Poor growth before birth is associated with a high risk of death and brain injury. There are many causes of poor growth, and current clinical monitoring has a limited ability to identify babies who will need intervention. Circadian rhythms are critical regulators of our physiology and behaviour over ~24-hour cycles, and we have found that they are present before birth. Our recent studies suggest that abnormal circadian patterns in the fetal heart rate recordings are associated with poor growth. We will undertake both clinical and pre-clinical studies to examine fetal circadian patterns in normal and growth restricted pregnancies to determine how and when circadian patterns are related to poor placental oxygen levels. These studies will test for the first time how circadian rhythms can be used to identify high-risk babies with very low oxygen levels before birth.