We used a whole genome approach to identify major functional gene categories (including xenobiotic transporters and metabolizing enzymes) whose expression depends on gestational age. STUDY DESIGN: We compared gene expression profiles of 1st (45-59 days) and 2nd trimester (109-115 days), and C-section term placentae. RESULTS: In 1st trimester placentae, genes related to cell cycle, DNA, aminoacids and carbohydrate metabolism were significantly overrepresented, while genes related to signal transduction were downregulated. In the organism defense category, we identified genes involved in chemical response, metabolism, and transport. Analysis of signal transduction pathways suggested, and subsequently confirmed independently, that the Wnt pathway was regulated by gestational age. CONCLUSIONS: Our study will serve as a reference database to gain insight into the regulation of gene expression in the developing placentae and, thus, allow comparisons with placentae from complicated pregnancies such as those in women experiencing gestational diabetes, pre-eclampsia and teratogenic sequelae.