IT IS PROBABLY THE LEAST DISCUSSED OF the major M words in women's lives. Marriage, motherhood, even menstruation have long been fodder for conversations among female intimates, but menopause has dared not speak its name. When Gail Sheehy began experiencing her own menopause five years ago, she found that reticence disturbing. "The subject has a stigma about it. It labels you as older," she says. "It's almost as if people feel that by talking about it they might catch it."

Now Sheehy, 54, has set out to end the shame—and the ignorance that fosters it. Her new book, The Silent Passage: Menopause, is based on interviews with dozens of medical experts and more than 100 American women across the socioeconomic spectrum. It offers information on hormone-replacement therapy, a standard treatment for menopause, and urges the millions of baby boomers who will become menopausal in the 1990s to view the change as "the gateway to a second adulthood" rather than a harbinger of the end.

The topic-has obviously struck a chord. Sheehy's book began last October as an article in Vanity Fair, where it prompted a deluge of mail. And the book itself is already into its fourth printing. "Before Ms. Sheehy recently went public..." wrote Carolyn Heilbrun in the weekly New York Observer, "...the subject was not much discussed at dinner tables....That time is emphatically past."

Going public about her own mood swings and hot flashes, as Silent Passage does in some detail, was not an easy decision for Sheehy. Her husband, M magazine editor-in-chief Clay Felker, had his doubts. "He didn't object, but he said, 'Gail, you have a tendency to be very open. Try to be discreet,' " she says. "But how could I ask other women to go public if I wasn't willing to?"

While the actual symptoms of menopause vary from person to person, Sheehy's experience was in some ways typical of her peer group. "Professional women today are used to controlling so much about their environment, and then here comes this big thing they can't control," she says. "I had assumed that since I have a very happy, active life and take my vitamins, menopause would be a breeze."

The reality was sharply different. Sheehy's first intimations of the change of life came one night in 1987 when she was safely ensconced in her Manhattan duplex. "My husband and I were reading in our funny old bathrobes, with jazz on the stereo smug as the dickens," she says. "Suddenly, I had this feeling of static in my brain; I felt off balance. Then I started having heart palpitations."

The symptoms subsided, only to be followed by others in the ensuing months. She had migraines, extreme fatigue, outbursts of temper and a decrease in sexual desire, changes her gynecologist told her weren't menopause because she was still menstruating. She now knows that she was in perimenopause, which leads into the change of life, "and it's normal to have the worst symptoms while you're still cycling regularly," says Sheehy, who eventually found a doctor who diagnosed her condition and eased her symptoms with hormone therapy.

Later, when Sheehy began consulting physicians for her book, she found that her first gynecologist wasn't unusually ill-informed; medical knowledge about menopause remains limited. Treatment with hormones—estrogen often combined with progestin—appears to lower the incidence of osteoporosis and heart disease in postmenopausal women while possibly raising the risk of breast and endometrial cancer. That makes the decision about whether to medicate a perplexing one. Yet no major studies have fully assessed the magnitude of those risks. "Our tax dollars have supported massive research into heart disease among men," Sheehy writes, but when if comes to older women's health, "it is as if we are still dependent on leeches and roots and shamans...."

That imbalance appears to be changing. Recently, at least two national studies on women's health in their postreproductive years have started. Sheehy hopes her book may increase awareness and spur further research. "That's partly why I wrote it," she says.

She is no stranger to writing that opens eyes. The daughter of a Mamaroneck, N.Y., advertising executive and a housewife turned motel owner, Sheehy graduated from the University of Vermont and landed her first serious journalism job writing gritty stories-including an exposé of New York City's public maternity clinics—for the women's page of the old Herald Tribune. She went on to a free-lance career and has published 10 books, including the best-selling Passages (1976) and Pathfinders (1981), both of which explore the crises that characterize various stages of adult life. Menopause was hardly mentioned in either book. "It wasn't part of my consciousness then," Sheehy says.

Today, Sheehy is a contributing editor at Vanity Fair, where she specializes in political profiles. Happily married since 1984, she is the mother of two grown children (Maura, 28, the product of a first marriage to internist Albert Sheehy, which ended in divorce, is a writer; Mohm, 21, whom Sheehy and Felker adopted from a Thai refugee camp in 1984. is a student at Wellesley). And Sheehy has discovered firsthand the good news about menopause: Once it's over, what Margaret Mead called "postmenopausal zest" can begin.

"You're no longer defined as a sexual object or a babymaker, and that's liberating," Sheehy says. "Menopause is a normal, temporary transition into possibly the most satisfying half of your adult life. Keep that in mind," she adds with a laugh, "the next time you break out in a sweat in public."