In 54 women the plasma prothrombin was studied from as early as the fifth week of pregnancy to the tenth week following delivery. Forty-four, or 81 per cent, at some time during pregnancy had a prothrombin of 110 per cent or above. In general, the levels were slightly lower in the first trimester, tended to rise during the last six months, and fell slightly in the last week before delivery. There was a tendency to rise during the first week of the puerperium and to fall gradually toward normal during the following five weeks.