The first trimester is a critical window of maternal-fetal communication for pregnancy. RNA-sequencing of matched maternal decidua (4) and placenta (4) identified 91 sexually dimorphic receptor-ligand pairs across the maternal-fetal interface, 32 in females and 59 in males. Overall design: Matched decidua (4) and placenta (4) samples from first trimester singleton healthy pregnancies included 2 female conceptions and 2 male conceptions.