A New York Times science writer reports on an astonishing medical frontier: the diagnosis and surgical and medical treatment of fetuses. The core of doctors who began to pioneer fetal medicine in the late 1970's and early 80's formed a close, private club nicknamed the Fetal Invaders. That's one of those jokes doctors employ to deflect the enormity of what they're doing. Increasingly, doctors are able to diagnose genetic disorders like Tay-Sachs disease or Down's syndrome or medical anomalies like underdeveloped lungs and blocked urinary tracts in ever-more immature fetuses. Also increasingly, thanks to sophisticated sonogram techniques, they can offer treatment to the fetus--or terrible choices to the parents. Kolata explores the ethical dilemmas as well as the medical opportunities in a series of interviews with the very small group who specialize in fetal therapy, and with some of their patients. In particular, she talks to a group of mothers who had the choice of having ""fetal reductions."" That is now-common procedure whereby three or more fetuses--often a result of fertility treatments--are ""reduced"" to two. Kolata also explores fetal-tissue transplants, plastic surgery, and genetic manipulation--all procedures in experimental stages. Somewhat repetitious, but easy reading that's direct and authoritative--and that raises troubling questions as we tinker earlier and earlier with new life.