The oligohydramnios sequence is a set of related conditions resulting from marked decreased in amniotic fluid. This typically results from bilateral renal agenesis (34%) (fig. 1), or chronic leakage of fluid. Other conditions such as bilateral multicystic renal dysplasia (34%), unilateral renal agenesis and contralateral multicystic renal dysplasia (9%), renal tubular dysgenesis, autosomal recessive (infantile) polycystic disease, non-functioning but morphologically normal renal tracts (3%), abherrant placental implantation (on a uterine septum for instance)  … can lead to the same set of findings[1].

50% of these fetuses have anomalies that are not part of the oligohydramnios sequence (Vacterl, Meckel, Smith-Lemli-Opitz).

In the oligohydramnios sequence, the decrease of fluid causes lung hypoplasia and fetal compression. The fetal compression results in abnormal limb positions with dislocations and an abnormal face (flat with low set ears). After delivery these newborn die of pulmonary insufficiency (via pneumothoraces).

Figure 1: Oligohydramnios (anhydramnios) in an early 2^nd trimester fetus with bilateral renal agenesis.

Reference:

[1] Newbould MJ, Lendon M, Barson AJ: Oligohydramnios sequence: the spectrum of renal malformations. Br J Obstet Gynaecol 1994 Jul;101(7):598-604