Oligohydramnios

Oligohydramnios causes fetal deformation and is well
recognised in fetal renal agenesis (Potter sequence). The
absence of urine production by the fetus results in severe
oligohydramnios, which in turn causes fetal deformation and
pulmonary hypoplasia. Oligohydramnios caused by chronic
leakage of liquor has a similar effect.
A normal fetus may be constrained by uterine
abnormalities, breech presentation or multiple pregnancy. The
prognosis is generally excellent, and the risk of recurrence is
low except in cases of structural uterine abnormality.

Dysplasia

Dysplasia refers to abnormal cellular organisation or function
within a specific organ or tissue type. Most dysplasias are caused
by single gene defects, and include conditions such as skeletal
dysplasias and storage disorders from inborn errors of
metabolism. Unlike the other mechanisms causing birth
defects, dysplasias may have a progressive effect and can lead to
continued deterioration of function.

Classification of birth defects

Single system defects
Single system defects constitute the largest group of birth
defects, affecting a single organ system or local region of the
body. The commonest of these include cleft lip and palate, club
foot, pyloric stenosis, congenital dislocation of the hip and
congenital heart defects. Each of these defects can also occur
frequently as a component of a more generalised multiple
abnormality disorder. Congenital heart defects, for example,
are associated with many chromosomal disorders and
malformation syndromes. When these defects occur as isolated
abnormalities, the recurrence risk is usually low.

Multiple malformation syndromes

When a combination of congenital abnormalities occurs
together repeatedly in a consistent pattern due to a single
underlying cause, the term “syndrome” is used. The literal
translation of this Greek term is “running together”.
Identification of a birth defect syndrome allows comparison of
cases to define the clinical spectrum of the disorder and aids
research into aetiology and pathogenesis.

Sequences

The term sequence implies that a series of events occurs after a
single initiating abnormality, which may be a malformation,
a deformation or a disruption. The features of Potter sequence
are classed as a malformation sequence because the initial
abnormality is renal agenesis, which gives rise to
oligohydramnios and secondary deformation and pulmonary
hypoplasia. Other examples are the holoprosencephaly
sequence and the sirenomelia sequence. In holoprosencephaly
the primary developmental defect is in the forebrain, leading
to microcephaly, absent olfactory and optic nerves, and midline
defects in facial development, including hypotelorism or
cyclopia, midline cleft lip and abnormal development of the
nose. In sirenomelia the primary defect affects the caudal axis
of the fetus, from which the lower limbs, bladder, genitalia,
kidneys, hindgut and sacrum develop. Abnormalities of all
these structures occur in the sirenomelia sequence.

renal aplasia

Murcs association is the name given to the non-random
occurrence of Mullerian duct aplasia, renal aplasia and
cervicothoracic somite dysplasia. In the Charge association the
related abnormalities include colobomas of the eye, heart
defects, choanal atresia, mental retardation, growth
retardation and ear anomalies.

Complexes

The term developmental field complex has been used to
describe abnormalities that occur in adjacent or related
structures from defects that affect a particular geographical
part of the developing embryo. The underlying aetiology may
represent a vascular event, resulting in the defects such as those
seen in hemifacial microsomia (Goldenhar syndrome), Poland
anomaly and some cases of Möbius syndrome.