library_construction_protocol: Mice with timed pregnancies were perfused with 60cc sterile PBS and RNA from whole fetal-placental units (E8.5-9.5) or whole placentas (E12.5-13.5) was isolated using Qiagen kits. Messenger RNA was purified from total RNA using poly-T oligo-attached magnetic beads. After fragmentation, the first strand cDNA was synthesized using random hexamer primers followed by the second strand cDNA synthesis. The library was ready after end repair, A-tailing, adapter ligation, size selection, amplification, and purification.