Jon Cohen and his wife, Shannon, never thought much about fertility. Their first child, Erin, was conceived easily, and Shannon's pregnancy progressed without a hitch. But when the couple decided to try again four years later, when both were 37, they ran into trouble. First they had difficulty conceiving; then Shannon had four consecutive miscarriages. "We felt like failures," says Jon.

The experience prompted Jon to delve into the science of miscarriage, which he maps out in his new book, "Coming to Term" (Houghton Mifflin. $24). Despite the guilt and secrecy, miscarriage is far from uncommon. Anywhere from 30 to 50 percent of all pregnancies are lost before women are even aware they've conceived, and at least one in seven known pregnancies ends in miscarriage. Older women are especially vulnerable--as more forty-somethings launch into baby-making, miscarriage could strike in even greater numbers. Researchers will never be able to treat the heartache, but they are beginning to unravel the biological triggers, and they're testing new approaches to prevention. Today "we have more technological tools," says Dr. Mary Stephenson, director of the University of Chicago's new Recurrent Pregnancy Loss Program.

Most miscarriages are caused by an abnormal number of chromosomes. Nature usually expels these pregnancies before the end of the first trimester. Sometimes the embryos are perfectly healthy, but women miscarry for other reasons: a misshapen or scarred uterus, insulin or hormonal imbalances, immunological problems or chronic infections in the uterus.

You can take steps to lessen the likelihood of pregnancy loss. Start by taking a multivitamin with folic acid, don't smoke and maintain a healthy body weight. About one in five women with recurrent miscarriage suffers from antiphospholipid syndrome, a clotting problem that can interfere with implantation. Doctors are testing the ideal dose and type of heparin, a blood thinner, as treatment. Progesterone suppositories may help strengthen the lining of the uterus in women with low hormone levels. Some researchers also believe that women with unexplained miscarriages after a healthy first child may experience an immune response to a subsequent pregnancy. Stephenson and her colleagues are now testing the effectiveness of an intravenous immunoglobulin therapy.

Losing a baby is as emotional as it is physical. Take time to grieve and, if you feel comfortable, talk to friends and family. "I guarantee you, somebody in your Sunday-school class, somebody you work with, has had a miscarriage," says Dr. William Kutteh, director of the recurrent-pregnancy-loss clinic at Fertility Associates of Memphis. For information and support groups, see theafa.org (888-917-3777), nationalshareoffice.com (800-821-6819) or resolve.org (888-623-0744). Even after recurrent miscarriages, you can still have a healthy pregnancy. An expert told Jon and Shannon that she had only a 3 percent chance of carrying a baby to term. "It was hokum," says Jon. After her four miscarriages, Shannon delivered two healthy boys.