Heparin switch in 3rd Trimester

Alyssa • Single Mother of Two.
For those of you ladies that were on a blood thinner and then were switched to Heparin, did you have to get some kind of blood test first? I came to get my Glucose test done, they mentioned needing a Heparin test and then moved me along. It's the first I heard of it, just wondering what it's for.