Some women are at risk for developing blood clots in their legs or lungs after they deliver a baby (postpartum). You may be approached about the pilot PARTUM trial if you are pregnant or have given birth within 48 hours and have risk factors for blood clots. If you choose to take part, before you leave the hospital you will be given a bottle of the medication (aspirin or placebo), to take home with you. The medication dosage is one pill per day for a total of 6 weeks.