okay to cut out the vitamins for a couple of weeks--in case your child may be reacting to something in them. If he takes any solids, they should be limited to the least allergic ones--pears, bananas, rice, potatoes, baked chicken and lamb--and then only if you have given him each one for five days before offering him something new.

Your own diet also should be restricted to these foods, with the fruit eaten fresh and the rest cooked simply and served with only salt and pepper. These restrictions should make him much calmer within a few days if he is upset by something you have been eating or drinking. You then add something new to your diet each day and watch his reaction carefully. It might be as simple as that.

And if it isn't? Look back again to our foremothers and the tricks they used. Warm water is an old standby. Either give it to your child to drink after nursing to help break up the mucous milk can cause, or fill a rubber bottle with warm water and lay it on his belly. He then can be swaddled tightly from the waist down, which soothes some children. Others are consoled if they're fed more frequently and in smaller amounts, or given long, warm baths.

Rock him very gently for 5-10 minutes, then put him down on his tummy wherever you want him to sleep. Rub his back some more to help him burp the gas away. Tiptoe away when he gets quieter but go back again in 10-15 minutes to console him some if he continues to cry. That's about as long as a young child should be left to weep alone.

A crib is not the only place a baby can sleep. A carriage is a well-tested alternative, since the baby will make it rock when he cries, and this in turn will rock him--perhaps to sleep. You also can carry him around in a front or back pack: The closeness of your body will let him doze for a time.

Some mothers report that a child has less gas if he sleeps sitting up, so they use an infant seat, while others use an automatic wind-up swing. For those who have a colicky infant, the CribCuddle (about $30) is supposed to help. This plushy, battery-operated sling is hung in the crib like a hammock and reverberates with the sound and sensation of a heartbeat to soothe the fussy child.

In lieu of a hammock like that for yourself, the best you can do for everyone is to take some time off. Look for a kind friend to mind the children for a couple hours, once or twice a week on a regular basis. This isn't time to be spent shopping or cleaning house but in reading, sleeping, going to the movies or soaking in the tub. Your own respites will make all child care easier and more relaxed. Your children will sense this.

This shouldn't imply that colic or fussiness is caused by ineptitude of parents or some subconscious rejection of their children. That's an old-fashioned idea. The kind of crying you describe is real discomfort, but because it isn't serious (except to parents), scientists seem to think they have more important problems to solve. The best thing you can do is try to find the solutions for yourself.

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