Miscarriages and preterm births are common and serious events affecting women, families, and healthcare systems on many levels. One of the risk factors for miscarriage and preterm birth is bleeding in the first trimester of pregnancy. Progesterone, a hormone that plays a key role during pregnancy, has been proposed as a possible medication to be used in pregnancy to prevent miscarriage and preterm birth among women who have bleeding in their first trimester of pregnancy. Unfortunately, unless sound clinical evidence is obtained through a clinical trial, whether or not progesterone can indeed prevent miscarriage and preterm birth remains uncertain and thus is not a recommended treatment in women with early pregnancy bleeding. The purpose of our study is to evaluate the effect of progesterone for the prevention of miscarriage and preterm birth among women with early pregnancy bleeding. We will carry out a clinical trial in which 850 women will be randomized to receive either progesterone supplementation (425 women) or a similarly appearing placebo (425 women) and the outcome of their pregnancy will be compared.