The beast first showed its face benignly, in the late-June warmth of a California swimming pool, and it would take me more than a year to know it for what it was. Willie and I were lolling happily in the sunny shallow end of my in-laws’ pool when he—then only seven—said, “Mommy, you’re getting thinner.”

It was true, I realized with some pleasure. Those intractable 10 or 15 pounds that had settled in over the course of two pregnancies: hadn’t they seemed, lately, to be melting away? I had never gained enough weight to think about trying very hard to lose it, except for sporadic, failed commitments to the health club. But I’d carried—for so many years I hardly noticed it—an unpleasant sensation of being more cushiony than I wanted to be. And now, without trying, I’d lost at least five pounds, perhaps even eight.

I suppose I fell into the smug assumption that I had magically restored the lucky metabolism of my 20s and 30s, when it had been easy for me to carry between 110 and 120 pounds on a frame of five feet six inches. True, in the months before Willie’s observation, I’d been working harder, and more happily, than I had in years—burning more fuel through later nights and busier days. I’d also been smoking, an old habit I’d fallen into again two years earlier, bouncing back and forth between quitting and succumbing, working up to something like eight cigarettes a day.

Of course Willie noticed it first, I now think: children major in the study of their mothers, and Willie has the elder child’s umbilical awareness of me. But how is it that I didn’t even question a weight loss striking enough for a child to speak up about? I was too happy enjoying this unexpected gift to question it even briefly: the American woman’s yearning for thinness is so deeply a part of me that it never crossed my mind that a weight loss could herald something other than good fortune.

As it happened, I took up running about a month later, in concert with quitting smoking for good. By the end of the summer I was running about four miles a day, at least five days a week. And with all that exercise I found I could eat pretty much anything I wanted without worrying about my weight. So more weight melted away, and the steady weight loss that might have warned me something was going badly wrong disguised itself instead as the reward for all those pounding steps I was taking through the chill of early fall, the sting of winter, the beauty of spring’s beginning. I went from around 126 pounds, in the spring of 2000, to about 109 a year later.

Somewhere in there my period became irregular—first it was late, then it stopped altogether. Well, I’d heard of this: women who exercise heavily sometimes do become amenorrheic. I discussed it with my gynecologist in January, and he agreed it was no real cause for alarm. He checked my hormone levels and found I definitely hadn’t hit perimenopause, but what I most remember about that visit is the amazed approval with which he commented on the good shape I was in.

Around that time—I can’t pinpoint exactly when—I began to have hot flashes, almost unnoticeable at first, gradually increasing in intensity. Well, I said to myself, I must be perimenopausal after all; a gynecologist friend told me that hormone levels can fluctuate so much that the test my doctor had done wasn’t necessarily the last word on the subject.

Then one day in April I was lying on my back, talking idly on the telephone (strangely, I don’t remember to whom), and running my hand up and down my now deliciously scrawny stomach. And just like that I felt it: a mass, about the size of a small apricot, on the lower right side of my abdomen. My mind swung sharply into focus: Have I ever felt this thing before, this lump? Well, who knows, maybe this is a part of my anatomy I was just never aware of before—I had always had a little layer of fat between my skin and the mysteries of the innards. Maybe there was some part of the intestine that felt that way, and I had just never been thin enough to notice it before.