1. This study was designed to examine the changes in kidney morphometry during gestation in fetuses that were either appropriate or small for gestational age and the relationship between umbilical vein plasma active renin and kidney morphometry.

2. Serial ultrasound measurements of various morphometric and renal indices were performed in a cohort of 87 singleton fetuses from 22 to 38 weeks gestation. Blood was collected from the umbilical vein at delivery and active renin was measured from the plasma based on angiotensinogen I generated during incubation with plasma and ox renin substrate.

3. Growth in the longitudinal plane of fetal kidneys was similar in the small- and appropriate-for-gestational age groups; however, growth in the anterio-posterior, transverse and circumference planes of the kidneys was significantly slower in the small-for-gestational-age group after 26 weeks gestation. Differences in growth rate in the two groups were most marked between 26 and 34 weeks and persisted until delivery when the anterior-posterior diameter was significantly larger (P < 0.00001) in the appropriate-for-gestational-age group (26.1 ± 2.5 mm compared with 19.8 ± 2.6 mm). The mean umbilical vein active plasma renin concentration at delivery was significantly higher (P < 0.05) in the small-for-gestational-age group (274.4 ± 32.9 μ-units/ml plasma) than in the appropriate-for-gestational-age group (164.9 ± 28.3 μ-units/ml plasma). In addition, there were statistically significant inverse correlations between renin concentration and birthweight (r = − 0.55, P < 0.001) and between renin concentration and kidney anterior-posterior diameter (r = −0.67, P < 0.001).

4. Fetal renal growth was slower in small than in appropriate-for-gestational-age fetuses. The period of 26–34 weeks gestation was that during which maximum fetal renal growth occurred. Umbilical vein plasma renin levels were higher in small-for-gestational-age fetuses. The findings of slow fetal renal growth rate and associated high renin concentrations seen in small-for-gestational-age fetuses could be implicated in an irreversible reno-vascular pathology leading to adult hypertension. We suggest that 26 to 34 weeks could be the ‘critical period’ during which the insult that leads to in-utero programming for the development of adult hypertension occurs.

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