A sequence is where a single initial aberration leads to a series of anomalies in the body. The best example of a sequence is 'Potter Sequence'.
Conditions like renal agenesis, maternal hypertension lead to oligohydramnios. The amniotic fluid plays a role in the lung development in the second half of pregnancy & so, there is a higher incidence of pulmonary hypoplasia in such foetuses which is the main cause of their death.
Oligohydramnios leads to direct pressure of the uterine wall on the developing foetus leading to flattened nose, recessed chin & low set ears. This is Potter sequence- sequence here because the initial aberration- oligohydramnios leads to & explains all the changes occurring later.
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