Whatever it is you think of the smoking fetus, it certainly makes you W think. The 30-second American Cancer Society public service spot purporting to show an unborn baby smoking a cigarette is strong medicine. So strong CBS and NBC are refusing to air it. ABC is showing it, however, as are local stations all over the country. "And it is the locals," says Irv Rimer, Cancer Society vice president for public affairs and producer of the spot, "that are always more receptive to our material."

The spot shows a startlingly realistic, well-developed fetus slowly bringing a cigarette to its delicate mouth. As it inhales and then exhales a whiff of smoke, a woman's voice is saying, "Would you give a cigarette to your unborn child? You do every time you smoke when you're pregnant. Pregnant mother, please don't smoke."

The people who care about the diseases caused by smoking -- mainly the American Lung Association, the American Cancer Society, the American Heart Association and the federal health establishment -- are watching uneasily as lung cancer replaces breast cancer as the leading cancer killer of women in this country. That already has happened in Texas, Louisiana, California and several other states, with the rest of the country expected to close the gap by midyear.

The battle between the tobacco industry and the antismoking forces is particularly intense in its competition for women. As cigarette companies target ads towards the fastest growing segment of new smokers in the country, the antismoking groups are trying to communicate the risks, both to the women's own health and to the health of their children, born and unborn.

Studies continue to document that the damage is real.

"A lot of this information has been around for several years," says Virginia Ernster, professor of epidemiology at the University of California at San Francisco. "But somehow there is a tremendous ignorance. When they are asked, women are unable to specify the risks of smoking to their unborn child, such as the increased risk of stillborn babies and of miscarriages. And still, about 30 percent of pregnant women continue to smoke.

"Yet this is a good group to target," Ernster says, "because it is an early time in their smoking history with years ahead of them. It is an ideal intervention point."

According to the American Lung Association, studies have established these A smoking-related risks:

* Risk of miscarriage is 170 percent higher in heavy smokers -- defined as 10 or more cigarettes a day.

* Risk of premature birth, 300 percent higher.

* Risk of stillbirth, 55 percent higher.

Prenatal cigarette smoking has also been linked to birth defects such as hare lips and cleft palates, infant death -- especially in the first 28 days -- and respiratory distress syndrome. It is also linked to an abnormality known as placenta previa. And parental smoking after birth may be linked to sudden infant death syndrome.

Finally, infants of smoking mothers tend to be smaller at birth -- a fact first demonstrated some 25 years ago. If they are raised in a smoking household, they may be shorter and smaller than children of nonsmokers and may score lower on math and verbal tests. They are more likely to be hospitalized in their early years, have more colds, flus and pneumonias and ultimately more obstructive airway diseases.

A recent lung association brochure keyed to pregnant women and new mothers states:

"The smoke from the burning end of your cigarete has twice as much nicotine as the smoke you inhale . . . and five times as much carbon monoxide. And that smoke will make it hard for your baby to breathe."

Nicotine is addictive and causes increases in heart rates and blood pressure. It also affects the central nervous system and possibly brain chemistry. Carbon monoxide is a poison. It displaces oxygen in blood cells and causes oxygen starvation throughout the body. A pregnant woman who smokes two packs of cigarettes a day deprives her fetus of 25 percent of its oxygen supply.

Smoking does other nasty things to mothers as well. For example, there is a link between smoking and early menopause, between smoking and early wrinkles. Nicotine gets into breast milk. Smoking while on the pill heightens the risk of cardiovascular disease. Smoking may also be linked to cervical and other cancers.

Yet the advertising war rages unabated.

"It is really ironic," says epidemiologist Ernster, "that as more and more data document the health hazards of smoking for women and their children, the magazines directed at women are becoming more and more dependent on cigarette advertising."

Ernster, who has been collecting advertisements aimed at women dating back to the 1920s, notes that for some of the most popular women's magazines cigarette advertising now accounts for about 15 percent of total advertising revenues. Notably, she points out, neither Good Housekeeping nor Seventeen accepts cigarette ads. But other publications aimed at young women do -- Vogue, Mademoiselle, Glamour. McCall's and Ladies' Home Journal accept cigarette ads as well; Readers Digest and The New Yorker do not.

Her advertisement collection demonstrates that smoking was linked to liberation of women way back in the 1920s, when "smoking women were considered to be undermining the moral fabric of society. They would even be expelled from college for smoking. Defying that taboo made cigarettes one of the badges of liberation."

Ernster points out that cigarette manufacturers have found other artful ways to get their commercial messages onto television screens, from which they have been technically banned since 1970.

One of best publicized of these was the advertisment for the British cigarette Kim, displayed on the costume of tennis champ Martina Navratilova at the 1982 Wimbledon tournament. Navratilova's costume was designed to look like the Kim package.

It caused considerable discussion -- even in Parliament -- and the cigarette manufacturer, the British American Tobacco Co., agreed to drop the name from the costume in subsequent games.

In this country, Virginia ("You've Come a Long Way, Baby") Slims uses a similar technique via its traveling women's tennis tourney. That cigarette, manufactured by Philip Morris Inc., has earned itself an entire opposition organization, with chapters all along the tournament route. Group Against Smoking Pollution (GASP) was set up to to call attention to what they see as the cigarette sponsor's efforts to flout the TV ban by dressing ball girls and boys in Virginia Slims T-shirts, and by what they regard as a "cynical" attempt to link smoking to women's health in the public mind -- by linking it to a healthy sport. GASP protesters, including members of Seventh-day Adventist youth groups, area cancer and lung associations and, for the first time, the Women's Health Network, picketed the tennis tournament over the weekend , distributing antismoking materials.

"The thing about lung cancer versus breast cancer," says Ernster, "is that the former is almost entirely preventable. Cigarettes account for 350,000 preventable deaths a year in this country -- in men and women and from all smoking-related diseases. That is the equivalent of two jumbo jet liners crashing every day. Wouldn't that be news?" Resources

* Coalition on Smoking OR Health, an umbrella antismoking policy organization, 1302 18th St. NW, Suite 603, Washington, D.C. 20036.

* D.C. Lung Association, 475 H St. NW, Washington, D.C. 20001, or call 682-LUNG for new booklets on smoking and pregnancy, tips on quitting and support programs.

* GASP of Northern Virginia: 569-4570.

* D.C. Chapter, American Cancer Society, 1825 Connecticut Ave. NW, Washington, D.C. 20009.