Prior to 16 weeks' gestations, the amount of amniotic fluid is dependent on the transmembrane flow. After that, fetal urine production is the predominant mechanism that determines the amniotic fluid volume. The fetus continuously swallows amniotic fluid, which is reabsorbed by the GI tract and then reintroduced into the amniotic cavity by the kidneys. Oligohydramnios occurs if the volume of amniotic fluid is less than normal for the corresponding period of gestation. This may be due to decreased urine production secondary to bilateral renal agenesis, obstruction of the urinary tract, or, occasionally, prolonged rupture of membranes [2, 3] . The resulting oligohydramnios is the cause of the deformities observed in Potter syndrome. The mechanism of lung hypoplasia in this condition is not clear. It is believed that adequate space in the fetal thorax and the movement of amniotic fluid into the fetal lungs is required for the normal development of lungs.

Genetics

The genetic aspect of renal malformation has not been fully understood yet and there is a lot of current research work in this field.

During nephrogenesis, multiple genes, transcription factors, and growth factors control the essential interaction between the ureteric bud and the metanephric mesenchyme. For example, the Lim1 and Pax2 transcription factors are essential for the formation of the mesonephric duct, from which the ureteric bud develops. Lim1 -deficient mice have complete renal agenesis. If the Pax2 gene is deficient, there is deletion of the caudal portion of the mesonephric duct, which results in renal agenesis [4, 5] .

WT-1, a zinc-finger transcription factor expressed in the metanephric mesenchyme, is essential for ureteric bud outgrowth. Homozygous null-mutants for WT-1 have complete renal agenesis. Similarly, transcription factor EYA1 from the metanephric mesenchyme is required for ureteric bud outgrowth, and deficiency of this protein is shown to cause branchio-oto-renal syndrome [6] .

The glial cell line–derived neurotrophic factor (GDNF) from the metanephric mesenchyme binds to the C-ret receptor on the branching ureteric bud and is responsible for the branching and elongation of the ureteric bud. Inactivation of either GDNF or the C-ret receptor leads to renal agenesis. Heterozygotes may have a unilateral renal abnormality while the contralateral kidney has normal development [7] .

Limb deformity (ld) gene codes for 4 different spliced formin genes, which are expressed in the mesonephric duct and branching ureteric ducts. Mutation of the ld gene leads to limb deformity with renal agenesis. Mutation of the formin IV gene leads only to kidney abnormalities. Homozygous mutation of the alpha-8 integrin subunit produces abnormalities similar to ld mutation with deformities including renal aplasia, dysplasia, or hypoplasia [8] .

Transcription factors such as EMX-2, BF-2, fibroblast growth factor 7 (FGF 7), epithelial growth factor receptor (EGF-R), GDNF, retinoic acid receptor alpha, and beta 2 are involved in the branching of the ureteric bud. A heterozygous mutation defect of the growth factor bone morphogenetic protein 4 (bmp 4) leads to renal hypoplasia or dysplasia, ureterovesicular junction obstruction, hydronephrosis, or the bifid/duplex kidney. This is a defect of ureteric branching and not induction of the ureteric bud; thus, renal aplasia does not occur [9, 10, 11] .

The autosomal recessive mutations of genes in the renin-angiotensin pathway result in renal tubular dysgenesis due to failed development of proximal tubules. These are heterogeneous mutations of renin, angiotensin, angiotensin converting enzyme, or type 1 angiotensin II receptor. [12, 13] . Some reports have suggested that the clinical picture of renal tubular dysgenesis is similar to the infants born to mothers who had received angiotensin converting enzyme inhibitor or angiotensin II receptor blockers during pregnancy [14, 15] .

Hepatocyte nuclear factor (HNF)-1beta gene (TCF2) is normally expressed in the Wolffian duct, metanephric tubules, and Mullerian during fetal life. This gene originally described in relation to maturity-onset diabetes, has been now recognized as a cause of cystic renal dysplasis [16, 17, 4] . Uroplakins IIIa is a protein expressed on the mammalian urothelia and has been suggested to be involved in defects of early kidney development like renal hypoplasia/dysplasia. [18]

Recognized genetic disorders such as renal coloboma syndrome (PAX2 mutation) and branchio-oto-renal syndrome (EYA1 mutation) are therefore associated with renal agenesis or dysplastic kidney abnormalities [6, 19] .

Some of the other reported Mutations associated with Renal Hypodysplasia are, FREM 1 (Causes Bifid Nose, Renal Agenesis and Anorectal Malformation) [20] , FRAS 1/FREM 2 (Fraser syndrome) [21, 22] , LRP4 (Cenani-Lenz syndrome) [23] , GLI3 (Pallister-Hall syndrome) [24] , SALL1 (Townes-Brocks syndrome, which has triad of imperforate anus, dysplastic ears and thumb malformation) [25] , KAL1 (Kallman syndrome) [26] , GATA3 (Renal hypodysplasia is associated with Hypothyroidism and sensory-neural deafness) [27] .

There are case reports on families with both unilateral and bilateral renal agenesis. This condition, termed hereditary renal adysplasia (HRA), is an autosomal dominant trait with incomplete penetrance and variable expression. Associated non-urogenital anomalies have been reported in HRA [28] .