Why baby aspirin?!

My friend just found out she’s pregnant, she’s miscarried early on twice before this pregnancy. She thinks she’s abt 4 weeks and they did her blood work today she won’t know the results until tomorrow however they told her to take baby aspirin the 81mg? Why is this? They never told her why just told her to start taking it. Anybody take this in early pregnancy? And how many does she need to take a day, they really didn’t tel her anything other than to take it.