**1. Introduction**

60 Basic Nephrology and Acute Kidney Injury

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and standard clearance techniques for measuring glomerular filtration rate in children with and without vesico-ureteric reflux, *Clin Nephrol,* Vol.7, No.6, pp. 262Preterm birth is the leading cause of morbidity and mortality in the neonatal period (Ward and Beachy 2003) and in childhood overall (McCormick, 1985). Over recent decades both the incidence of preterm birth and survival rates of preterm infants has increased, with babies born as early as 25 weeks gestation now having about an 80% chance of survival (Kutz et al., 2009). Preterm birth is defined as birth prior to 37 weeks of gestation; it can be further subclassified as moderately preterm (birth between 32 and 37 weeks gestation) very preterm (birth < 32 weeks gestation) and extremely preterm (birth at < 28 weeks gestation) (Tucker and McGuire, 2004).

Due to the immaturity of the organs at the time of birth, preterm infants exhibit an increased risk of developing a number of postnatal complications including renal insufficiency and in severe cases renal failure (Drukker and Guignard, 2002; Choker and Gouyon, 2004); the mortality rate in these infants is very high (Drukker and Guignard, 2002; Andreoli, 2004). There is also evidence that preterm birth adversely affects nephrogenesis (the formation of nephrons) in the developing kidney; if this is the case, this has the potential to not only adversely affect renal function in the early postnatal period but to also increase the risk of renal disease later in life. Certainly, there are many studies linking a reduced nephron endowment early in life with hypertension (Keller et al., 2003; Luyckx and Brenner, 2005) and vulnerability to secondary renal insults in adulthood (Nenov et al., 2000; Zimanyi et al., 2006; Hoppe et al., 2007). In this regard, there is substantial recent epidemiological evidence linking preterm birth with an increase in blood pressure in adulthood (Siewert-Delle and Ljungman, 1998; Kistner et al., 2000; Kistner et al., 2002; Doyle et al., 2003; Bonamy et al., 2005; Hack et al., 2005; Johansson et al., 2005; Dalziel et al., 2007; Cooper et al., 2008; Keijzer-Veen et al., 2010b); these observations may be due to a reduced nephron endowment in preterm individuals.

In this chapter, we review the current knowledge of the effects of preterm birth on nephrogenesis in the developing kidney and on renal function postnatally.
