Newlyweds Tom and Dorothy Barrett, both in their mid-twenties, came to learn as much as they could about pregnancy before taking the step to have a child. Sue and David Lubrowski, in their early thirties and married for a decade, were there to try to decide whether to start a family before they switch careers.

With 26 other prospective parents, these two couples attended a recent three-hour class at Alexandria Hospital on preparing -- well before conception -- for pregnancy. Some came to find out if they might be in a high-risk pregnancy group. Others wanted to learn what to eat to prepare their bodies for parenthood. Mostly, they just wanted to discover what having a child might entail.

"Even though I am a nurse," one woman told the class, "I'm here tonight because I know little or nothing about being pregnant."

Pre-pregnancy classes -- or preconception classes, as they are sometimes called -- are one of the latest consumer health trends fueled by over-achieving baby-boomers preparing for parenthood with the ardor once reserved for getting into Harvard or becoming a partner at the law firm.

But this is no mere fad: Studies show that birth defects can be significantly reduced with preconception visits to the doctor. The American College of Obstetricians and Gynecologists (ACOG) now advises that prospective mothers see their doctors three to six months before becoming pregnant.

The idea seems to be catching on. At Alexandria Hospital, classes taught by nurse Leith Mullaly are booked through April. Couples come from as far away as Baltimore and West Virginia. At Georgetown University Hospital, where obstetrics and gynecology chief Dr. John Queenan has been a leader in the drive to make pregnancy a 12-month proposition, couples can take pre-pregnancy classes, and women can get special preconception physical examinations and laboratory tests.

At George Washington University Hospital, classes focus predominantly on high-risk patients -- women with diabetes or other medical problems who face greater potential complications during pregnancy. But "about half of all patients come in now for a consultation before getting pregnant," said Dr. Allan B. Weingold, chief of obstetrics and gynecology at GW.

"There are a huge number of people in the U.S. who want to do everything they can to optimize the outcome of pregnancy," said Georgetown's Queenan. Yet until recently, physicians generally told women to wait until they missed two menstrual periods before coming in for an examination.

By then, the fetus is tiny but fully formed, and the time of "greatest sensitivity to malformations" is already past, Queenan said.

"What we end up doing in obstetrics is catch-up, certainly not primary prevention," said nurse Merry-K Moos, who heads a similar program at the University of North Carolina at Chapel Hill.

Yet some simple changes in diet and life style before and during pregnancy can have big payoffs in producing healthier babies. "There are certain conditions which show clearly that what is done before the pregnancy is as important as what is done during the pregnancy," Queenan said.

Most prominent among these are neural tube birth defects -- serious malformations of the brain and spinal cord -- which afflict 5,000 to 6,000 of the 3 million babies born each year in the United States and cost thousands of dollars per child to treat.

The risk of developing neural tube defects such as spina bifida (an often crippling spine malformation) and anencephaly (in which the brain and skull do not develop) can be significantly reduced when multivitamins and folate, a type of B vitamin, are given to prospective mothers a month before pregnancy and are continued through the second month of pregnancy, one British study found. For women who had given birth to one child with this type of defect, the chances of a second pregnancy producing a child with this type of defect dropped from 4.2 percent to less than 1 percent with the vitamin supplements, the study found. If the mother had given birth to two children with neural tube birth defects, the risk of recurrence dropped from about 10 percent to 2 percent.

Other potential complications that may be avoided by having pre-pregnancy examinations include:

*Rubella. Fifteen percent of women in their reproductive years are susceptible to rubella, or German measles. "If susceptibility were determined before pregnancy, it would be possible to receive rubella immunization and then become pregnant after a safe waiting period," Queenan said.

*Rh factors. One in 100 women have antibodies in their blood that can prompt their immune systems to attack the fetus' red blood cells. Doctors can often prevent this condition by giving an injection of Rh immune globulin, which prevents the mother's body from producing more of these antibodies.

*General health problems. Breast lumps, cervical polyps, ovarian cysts and abnormal pap smears are best identified and treated before pregnancy. Laboratory tests also can "permit us to treat such conditions as anemia, urinary tract infections or gonorrhea before pregnancy," Queenan said.

*Consulting a doctor before conceiving a child also gives couples a chance to learn about proper nutrition and exercise. "Dieting and strict weight control are no longer recommended for most women during pregnancy," said Dr. William T. Mixson, ACOG president. The organization's current recommended weight gain for most women during pregnancy is about 25 pounds. "So it's a better idea to get your weight to a reasonable level for you and start a program of good nutrition before pregnancy."

Current research indicates that "it's easier to continue an exercise program begun before pregnancy, rather than trying to shape up a body adjusting to pregnancy," Mixson said.

Preconception planning also provides an opportunity to change habits -- such as smoking cigarettes, using recreational drugs and drinking alcohol -- which can be extremely dangerous to the unborn baby. Alcohol-related birth defects alone afflict an estimated 40,000 newborns each year, reports the National Institute on Alcohol Abuse and Alcoholism (NIAID). The most severe is fetal alcohol syndrome, a cluster of congenital defects that irreversibly damages major organs, including the brain.

Research shows that even one to two ounces of alcohol a day can harm a developing baby -- findings that prompted the U.S. surgeon general to issue an advisory in 1981 urging pregnant women and women considering pregnancy to abstain from drinking alcohol.

Planning for pregnancy also gives prospective parents a new awareness about potential health hazards in their home and office that could be harmful to their unborn child. The North Carolina program reminds women to wear gloves and a mask when using paint strippers, oven cleaners and bug sprays and thereby reduce exposure to common chemicals. Reducing on-the-job exposure to radiation and chemicals is also emphasized by most pre-pregnancy classes, and warnings are issued about minimizing the use of X-rays throughout pregnancy, but especially in the first few weeks of gestation.

Whether all this information cuts health care costs is uncertain. The concept is so new that studies have not yet been done to examine cost-effectiveness.

But as Georgetown University's Dr. Jerry Martin noted, "one premature baby doesn't take long to run up huge hospital bills, and one baby with birth defects can quickly consume $500,000 to a $1 million."

Moving standard blood tests now performed during pregnancy to the preconception period also "does not increase the cost," said Queenan. "It only changes the timetable."

There's also one more benefit that many prospective parents consider priceless: the luxury of deliberating about having a child -- something not possible in previous generations.

"The decision of whether or not to become a parent is probably one of the more important decisions you'll ever make," Mullaly told her class at Alexandria Hospital. "And yet until very recently, people just did it without thinking."