The study used a particularly sensitive and accurate measurement technique that Dr. Wilcox said ''was at least 50 times more sensitive'' than methods used in previous studies.
But Dr. Wilcox said that even this method underestimates the miscarriage rate by an unknown amount since some embryos are so defective that they never make human chorionic gonadotropin and are miscarried within days of fertilization. #221 Women Studied The researchers studied 221 women who were trying to become pregnant. The study showed, as have previous studies, that there is about a 25 percent chance each month that a woman who is trying to become pregnant will conceive. But once an egg is fertilized, according to the study, the pregnancy has a 31 percent chance of ending in a miscarriage. In the study, 22 percent of the women who had miscarriages never had any clinical signs of pregnancy, such as a growing uterus.
Experts said that it was important to have reliable information on early miscarriages to assess the success of in vitro fertilization, a method in which a woman's eggs are removed from her body and mixed with sperm in the laboratory; the growing embryo is then implanted in her uterus.
Dr. Joseph Schulman of the Genetics and IVF Institute of Fairfax, Va., a private clinic that specializes in in vitro fertilization, said he and others have found that about 30 percent of the embryos they implant are miscarried, a rate that is close to the natural rate detected by Dr. Wilcox.