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 | fungal infection | zoonotic infection | toxoplasmosis | Malaria | maternal diabetes | maternal hypertension | maternal hyperthermia | Maternal Inflammation | Maternal Obesity | hypoxia | biological toxins | chemicals | heavy metals | air pollution | 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
  Critical Period Links: critical period | abnormal development | Critical Periods table | Image - Critical Periods table | Genital | Opioids | Neural | Thalidomide | Environmental

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

Abnormal AusData81-92.png

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
patent ductus arteriosus‎/common truncus 13,876 301
transposition of the great vessels 3,333 1,252
Tetralogy of Fallot 2,518 1,657
atrial septal defects/ventricular septal defects 2,122 1,966
hypoplastic left heart 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)

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.

Australian Congenital Anomalies Monitoring System

Congenital Anomalies in Australia 2002-2003  
Congenital anomalies in Australia 2002-2003[2] was published in 2008 as part a new revised series on 33 selected congenital anomalies also monitored internationally by the International Clearinghouse of Birth Defects Surveillance and Research.
  • Hypospadia is the most commonly reported condition at birth, but severity of the condition is not reported to the national data collection.
  • Trisomy 21 (Down’s syndrome) is the next most commonly reported condition at birth (11.1 per 10,000 births), but many affected pregnancies are detected early and managed by early termination. An estimated 63.6% of the fetuses diagnosed with trisomy 21 were managed by terminations of pregnancy or were fetal deaths. When terminations of pregnancy were included, the estimated rate for trisomy 21 was 26.3 per 10,000 pregnancies. Trisomy 21 was more common with advancing maternal age.
    • Other chromosomal abnormalities such as trisomy 13 and trisomy 18 also had a large proportion of fetal deaths or terminations of pregnancy and were more common in women aged 40 years or older.
  • Neural tube defects were diagnosed in about 4.2 per 10,000 births.
  • More males than females diagnosed with congenital anomalies for many of the reported conditions (e.g. hydrocephalus, most of the reported congenital heart diseases, oesophageal atresia and polycystic kidneys).
  • Anencephaly - younger women had a higher rate compared with older women, most of the women who gave birth to a baby with anencephaly (77.2%) were in the 20–34 years age group.
Links: Human Abnormal Development | Hypospadia | Trisomy 21 | Trisomy 13 Trisomy 18 | Neural Tube Defects | Folic Acid and Neural Tube Defects | Hydrocephalus | Cardiovascular Abnormalities | Polycystic Kidney Disease | Anencephaly | Australian Statistics | Reports
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.
External Links: AIHW Report Page | NPESU Report Page | International Clearinghouse of Birth Defects Surveillance and Research
Links: Australian Congenital Anomalies Monitoring System (ACAMS)

International Classification of Diseases

The International Classification of Diseases version 11 ( ICD-11) is the standard diagnostic tool for epidemiology, health management and clinical purposes. This includes the analysis of the general health situation of population groups. It is used to monitor the incidence and prevalence of diseases and other health problems.

 ICD-11 20 Developmental anomalies - Structural developmental anomalies primarily affecting one body system
System  ICD-11 Embryology page link
nervous system neural abnormalities
eye, eyelid or lacrimal apparatus vision abnormalities
ear hearing abnormalities
face, mouth or teeth tooth abnormalities
neck head abnormalities
respiratory system respiratory abnormalities
circulatory system cardiovascular abnormalities
diaphragm, abdominal wall or umbilical cord diaphragm abnormalities | placenta abnormalities
digestive tract gastrointestinal abnormalities
liver, biliary tract, pancreas or spleen liver abnormalities | gall bladder abnormalities | pancreas abnormalities
urinary system renal abnormalities
female genital system genital abnormalities
male genital system genital abnormalities
breast mammary abnormalities
skeleton musculoskeletal abnormalities
skin integumentary abnormalities
adrenal glands adrenal abnormalities
 ICD-11 abnormal development | prenatal diagnosis | neonatal diagnosis


Note - linked terms in the list below are external resources to the current class content.

  • 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)
  • iodine - required for brain development.
  • 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.


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Cite this page: Hill, M.A. (2024, June 23) Embryology Foundations Practical - Critical Periods. Retrieved from

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© Dr Mark Hill 2024, 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
  2. Abeywardana S & Sullivan EA 2008. Congenital anomalies in Australia 2002–2003. Birth anomalies series no. 3 Cat. no. PER 41. Sydney: AIHW National Perinatal Statistics Unit.