A sequence is a predictable series of anatomically related anomalies that develop as a consequence of a known primary disturbance (ie, cause-and-effect relationship).  In this case, the neonate has Potter sequence, a cascade of deformations due to severe oligohydramnios.

Low amniotic fluid is typically the result of a genitourinary tract anomaly (eg, bilateral renal agenesis) leading to impaired fetal urine production or preterm prelabor rupture of membranes.  Regardless of the cause, insufficient amniotic fluid causes fetal constraint and compression, and this abnormal mechanical force underlies the related anatomic defects of Potter sequence.  Postnatal features include the following:

• Pulmonary hypoplasia:  Restricted fetal thoracic expansion leads to alveolar collapse and impaired lung development.  This is the most severe consequence of Potter sequence, manifesting as respiratory distress or decreased breath sounds; it is often fatal.

• Flattened facies:  Extrinsic compression leads to the characteristic facial appearance of a flattened nasal bridge, low-set ears, and a recessed chin.

• Skeletal anomalies:  Limited fetal movement leads to limb abnormalities such as contractures and clubfoot, as seen in this case.

Potter sequence has been inaccurately referred to as Potter syndrome.  A syndrome is an aggregate of anomalies with a known underlying cause, usually genetic, such as Down syndrome.  However, in a syndrome, the anomalies develop independently from one another rather than sequentially due to causal interaction.

(Choice A)  An association is 2 or more anomalies that occur together more often than would be expected by chance alone but without a known cause.  An example is VACTERL association (Vertebral anomaly, Anorectal malformation, Cardiovascular anomaly, Tracheoesophageal fistula, Esophageal atresia, Renal and/or radial anomaly, Limb defect).

(Choice B)  Dysplasia refers to the abnormal growth or development of a cell, tissue, or organ (eg, congenital hip dysplasia).

(Choice C)  Mosaicism is the occurrence of 2 or more genetically distinct cells within the same patient (eg, mosaic Down syndrome).

(Choice D)  Polymorphism refers to genetically distinct normal phenotypes within a population (eg, eye color).

Educational objective:
A sequence is a predictable series of anatomically related anomalies that develop as a consequence of a known primary anomaly (ie, cause-and-effect relationship).  Oligohydramnios leads to fetal compression and the predictable cascade of findings in Potter sequence (eg, pulmonary hypoplasia, facial and limb anomalies).