Foundations Practical - Critical Periods

From Embryology
Foundations Practical: Introduction | Week 1 and 2 | Week 3 and 4 | Week 1 to 8 | Week 9 to 36 | Neonatal | Critical Periods | Additional Resources | Quiz
Human critical periods of development

Abnormal Development

We have now had a very quick trip through more than 9 months of development in nearly 2 hours. Before we finish it is worth thinking about times in development when things may go wrong.

Environment Effects

Maternal Diet
These are developmental times (stages) sensitive to insult, be aware of these times, the causes and the potential effects.

Research has identified many environmental factors now known to negatively impact upon normal development (teratogens) and we have also discovered key factors required for normal development (maternal diet).

Maternal diet, such as folate and iodine, has key roles to play in normal development.


Fetal Alcohol Syndrome (FAS) facial features[1] This disorder was clinically described (USA) in humans about 30 years ago (1973), while historically alcohol's teratogenic effects were identified in the early 20th century in a mix with the prohibition cause of the period.

Environmental Links: Introduction | Low Folic Acid | Iodine Deficiency | Nutrition | Drugs | Australian Drug Categories | USA Drug Categories | thalidomide | herbal drugs | Illegal Drugs | smoking | Fetal Alcohol Syndrome | TORCH | viral infection | bacterial infection | Zoonotic Infection | Toxoplasmosis | Malaria | Maternal Diabetes | Maternal Hypertension | maternal hyperthermia | Maternal Inflammation | Maternal Obesity | Hypoxia | Biological Toxins | Chemicals | heavy metals | radiation | Prenatal Diagnosis | Neonatal Diagnosis | International Classification of Diseases | Fetal Origins Hypothesis

Critical Periods

Critical periods of development refer to times when genetic or materal effects can impact upon the developmental process. The timing of these effects will impact on different systems at different times.

Critical Periods of Human Development
Conceptus Embryonic development (weeks) Fetal period (weeks)
Early zygote.jpg Week2 001 icon.jpg Stage9 sem4c.jpg Stage13 sem1c.jpg Stage15 bf1c.jpg Stage17 bf1c.jpg Stage19 bf1c.jpg Stage23 bf1c.jpg
Stage2.jpg Heart
Upper limbs
Lower limbs
CSt3.jpg Palate
Week2 001 icon.jpg External genitalia
Loss Major abnormalities Functional and Minor abnormalities

Links: Embryonic Development | Timeline human development | Movie - Human Development annotated cartoon | Human - critical periods | Human Abnormal Development

Systems with long periods of development or complex developmental origins are more susceptible to developmental abnormalities.

  • Which systems take a long time to develop?
  • Which systems are complex in origin?

Australian Statistics

Australian abnormalities graph allsystem.png Trisomy21male.jpg

Australian Congenital Anomalies Monitoring System (ACAMS) contains data based on notifications of major congenital anomalies to birth defects registers in New South Wales, Victoria, Western Australia and South Australia and on data collected on congenital anomalies in Queensland, Tasmania and the Australian Capital Territory. Information is included on live births and stillbirths of 20 weeks gestational age or more or 400 grams birthweight or more (including induced abortions) with a congenital anomaly. Congenital anomalies are coded using the British Paediatric Association Classification of Diseases (ICD-9-BPA), which is based on the International Classification of Diseases, 9th Revision (ICD-9). ACAMS

USA Statistics  
USA Selected Abnormalities (CDC National estimates for 21 selected major birth defects 2004–2006)  
Birth Defects Cases per Births (1 in ...) Estimated Annual Number of Cases
Anencephaly 4,859 859
Spina bifida without anencephaly 2,858 1,460
Encephalocele 12,235 341
Anophthalmia/microphthalmia 5,349 780
Common truncus 13,876 301
Transposition of great arteries 3,333 1,252
Tetralogy of Fallot 2,518 1,657
Atrioventricular septal defect 2,122 1,966
Hypoplastic left heart syndrome 4,344 960
Cleft palate without cleft lip 1,574 2,651
Cleft lip with and without cleft palate 940 4,437
Esophageal atresia/tracheoesophageal fistula 4,608 905
Rectal and large intestinal atresia/stenosis 2,138 1,952
Reduction deformity, upper limbs 2,869 1,454
Reduction deformity, lower limbs 5,949 701
Gastroschisis 2,229 1,871
Omphalocele 5,386 775
Diaphragmatic hernia 3,836 1,088
Trisomy 13 7,906 528
Trisomy 21 (Down syndrome) 691 6,037
Trisomy 18 3,762 1,109
Links: Human Abnormal Development | CDC Birth Defects - Data & Statistics | USA Statistics | Victoria 2004 | USA 2006 | Europe 2010


Trisomy 21 (Down's syndrome)

With increasing maternal age comes the increased risk of age related genetic abnormalities, in particular the most common chromosomal abnormality (aneuploidy) trisomy 21.

In order to detect some genetic abnormalities many countries offer genetic screening programs that include maternal serum screening (MSS, for detection of Down's syndrome and neural tube defects), embryonic and newborn screening (for phenylketonuria (PKU), hypothyroidism, cystic fibrosis and metabolic disorders).

Links: Genetics | Prenatal Diagnosis | Neonatal Diagnosis | Trisomy 21 | Genetic risk maternal age

Developmental Origins of Health and Disease

The fetal period is also potentially sensitive to environmental effects that impact upon interuterine growth.

Along these lines there is growing evidence that low birth weight, and therefore inhibited fetal growth/development, has postnatal effects on lifelong health outcomes. The theory was originally called the Barker Hypothesis, it became the Fetal Origins Hypothesis, and is currently the Developmental Origins of Health and Disease (DOHaD).

This is now known as the Developmental Origins of Health and Disease (DOHaD).

Normal distribution curve on graph
Normal distribution curve (red)


  • folate/folic acid - (Latin, folium = leaf) (= Folate, Pteroylglutamic acid) A water soluble vitamin, found in many fruits (particularly oranges, berries and bananas), leafy green vegetables, cereals and legumes, which can prevent neural tube defects (NTDs). The cellular roles of folate include: DNA synthesis, amino acid metabolism and methylation of genes, proteins and lipids via S-adenosylmethionine-mediated one-carbon transfer reactions. (More? folate)
  • teratogen - (Greek, teraton = monster) Any agent that causes a structural abnormality following exposure during pregnancy. The overall effect depends on dosage and time of exposure. Absolute risk - the rate of occurrence of an abnormal phenotype among individuals exposed to the agent. (e.g. fetal alcohol syndrome) Relative risk - the ratio of the rate of the condition among the exposed and the non-exposed. (e.g. smokers risk of having a low birth weight baby compared to non-smokers) A high relative risk may indicate a low absolute risk if the condition is rare.

Foundations Practical: Introduction | Week 1 and 2 | Week 3 and 4 | Week 1 to 8 | Week 9 to 36 | Neonatal | Critical Periods | Additional Resources | Quiz

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Cite this page: Hill, M.A. (2018, June 21) Embryology Foundations Practical - Critical Periods. Retrieved from

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© Dr Mark Hill 2018, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
  1. Wattendorf DJ & Muenke M. (2005). Fetal alcohol spectrum disorders. Am Fam Physician , 72, 279-82, 285. PMID: 16050451