Q. "After going to doctors for three years, my husband and I finally expect our first child, due in September!" writes a Palisades mother.

"By now I've spent so much time thinking about this baby I'm beginning to feel spooked. Every week I worry about something new. This week it's PKU. Are all newborns tested for this? Are there other problems like it?"

A. There are about 300 metabolic-diseases that, like PKU, can cause retardation, but rest easy, these enzyme errors are very rare.

PHENYLKETONURIA (PKU), which causes severe retardation and behavioral problems, strikes only one child in 15,000 and hypothyroidism, which can retard children and cause dwarfism and respiratory problems, too, occurs only once in 3,800 births.

Although metabolic problems cause only a tiny fraction of retardation, they are the only ones whose effects can be prevented or greatly minimized. This is done by an increase in certain vitamins, a change of diet or, at the least, infant stimulation. But the disease must be caught early.

Will your baby automatically be tested for these problems? It depends on where she is born.

Maryland and Virginia, like most states, require tests for PKU and hypothyrodism, but the District does not. The program was dropped in 1970 after the city lab found no evidence of it in 80,000 tests.

Despite the cost of testing, a General Accounting Office report says that each dollar spent to find retardation in newborns saves $20 a year in care. Studies in New York and Massachusetts back this up.

Now a bill is awaiting congressional review that will reinstate the District program in October, and other tests can be added if the City Council votes for them.

Even without mandatory screening, half of the city's hospitals give the hypothyroidism test and most check for PKU. However, D.C. General -- where one baby in four is considered "highrisk" at birth -- checks for neither.

At the other end of the spectrum, Georgetown University Hospital probably has the most advanced metabolic tests, screening its newborns for six disorders.

In this program, the baby's heel is pricked to get two drops of blood on filter paper. It is then analyzed by the year-old, non-profit Infant Metabolic Diagnostic Laboratory, 2233 Wisconsin Ave. NW. Here tiny discs are punched out of the bloodstains and run through computers to identify the two main diseases as well ad homocystinuria, maple syrup urine disease and galactosemia.

The sixth test has just been added -- an ascending paper chromotogram -- which checks for some still-rarer causes of mental retardation. This new lab will be dedicated to its donor, Jacob Burns, on Sunday by Dr. Mary Coleman, director of the Children's Brain Research Clinic, and Dr. Margaret Giannini, head of the new National Institute for the Handicapped. m

As sophisticated as these tests are, they only isolate .05 percent of all retarded babies, even with the follow-up test given for PKU. However, Irving Levine, the lab director, says the figure may jump dramatically if their research is as promising as it looks.

"Maybe, just maybe, we'll be able to identify 50 percent of all retarded babies -- or more," says Levine.

These early findings are based on two non-specific tests being developed by the lab. If they continue to prove successful, they will be given to infants as "first stage screening," to see if other tests are necessary, he says.

The first of these two screens, now used on the Georgetown Hospital newborns, checks the uric acid ratio to see if it is abnormal, as it is in a fourth of all retarded children. The second test, still not fully researched, measures the level of serotonin in the whole blood, which is abnormal in 22 forms of retardation.

Once a retarded baby is identified -- and there are two of them in every 100 children born -- parents at least can start in immediate infant-stimulation program. This alone can improve the child's IQ as much as 40 points, which can mean the difference between an institution and a job.

As important as postnatal tests are, it's what you do in these nine months that counts the most. To improve your odds for a healthy baby -- mentally and physically -- eat good, pure, nutritious food: take your vitamins: give up caffeine; avoid medicines; quit smoking -- and drink as much as you want, as long as it's milk, water or fruit juice.

You can read more about a good prenatal care in what continues to be a parent's best source of information: the U.S. government. For a free reprint, "When Your Baby's Life Is So Much Your Own," send a card to the Comsumer Information Center, Dept. 525H, Colo. 81009.