ABSTRACT
The era of prenatal diagnosis dates from the late 1970s when case reports of the prenatal detection of urologic malformations first began to appear in the literature. Since that time prenatal diagnosis has rapidly become an established and routine feature of clinical pediatric urology throughout the developed world. Many aspects of prenatal diagnosis and the natural history of urinary tract malformations have been clarified by studies published in the last two decades, but some important questions relating to long-term outcomes remain unresolved. A detailed consideration of the investigation and practical management of the various anomalies detected by prenatal ultrasonography is detailed in the relevant chapters elsewhere in this book. This chapter will therefore focus predominantly on the following areas of fetal and perinatal urology:
■ pathophysiology of urinary tract malformations ■ current status of fetal intervention ■ value of prenatal ultrasound in screening ■ a rational approach to early postnatal investigation ■ significance of mild dilatation (‘pyelectasis’)
Detailed embryology is beyond the scope of this chapter, and is covered elsewhere. A pivotal event in functional development occurs at around 32 days of gestation when the ureteric bud fuses with the metanephric mesenchyme to initiate the formation of excretory nephrons. Nephrogenesis proceeds by a process of reciprocal induction until the 36th week of pregnancy, with the glomeruli, proximal tubules, and loops of Henle being derived from the metanephros, whereas the collecting ducts, calyces, and renal pelvis are derivatives of the ureteric bud. In man, nephroge-
nesis ceases at 36 weeks, and thereafter the number of nephrons remains fixed at between 0.7 and 1 million per kidney. Major developmental defects dating from the first trimester of gestation are characterized by agenesis or dysplasia (arrested differentiation that is characterized histologically by immature tubules and the presence of aberrant mesenchymal derivatives such as smooth muscle and cartilage).
