Q. My husband must be the most nervous father-to-be in Washington. He worries about how much I sleep and if I eat right and did I take my vitamins this morning and am I still smoking a half-pack of cigarettes a day? Even an aspirin worries him, and now the latest: Why don't I stop drinking?

In no sense do I have a drinking problem, as he is the first to admit, but he is afraid any alcohol might hurt the baby.I think having a baby is the most natural thing in the world. How can I make him understand that I don't need a nanny?

A. Loves makes a husband act in the damndest ways, especially when there are suddenly two of you to love.

Of course you don't want (or need) a nanny to remind you to sleep enough, to eat fresh, nutritious food and to take your vitamins, but some facts might make you reconsider your business-as-usual attitude.

There are two points to remember. First, everything you eat, drink or smoke reaches your baby; the placenta is a conduit, not a filter. Second, the safety of many things is unknown for unborn children or pregnant mothers, since researchers aren't encouraged to experiment on them.

There are enough restrospective studies however, to give reliable warnings.

This is how we know that alcohol is perhaps the worst offender - since it is the most socially acceptable drug - but while its danger may not be as bad as your husband fears, it cannot be dismissed.

This is because alcohol is not only toxic, but it's a teratogen, our third biggest cause of birth defects. It is also the only one of the big three whose effects are preventable. At least 1,500 babies will be born with fetal alcohol syndrome in this country in the next 12 months, most of them to alcoholic mothers.

Since alcohol attacks the central nervous system, which is completely exposed in the fetus' first weeks of life, the FAS baby may have brain lesions - sometimes fatal - or be retarded. He may have facial irregularities, a small head, ill-formed arms, legs and genitals, almost surely a low I.Q., and eyes with narrow slits.

Less alcohol, of course, is less damaging. If a woman in the first critical month or two of pregnancy drinks 1 1/2 ounces of 86-proof whiskey every night - or 5 ounces of wine or 3 ounces of sherry or a 12-ounce beer - she is only drinking a half-ounce of absolute alcohol and it probably takes three ounces day to produce an FAS baby.

The problem is that no one knows how much is too much, since all people don't have the same tolerance. Some researchers suspect even one ounce a day - the maximum allowed by the National Council of Alcohol Abuse and Alcoholism - can be dangerous and two ounces of absolute alcohol a day is a clear risk.

Even 45 drinks a month is considered a lot to scientists. Say the woman has a nightly glass of wine with dinner and then a six-pack of beer at an allday picnic. Or maybe a fancy day on the Washington party circuit with a Dubonnet and a glass of wine at a luncheon and that night, a drink before dinner, two glasses of wine with it and then a brandy.

The woman who is used to drinking can tolerate this fairly well, even though the toxicity remains the same. The baby, however, hasn't had so much practice. It still takes the mother an hour to metabolize each glass of wine and all the while the bones, the muscles the organs and the brain of the baby are bathed in alcohol.

When a woman has more than 60 drinks a month, particulary when some of the doses are heavy, she has one chance in 10 of having a baby with some FAS symptoms.

At least 3,500 babies a year are born with them - babies who fuss a lot and have poor muscle tone, who will be shorter and, although their I.Q.'s are normal, will have minimum brain dysfunction. This will be marked by hyperactivity - a condition shared by 5-7 million American schoolchildren - and problems dealing with memory, concentration and an ability to break the kind of codes found in arithmetic. While many children are born with these symptoms, alcohol of course, may be just one of the causes. It is, however, the most insidious, since so many more women now drink.

Because FAS and its lesser symptoms are so serious, the government is looking for better ways to warn women. If a publicity campaign doesn't work, labels may be required on beer, wine and whiskey, warning pregnant women that alcohol causes birth defects, or even warning all women of child-bearing years to beware since the biggest danger probably happens before pregnancy is recognized.

Even though you limit your intake to two drinks a day, we think you should measure your glasses too. In the past 25 years wine and cocktail glasses have almost tripled in size and quite often the contents have too, since jiggers are seldom used any more.

Although five ounces of wine may sound like a lot, they are lost in those big balloon glasses, and as for 1 1/2 ounces of bourbon - just try pouring three tablespoons in a tumbler and see how little it is. No one should ever kid themselves about how much they drink, especially a pregnant woman. With a teratogen, you have to know.

Alcohol is not the only cause of birth defects. There are genetic factors, diseases in pregnancies, X-rays - and there are the drugs that come in pills instead of a cocktail.

Studies show that defects can be caused by the pill, if taken within three months of conception, or if the mother has taken estrogen, progestin, amphetamines, tranquilizers, mephrobamate or benzodiazepine in the first trimester. That's also the danger time for tetracycline, which causes a baby to have gray teeth.

The salicylate in aspirin in the last few months of pregnancy can make the labor last longer and the bleeding worse, while Demerol, given to suppress labor pains, can depress the baby's breathing. So can the oxytocic agents that are used to induce labor or lessen post-partum bleeding.

This is all spelled out, and free, in the Food and Drug Administration's reprint, Drugs and Pregnancy, from the Consumer Information Center, Dept. 653G, Pueblo, Colo. 81009.

Even your husband's concern about smoking has merit.

There are more miscarriages, more premature deliveries and more stillbirths among mothers who smoke, and the more they smoke, the higher the incidence.

Not only are the babies generally smaller if their mothers smoked in the last trimester, but an English study of 5,000 children claims that those whose mothers smoked in late pregnancy were shorter and had lower reading scores 7 and 11 years later.

Basically, chere , it is a matter of playing the odds. Nine months of caution improves them a lot.