Mechanisms of birth defects

Question 1

What is the cause of abnormal embryonic development ?

Answer 1

Embryonic development is a precisely timed sequence of events and controlled b cascade of activation of developmental genes thus birth defects occur due to abnormal development in a particular part of the embryo at a particular time

Question 2

What is a birth defects or congenital abnormality and how are they classified

Answer 2

*Any abnormality of a structure / function that is present although not necessarily obvious at birth .

Birth defects can either be minor /major
single/ isolated or multiple

Question 3

When can congenital defects develop during pregnancy ?

Answer 3

1. Pre-embryonic phase : From fertilization until the end of the second week of pregnancy
2. Embryonic phase: From the 3rd to 8th week of pregnancy .( this is the phase where major organs develop)
3. Fetal phase : From the 9th week ( phase of growth and maturity )

Question 4

T/F : 30% of all affected pregnancy by morphological errors are lost spontaneously

Answer 4

True

2.7% of babies in USA are born with congenital defects whereas In SA it is 6,8%

Question 5

Name the 4 mechanisms of structural birth defects

Answer 5

1.Malformation
2Deformation
3.Disruption
4.Dysplasia

Question 6

What is malformation and How does malformation cause structural defect?

Answer 6

Malformation is the abnormal structure caused by complete or incomplete development of an organ or limb.

Defect is permanent

It occurs in embryonic period / PERIOD OF ORGANGESESISI

It is caused by :

• chromosomal abnormalities
• Single gene abnormalities
• Multifactorial
• Teratogens eg alcohol ,rubella

> oCCURS DURING FETAL PERIOD

Reoccurrence risk depends on the cause

Multifactorial is the common cause and chromosomes are the least common cause of malformations

Question 7

What is deformation and how does it cause structural defect ?

Answer 7

• Normal underlying tissue
• Normal structure id pushed /pulled out of shape ( by external mechanical forces )
• Usually occurs later in pregnancy and often involves bones ,cartilage and joints

Causes
>abnormal uterus
>Multiple pregnancy
>oligohydramnios

> Occurs during fetal period

*Often correctable after birth

Recurrence risk is low

Question 8

What is disruption and how does it cause structural defects ?

Answer 8

A non-progressive congenital morphologic anomaly due to the breakdown of a body structure that had a potential to develop normally

Occurs when an external agent destroy the normal tissues of a body part of a developing fetus

defect is permanent (tissues are destroyed )

Cause
>ischaemia (amniotic bands ,fetal intravascular accidents )
>necrosis

> Period of organogenesis or fetal period

Recurrence is low since they do not have genetic basis

Question 9

What is dysplasia and how does it cause structural birth defects

Answer 9

Abnormal organization and functioning of a specific tissue type throughout the body eg skeletal dysplasia’s
*Abnormality continues as long as tissue grows

> permanent

> Occurs during organogenesis

Caused by single gene defects

Recurrence risk : Mendelian inheritance

Question 10

T/F; Many healthy people have one or two minor abnormalities .

> /= 3 anomalies ,even minor are more likely to be part of a syndrome , sequence or association

Answer 10

true

Question 11

What is a syndrome and give an example

Answer 11

Recognizable pattern of symptoms ,signs and dysmorphic features with a strongly suspected /known genetic cause.

Example : Down syndrome / trisomy 21

Question 12

What is a sequence and give an example

Answer 12

A sequence is the result of a cascade of events in which one abnormality leads to further abnormalities

eg Potter sequence

Question 13

What is association and give an example

Answer 13

A collection of major malformations that occur together more often that would be expected by chance and cannot be recognized as a syndrome or sequence .

eg VACTERL association (no genetic cause has been identified)

Question 14

Summary

Answer 14

• Three developmental phases of pregnancy’s include : pre-embryonic ,embryonic ,fetal phase
• A birth defect /congenital abnormalities is any abnormality of the structure or function that is present birth ,it might not necessarily be obvious
• There are high rates of abnormalities in SA (6.8%)
• Four main mechanisms : malformations ,disruptions ,dysplasia ,deformation
• Understanding the mechanisms causing birth defects assist in the comprehensive management and counselling of a patient

*Birth /congenital defects can be categorized as : associations ,syndromes or sequences