By JANE E. BRODY

Pregnancy has long been assumed to be a time of expectant joy, at least for women whose pregnancies are planned and who look forward to motherhood. And indeed, it is a happy time for most.

But not all. A significant minority - 10 to 20 percent, depending on who is counting - suffer moderate to severe depression during pregnancy, which translates to 80,000 women a year in the United States. All too often the problem goes unrecognized by the women and their doctors.

Some depression symptoms - fatigue, change in appetite and lack of energy - overlap normal signs of pregnancy, prompting some women to ignore them. Others are embarrassed to mention their depressed feelings to their doctors since they're supposed to be thrilled to be pregnant.

But even when pregnancy-related depression is recognized and acknowledged, women and their doctors can find themselves in a dilemma. After decades of warnings to avoid all manner of drugs, alcohol, nicotine and caffeine, pregnant women are often reluctant to take antidepressants even if their doctors will prescribe them.

Weighing the Risks

New studies examining possible effects of antidepressants on the fetus as well as the risks involved in failing to treat depression during pregnancy are likely to make decisions even harder. The decision to treat or not to treat must involve a careful assessment of known risks and benefits based on the best medical information available.

A depressed woman is more likely to be delinquent about prenatal care. She may miss doctor appointments, eat and sleep poorly, fail to take vitamins, have difficulty forming a healthy attachment to her unborn child, and experience stress that can expose the fetus to harmful levels of hormones and neurotransmitters.

To alleviate depressed feelings, some women may turn to alcohol or cigarettes, which themselves can harm an unborn child.

Untreated depression during pregnancy has been linked to higher rates of miscarriage, stillbirths, premature deliveries, intrauterine growth restriction and low-birth-weight babies. Though they often catch up early in life, babies born smaller than they should be for their gestational age face higher than average rates of high blood pressure and heart disease as adults.

The untreated depression can also damage a woman's relationship with her spouse and other children. The inability of a depressed person to cope well with stresses, however small, can result in undue irritability and impatience.

Finally, depression is unlikely to end once the baby is born. As Dr. Shaila Kulkarni Misri noted in her recent book, "Pregnancy Blues" (Delacorte Press, $23), "If depression goes untreated during pregnancy, it will worsen and more than likely continue postpartum."

Postpartum depression not only robs a woman of the joy of having a new baby, it can seriously impair her ability to nurse and care for the infant, and it prompts some women to harm the baby.

A woman taking antidepressants before becoming pregnant may assume that the safest course is to stop the medication until the baby is born or after she stops nursing. But a study published Feb. 1 in The Journal of the American Medical Association found that such an interruption greatly increased the chance that major depression would recur during the pregnancy.

Even stopping treatment only for the first 12 weeks, when the baby's organs are forming, increased the chance of a relapse, the researchers, headed by Dr. Lee S. Cohen, perinatal psychiatrist at Massachusetts General Hospital, found.

Dr. Misri emphasizes that pregnancy, "far from being protective against psychiatric illness, as many continue to believe, can actually trigger depression for the first time, exacerbate an already existing condition, or cause the relapse of a depression that had previously been under control."

She calls pregnancy-related depression "an equal-opportunity illness" that can strike any woman, rich or poor, socially well-connected or isolated, previously healthy or suffering from years of recurrent depressions.

Are Antidepressants Safe?

Two new studies have raised questions about the safety of leading antidepressants during pregnancy, the selective serotonin reuptake inhibitors, or S.S.R.I.'s, like Prozac and Zoloft.

One study, published in February in The Archives of Pediatric & Adolescent Medicine, examined 60 newborns who had been exposed in utero to S.S.R.I.'s. Eighteen of them showed mild to severe signs of "neonatal abstinence syndrome" - withdrawal from the drugs at birth.

Symptoms included high-pitched crying, disturbed sleep, feeding difficulties, tremor and muscular stiffness that lasted a week or two. The effects were dose-related, and the drug Paxil, a longer-acting S.S.R.I., was linked to the most severe symptoms.

The researchers, headed by Dr. Rachel Levinson-Castiel of the Schneider Children's Medical Center of Israel, recommended that infants exposed in utero to S.S.R.I.'s be kept in the hospital for observation for at least 48 hours or until symptoms subside.

A Circulation Disorder

The second study, published Feb. 9 in The New England Journal of Medicine, highlighted a rarer though more serious risk: the development of persistent pulmonary hypertension of the newborn, or P.P.H.N. With it, the baby fails to convert from fetal circulation, when oxygen and nutrients are supplied by the mother through the placenta. A baby with the disorder breathes normally but the blood supply bypasses the lungs because pressure there is too high.

The researchers, headed by Christina D. Chambers of the University of California, San Diego, examined 377 women whose infants had the disorder and 836 matched control women and their healthy babies. Fourteen infants with the disorder had been exposed to S.S.R.I.'s after the 20th week of gestation, compared with six infants in the control group. This meant that exposure to an S.S.R.I. after the midpoint of pregnancy raised the chances that the baby would develop the disorder by a factor of six.

An earlier study by the University of Pittsburgh found that about 1 percent of babies exposed to antidepressants in the last third of pregnancy developed serious respiratory problems.

The California researchers emphasized that the "absolute risk" that a baby exposed to S.S.R.I.'s would develop the hypertension disorder was extremely low: "about 99 percent of women exposed to one of these medications late in pregnancy will deliver an infant unaffected." Still, they urged that this risk be factored in when deciding to take an S.S.R.I. late in pregnancy.

The risk has to be weighed against a possibility of a relapse of depression and postpartum depression if the drug is stopped in the third trimester .

There are, of course, other drugs to alleviate depression, and the safest are the tricyclic antidepressants. Although these drugs have more discomforting side effects, neither they nor the S.S.R.I.'s cause birth defects or lasting developmental harm to children exposed in utero, long-term studies show.

These studies found no significant effect of in-utero exposure to the trycyclics S.S.R.I.'s on I.Q., language development or behavioral development among children followed to age 7.