UNSW Embryology

These notes cover abnormalities that can occur during development often described as congenital defects. There are many different ways that developmental abnormalities can occur the 2 major types are Genetic (inherited) and Environmental (maternal) derived abnormalities.

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While genetic abnormalites will have well-defined impacts upon development, maternally derived effects can be harder to define and variable depending on many different factors (timing, exposure level, and combination with many other factors).

Often not considered, is that pregnancy itself can also expose abnormalities in the mother (congenital heart disease, diabetes, reproductive disorders) that until then had gone undetected. In addition, these two major causes can also interact giving a broad spectrum of both major and minor abnormalities. This section of notes also includes, twinning and statistical information relating to abnormalities at birth from several different countries.

Genetic (inherited) - human chromosomes

Environmental (maternal) - young child with smallpox scars

Embryos with major genetic abnormalities, that impact on developmental processes, in general fail to develop and are spontaneously aborted. In other embryos, there are several known genetic abnormalities (on the basis of maternal age or family history) that can now be screened for with prenatal testing. (More? Genetic Abnormalities).

In terms of maternal/paternal family history, some ethnic backgrounds have been shown to have disease-associated genetic variants, though most common genetic diseases are consistent across ethnic boundaries. For example: Caucasians of northern European ancestry and cystic fibrosis (CTFR gene), Mediterranean, Asian and Far Eastern ancestry with beta-thalassaemia. (More? Genetic Abnormalities)

Many genetic developmental abnormalities involve only small DNA mutations affecting individual or a few genes, exceptions to this are those that involve abnormal segregation of chromosomes generating an abnormal chromosome number (complement) to the developing embryo.

The most common trisomy (the number indicates the affected chromosome) is Down syndrome (or trisomy 21) followed by Edwards syndrome (or trisomy 18) there are also less common trisomy of chromosomes 9, 13, 15.

Importantly, the occurance of these chromosomal abnormalities increases significantly with increasing maternal age and requires increased prenatal testing.

There are many pamphlets providing information about prenatal diagnosis (see NSW State Health Publication Checking your baby's health before birth).

Environmental abnormalities (often called maternal derived) relate to lifestyle, infection, maternal health and nutrition. In general, the more complex and the longer timecourse of development, the more sensitive the system is to these abnormalities. The following list of topics is in no specific order and links to more detailed pages. (More? Maternal Abnormalities)

Infections of all kinds viral, bacterial and protist collectively grouped under the acronym TORCH, which stands for: Toxoplasmosis, Other organisms (parvovirus, HIV, Epstein-Barr, herpes 6 and 8, varicella, syphilis, enterovirus) , Rubella, Cytomegalovirus and Hepatitis. See also a page on bacterial infections , viral infections, Polio , Parvovirus, Malaria and Toxoplasmosis.

(More? Environmental)

Infections (as well as other environmental conditions) can also lead to increases in maternal core body temperature, Maternal Hyperthermia, which has also been shown in animal models to be a potent teratogen.

Maternal drugs fall into many different classes and there is specific information for each of these groups. Importantly the developing fetus is exposed to different levels and and metabolises more slowly all drugs.

Lifestyle - alcohol (fetal alcohol syndrome), smoking, and illegal drugs (Cannabis/Marijuana, Methamphetamine/Amphetamine, Cocaine, Heroin, Lysergic Acid Diethylamide)

Maternal conditions - prescription drugs (therapeutic chemicals/agents, thalidomide limb development) and non-prescription drugs (herbal)

Environment, for example smoking, chemical, heavy metals. Other maternal maternal effects include changes in endocrine function (maternal diabetes, thyroid development).

In addition to these obvious environmentally derived abnormalities, there is growing evidence that the interuterine environment has a strong influence on later postnatal health and neurological development. This theory is based on the early statistical analysis of disease/longevity in babies with low birth weights in England by Barker, and was initially called the "Barker Hypothesis" and now called the Fetal Origins Hypothesis.

A better understanding of abnormal development can also be gained from looking at normal birth statistics. Statistics on prenatal diagnosis, birth and prenatal/postnatal development (growth, neural development) not only give us normal values for the population, but also identify health trends and potential genetic/environmental influences on development.

This site contains several resources for statistical data and Australian statistical data that has been derived from the Australian Institute of Health and Welfare National Perinatal Statistics Unit, UNSW (More? NPSU) which publishes "Congenital Malformations Australia" every 2 years. Be aware that some congenital abnormalities, by their nature, affect multiple systems.

In the USA, the Center for Disease Control (CDC) keeps and publishes relevant statistical information.

While in general many twins develop normally, the process of twinning is an abnormality that is also covered in this section of notes and in the Week 1 notes.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers

This quick link is to the abnormal development page for each system.