This thesis explores the complex tri-compartmental pharmacological interaction between the mother, the placenta, and the foetus. I argue that maternal physiological changes, such as a 50% increase in blood volume and doubled renal clearance, significantly alter drug concentrations and can lead to sub-therapeutic treatment. The placenta is presented not as a passive barrier, but as an active biochemical gatekeeper that regulates drug exposure through enzymes and efflux transporters. Central to my research is the "chronological imperative," which asserts that the timing of drug exposure—particularly during the critical window of organogenesis—is the primary determinant of structural or functional harm. I examine specific high-risk scenarios, such as the Potter sequence caused by ACE inhibitors, which illustrates how late-term exposure can lead to foetal renal failure and developmental compression. My findings highlight that maternal stability is the essential precondition for foetal safety, as untreated conditions often pose a greater risk than the medications themselves. Ultimately, I advocate for a shift toward precision medicine using advanced technologies like PBPK modelling and "placenta-on-a-chip" to replace trial-and-error prescribing with evidence-based care.