Q. "Yesterday our little girl Karen, who is 21 months, ran out of the yard and into the street," writes a father in Hillcrest Heights. "We ran after her, yelling at her, and she didn't even turn around. My wife says the baby never listens to her, although she pays me a fair amount of attention, and that she's getting more and more willful. I guess this is to be expected at this age, but I think it's more than that. A few hours after the street incident, Karen was in the living room. I called from the kitchen -- just as loud as I would to anyone else -- 'Karen, come here. I have a cookie for you.' She didn't even turn around. She never seems to come unless she sees you talking or unless you yell. I think she may be a little deaf.

"My wife refuses to consider it, maybe because her younger brother was born deaf -- really deaf -- and I just don't think she can face the idea yet. If it's true, for my wife's sake would it hurt to postpone the diagnosis? We expect another baby in March."

A. Yes, it would hurt to postpone the diagnosis. The sooner you find out if there is a problem, the sooner your child can be helped.

No handicap isolates a person more than deafness and yet it often can be corrected by surgery, and with special education and aids, it always can be helped.

A postponement also hurts your wife, for in her heart she is just as anxious about Karen as you are -- she's just a lot more afraid to find out. Every parent of a handicapped child feels guilty, and the deafness in her family is sure to make your wife feel even more responsible.

While heredity can cause poor hearing, so can a lot of other things, from German measles, the Rh factor and veneral disease in early pregnancy to high fevers, a sharp blow on the ear or a lot of firecrackers blasted nearby in childhood.

If loud noises gave Karen a high-frequency loss, it would explain why she responds to your voice better than her mother's. The high sounds are the first to go with most people, although with others it can be the low ones. That's why older children are told to turn down the volume of their music. When teen-agers play their stereos as loud as a rock band in a disco, they are getting 110 dBa's -- 20 more than the government considers safe for an 8-hour period. Although full hearing springs back about two hours after such exposures, like an old rubber band it doesn't spring back quite as well, for noise has a cumulative effect. In one study 61 percent of the college students tested had the same high-pitched hearing loss that afflicts the elderly, which means that the soft consonants merge until sat and pat sound alike and phones ringing and birds singing go unheard for the rest of their lives.

Begin by making an appointment for a 30-minute hearing test at the Washington Hearing and Speech Society, 1934 Calvert St. NW, a United Way agency available to everyone in the metropolitan area and which charges according to income.

Judyth Tinsley, executive director of the agency, says one person in 10 has some hearing loss, although the average person never has a hearing test.

If your child is deaf, it may mean an operation to repair a punctured eardrum or rebuild the bone structure of the middle ear or to drain it, for the Eustachian tubes in a child often get blocked.

If the conductive loss persists, or if there is neural damage -- which can't be repaired -- the WHSS will help your child get the hearing aid that suits her best and help you investigate the three distinct kinds of training your child can get if she still can't hear.

There is sign language (with different vocabularies) and finger spelling; oralism, which teaches the deaf to talk and read lips, and cued speech. Cued speech is probably the most promising avenue, but Margaret Hiltzelaw, the chief audiologist at WHSS, believes a child must have some residual hearing to use it well.

Although deaf children need help, their parents need some too. In Silver Spring there is the International Assn. of Parents of the Deaf and the National Assn. of the Deaf, which is made up primarily of the deaf and the professionals who work with them. There is also Deaf-pride, at 2010 Rhode Island NE, which is made up of the deaf, their teachers and their parents. Deafpride not only helps members through the maze of government problems, but it has workshops, retreats and group sessions, and teams older parents with younger ones in a "buddy system" of support.

All of this is to tell you and your wife that if Karen does have a hearing problem, you won't be in it alone. So hurry and have her checked and the new baby too, before leaving the hospital.