Abstract: We have generated a new deletion allele within a large cluster of imprinted genes which leads to a hypomorphic allele for Ascl2, a transcription factor essential for placental development and intestinal stem cell fate. Our analysis of hypomorphic Ascl2 mouse conceptuses revealed that the function of Ascl2 is dosage sensitive and that reduced levels of Ascl2 lead to highly disorganized placentae with several different cell types being affected. These mutant placentae can still support development to term although the associated embryos are growth retarded. Our deletion allele therefore provides a mouse model for intrauterine growth restriction, which affects more than 30,000 births each year in Canada.