Despite marking our age from the day of birth, our physiological journey starts 9 months before; assuming we went to full term gestation. During this time we build our existence upon the merging of two cells, growing an entire human being, the environment it lives in; the amniotic sac, and the organ of gas, nutrient and waste exchange; the placenta. An undeniably fantastic bioengineering feat, and one which provides the physiological foundation for our post-natal development to adulthood. However, our physiological journey from conception to birth can face adverse challenges, such as hypoxia and inflammation, leading to impaired development of organs and their functional control and injury, all of which has consequences for our post-natal physiology. Further, such adverse events may cause the fetus to be vulnerable to further challenges to its environment leading to death before birth, as is seen with stillbirth. To develop methods of detecting the at-risk-fetus to inform clinical decision making and targeting of perinatal treatments requires a greater understanding of the maturational changes in fetal physiology and pathophysiology. This includes the role of a key regulator of our own biology; circadian rhythms. This talk will discuss how a preterm fetus can survive a significant hypoxic insult and continue to develop to full maturity, but with evolving brain injury. It will examine how injury and/or impaired brain development may make the fetus more vulnerable to further episodes of hypoxia. The talk will then explore how we can utilise information about the temporal changes in fetal physiology and behaviour after hypoxia to develop potential clinical biomarkers to detect an adverse event and determine phases of injury. This is vital for facilitating implementation of targeted therapies and inform clinical decision making. Here, time of day is an important factor as the fetus has robust circadian and ultradian rhythms, but their nature and functional role are poorly understood. The talk will present new data about fetal cardiovascular and neurophysiological circadian and ultradian rhythms. It will detail how they change with maturation and are altered by hypoxia and inflammation. The talk will highlight the importance of the time of day when utilising fetal diagnostic and prognostic biomarkers and the implications for the timing of routine clinical assessments. Finally, the talk will end with reflection upon the challenges we face to improve our understanding about the first 9 months of our physiological journey if we are to optimise our physiological development from birth to adulthood.