Preterm birth (PTB) is defined as birth before the 37th week of pregnancy and affects 15 million infants per year. Presently, there is no clinical test to determine PTB risk. A 3D printed microfluidic device is being developed as a clinical test for PTB risk via detection of a panel of biomarkers. A significant step is extraction of the PTB biomarkers from blood serum. In this work, I developed 3D printed microfluidic devices in which monoliths can be polymerized. Using the monolith as a solid support to attach antibody, I show that ferritin, one of the PTB biomarkers, can be selectively extracted from human blood serum. This is the first study where a monolith has been formed in a 3D printed microfluidic device and used to perform an immunoaffinity extraction. This work is an important step in developing a clinical test for PTB risk. The realization of this work also demonstrates that 3D printing can be used to fabricate functional microfluidic devices.