I should have been thrilled, but I couldn't let go of my fears. After a year and a half of trying to conceive, I was three months pregnant. Finally, at 37, the baby I longed to give to my husband, Will, and to Quinn, our 6-year-old son, was on the way. The problem was I didn't feel pregnant: No swollen tummy, no tender breasts, none of the symptoms from the first time around. Still, after dropping off Quinn for the first day at his northern California kindergarten in the fall of 1998, I proudly told the other moms I was expecting. An hour later, lying on the examination table at my doctor's office, a nurse began moving a monitor across my belly. I listened for the sound of my baby's heartbeat. Silence. "This can't be!" I remember screaming. Later that day Quinn summed it up the best: "That baby is gone, isn't it, Mom?"

That day launched my odyssey through the little-discussed but well-populated world of secondary infertility. That's the science-y-sounding name given to those who can't conceive again after they've already had at least one child—in our case Quinn, a wonderful, wise boy and a constant source of joy. Because I conceived him at 31, as easily as drinking a little too much champagne on New Year's Eve, I never thought the word infertility would apply to me. Half-Sicilian, half-Irish, I come from a family of five kids. I figured Will and I had all the time in the world to have more.

Boy, was I wrong. What I hadn't considered—along with millions of other women, both with kids and without—is that age matters. My mother had her first child at 20 and her last—me—at 32. Growing up, I used to ask her why she was such an "old mom." By the time we jumped back into baby-making, I was 36 and oblivious to the fact that women between the ages of 35 and 39 have a one in five chance of encountering infertility (difficulty conceiving, problems sustaining a pregnancy—or both).

I was heartbroken after that miscarriage and a second one in December 2000. But I found it difficult to share my pain. When you have no kids and are having trouble trying, people are sympathetic. It's different when you already have a family. Friends and relatives would say things like, "You should be happy with one." Or "You're probably better off not having another, it's too much work." Some fertility doctors don't even allow you to bring a child to consultations—too difficult for childless patients to see. How could I explain that I ached for another child, that I wanted Quinn to have the joy of growing up with a brother or sister? How could I take time from the child I adored to grieve for babies I had lost?

My doctor said it was just a matter of time. I'd had a baby once, so I could have one again. But why was it taking so long? "Old eggs," she explained. That's shorthand for saying miscarriage rates rise as women age due to chromosomal abnormalities that naturally occur within some of their eggs. But I couldn't help but feel that something else was going on, since both my miscarriages happened when I was about 12 weeks along, the point at which that risk usually declines.

A friend of mine put me in touch with a woman named Julie, a writer and mother of two in Texas, who had also suffered secondary infertility. When Julie talked about how guilty she had sometimes felt, struggling to have a second child when she already had one, I felt for the first time in years that someone understood what I was going through. She told me I needed to go straight to an R.E.—a reproductive endocrinologist, who helps infertile and high-risk women get pregnant—before time ran out altogether.

The first step with my new doctor was endless tests and months of waiting as the specialist ruled out possible causes for our infertility, one by one. There were some funny moments, like the time Will—who was depressed about turning 50—called the lab and was told he had the highest-powered sperm they had ever seen. He grinned from ear to ear. How often does a man get to find out that he's superman?

Eventually, we figured out my problem: I had low progesterone. That's the hormone that helps support the pregnancy in the first 12 weeks until the placenta kicks in. If I had been given a progesterone cream suppository during my previous two pregnancies, it turned out, I might have been able to carry them to term—we'll never know for sure. While I was glad to have an answer, I was raw with anger. I had wasted four years and now, at 41, I had really old eggs. My doctor put me on Clomid—a drug that stimulates egg production—and for six months we tried artificial insemination.

No luck. We were coming to the end of the road. The drug treatments weren't working, although the hormones were making me an emotional wreck. Quinn, "my little man," often bore the brunt of it. We never told him we were trying to get pregnant, but he couldn't help but notice how often his mom left him with a sitter to go to the doctor or how snappish I could be when he asked for help on his homework. That's another thing about secondary infertility—it's not just about you and your husband.

After one more miscarriage, I decided to accept my fate. "Go live your life," my R.E. said. I was blue for a while, but I refocused on Quinn and Will, and the cloud started to lift. Will offered to take me away for a romantic weekend. Paris? No. Ottawa, the capital of Canada. But we did stay in a lovely old hotel that looked like a castle, held hands over dinner and felt young again. Weeks later I began to have what I suspected were the first symptoms of menopause, feeling bloated and tender. I was almost 42, after all. But Julie, ever hopeful, said, "Take a pregnancy test." You're kidding, I thought. Still, I went to the drugstore and picked one up. Bingo—a bright blue line!

Of course, no pregnancy of mine would be complete without a little drama. At 8½ months, while discussing my upcoming baby shower with a friend, I stepped on some trolley tracks in the pouring rain. I was electrocuted! The jolt threw me two feet forward (fortunately, I landed on my butt and not my stomach), and the skin on my fingers was singed. Rushed to the emergency room, for the first time in ages I had some good pregnancy karma: Electricity doesn't travel through amniotic fluid, so the baby was fine.

On July 1, 2003, Ava arrived, 7 lbs. 7 oz., with a sparkle in her eye. As they laid this beautiful creature on my chest, I finally felt the world could understand what I could never explain: Why wouldn't I want two? I looked at Ava and said my first words to her: "I knew you were coming."