Preterm delivery is the most common cause of infant morbidity and mortality in the United States. Some women have episodes of preterm labor during their pregnancy which can be temporarily stopped. These women, however, are at high risk for delivering before term. At this time, we do not have sufficient evidence to use any medication to help prevent these women from delivering early. Recently, preliminary studies have shown that progesterone may help prevent some women at high risk for preterm delivery from delivering early. Our study will investigate whether progesterone can help this specific group of women, women with arrested preterm labor, deliver healthy infants at term.