Skip to content
Explore atlas
PH-FD29

Developmental malformations

How organs are built in weeks 3 to 8 after fertilisation, how genes, teratogens or both derail that programme, why timing decides the defect, and how one malformation can cascade into others.

By the end, you can

  • Explain why the embryonic period is the most sensitive time for major structural malformations.
  • Describe the genetic, environmental and multifactorial causes of malformations, with examples.
  • Relate failures of closure, fusion, septation, canalisation and migration to the defects they produce.
  • Explain how an open neural tube defect raises alpha-fetoprotein and causes loss of function below it.
  • Distinguish a malformation, deformation, disruption, sequence and syndrome.
  • Recognise the classic appearances, the four types of morphological defect, the teratogen windows and the antenatal tests, and explain how folic acid, avoiding teratogens and surgical repair act on the mechanism.

What it is

Developmental malformations (congenital anomalies, birth defects) are structural defects present at birth that arise when an organ or body part forms abnormally. A malformation is an error in the developmental programme itself; a deformation is a normally formed part moulded by outside pressure; a disruption is the destruction of a part that was forming normally. Causes are genetic, environmental or, most often, a mixture, and for many no cause is found.

5 types to tell apart
Malformation
An intrinsic error in forming a structure, usually in weeks 3 to 8: spina bifida, cleft lip, a ventricular septal defect.
Deformation
A normally formed part moulded by mechanical force, usually late in pregnancy, such as a positional clubfoot when there is little amniotic fluid.
Disruption
A part that was developing normally is destroyed, for example by a constricting amniotic band or loss of its blood supply.
Sequence
One primary anomaly sets off a cascade of others, as in the oligohydramnios (Potter) sequence.
Syndrome and association
A syndrome is a set of anomalies with one underlying cause (trisomy 21); an association is a set that occurs together more often than chance without one known cause (VACTERL).

Normal function

Organs are built in a fixed order

In the third week after fertilisation, cells of the embryonic disc move inwards to form three germ layers: (skin and nervous system), (muscle, bone, heart, vessels and kidneys) and (the lining of the gut and its glands, and of the lungs). The in the midline signals to the ectoderm above it to become the neural plate.

Over the next five weeks every organ is built from these layers by a small set of processes: sheets of cells fold and close into tubes, separate swellings fuse, single tubes and chambers are divided by septa, solid cords hollow out (canalise), cells migrate long distances, and unwanted cells are removed by apoptosis. Signalling genes run each process, and each happens in its own window of time.

By about week 8 the main organs are in place. The fetal period that follows is mostly growth and maturation, although the brain keeps developing until well after birth.

Point to it on the model