The oviduct and uterus provide the environments for the earliest stages of mammalian embryo development. However, little is known about the mechanisms that underlie the formation of oviduct and uterine fluids, or the extent to which the supply of nutrients via these reproductive tract tissues matches the nutrient requirements of early embryos. After reviewing our limited knowledge of these phenomena, a new experimental paradigm is proposed in which the epithelia lining the endosalpinx and endometrium are seen as the final components in a supply line that links maternal diet at one end and embryo uptake of nutrients at the other. When considered in this way, the oviduct and uterine epithelia become, for a few days, potentially the most critical maternal tissues in the establishment of a healthy pregnancy. In fulfilling this ‘gatekeeper’ role, female reproductive tract fluids have a key role in the ‘developmental origins of health and disease’ concept.