Total RNA was isolated from one quarter of snap-frozen (liquid nitrogen) placenta at late gestation using the miRNeasy mini kit (Qiagen, UK), following tissue disruption in Qiazol (Qiagen, UK) in accordance with the manufacturer's instructions. RNA was treated with QIAseq FastSelect -rRNA HMR, to prevent any rRNA present from being converted into the sequencing library Stranded RNA-seq libraries were prepared from 500 ng of total RNA per sample, using the NEBNext Ultra II Directional RNA Library Preparation Kit for Illumina (NEB; E7760) and NEBNext Multiplex Oligos for Illumina (96 Unique Dual Index Pairs) (NEBNext; E6440). Libraries were quantified using the Qubit Fluorometer and the Qubit dsDNA HS Kit (ThermoFisher Scientific; Q32854). Library fragment-length distributions were analysed using the Agilent 4200 TapeStation and the Agilent High Sensitivity D1000 ScreenTape Assay (Agilent; 5067-5584 and 5067-5585). Libraries were pooled in equimolar amounts and final library quantification was performed using the KAPA Library Quantification Kit for Illumina (Roche; KK4824). The library pool was sequenced on the Illlumina NextSeq500 over three NextSeq500 High Output 150 cycle kits (Illumina; 20024907), to generate over 40 million pairs of 75-bp paired-end reads per sample.