A startling case in Japan has confirmed that pregnant women with cancer can pass the disease to their fetuses. These transmissions, normally blocked by the placenta, are rare, so the work likely won't change how doctors screen or care for pregnant women. But scientists say the case could help illuminate how cancer foils the body's immune system. In early 2007, a 28-year-old Japanese woman gave birth to a girl. Thirty-six days later, the mother was hospitalized with vaginal bleeding, which became uncontrollable. Doctors diagnosed leukemia, and she soon died. The baby developed normally until age 11 months, when a huge...