2. The Embryonic Period

• Occurs from 3rd - 8th week.

• Organogenesis occurs. It is a process in which every major organ takes shape, in at least a primitive form, develops. Such developments include:

  
  
  • The heart is beating four weeks after conception.
  • Primitive nervous system emerges during the 2nd month.
  • Sexual differentiation begins during the 7th and 8th weeks.
    

• Blastocyst differentiates into different layers:

  
  
  

  • Outer layer development includes the chorion membrane which would progress to form the placenta. A placental barrier is also formed to protect the developing child from harmful substances in the mother’s bloodstream.

    
    

  • Interior of the blastocyst give rise to other cell structure which would develop into specific tissues and organ

3. The Fetal Period

• Last from week 9 to birth
• In the 3rd month, distinguishable external sex organs appear, and bones and muscles develop; fetus begins to move.

• Between the 10th and 20th week, neurogenesis (the rapid proliferation of neurons) occur; neurons then engage in migration to form specialised functioning units e.g. into visual or auditory neurons.

  
  

• Age of viability is about 23 weeks after conception; at this age, the fetus has a chance of surviving outside the womb.

1. The Germinal Period

• Occurs in the first 2 weeks
• The zygote divides multiple times through mitosis, forming the blastocyst.
• At the end of week 1, the blastocyst implants into the uterus wall.

• Dosage and duration. The greater the level of exposure and the longer the exposure to a teratogen, the more likely it is that serious damage will occur

• Genetic makeup. Susceptibility to harm is influenced by the genetic makeup of the unborn child as well as the mother’s genotype. Some fetuses are more (or less) resistant to teratogens and some mothers are more (or less) able to detoxify teratogens.

• Critical period. The effects of a teratogenic agent are worst during the critical period when an organ system grows most rapidly

• Environment. The effects of a teratogen depend on the quality of both the prenatal and the postnatal environments.

Drugs: Thalidomide

• Thalidomide is a drug used widely in the late 1950s to relieve morning sickness
• Thalidomide had specific effects on development, depending on which structures were developing when the drug was taken.
• Taking thalidomide during the first two months of pregnancy resulted in stunted limbs or deformed sense organs in the unborn baby.
• If the mother waited until 35 or 36 days after conception before using thalidomide, her baby was usually not affected.

Tobacco

• Smoking restricts blood flow to the fetus and increases the risk of a miscarriage, premature birth, and low birth weight.
• Babies of smokers are also more susceptible to respiratory, cognitive and conduct problems.
• Heavier smoking also raises the risk of sudden infant death syndrome (SIDS), in which a sleeping baby suddenly stops breathing and dies.

Alcohol

• Alcohol readily crosses the placental barrier and disrupts neural development.
• The most severe effect of alcohol is a cluster of symptoms called Fetal alcohol syndrome (FAS), which causes damage to the central nervous system, resulting in:
  
  • Low IQ scores (many are mentally retarded)
  • Hyperactive behaviour and attention deficits.
  • Mental health problems later in life
  • Distinct facial abnormalities

Diseases and Infections

• Maternal rubella can lead to blindness, deafness, heart defects and mental retardation. It is most dangerous during the first trimester when the ears, eyes, heart and brain are rapidly taking form.
• Sexually transmitted infections such as HIV simplex and syphilis can cause damage to the eyes and brain.
• HIV-infected mothers can transmit the virus to their babies
  
  • (1) prenatally, if the virus passes through the placenta;
  • (2) perinatally, when blood may be exchanged between mother and child as the umbilical cord separates from the placenta; or
  • (3) postnatally, if the virus is transmitted during breast-feeding

Environmental Hazards

• Radiation from X-rays and cancer treatments can cause mutations, spontaneous abortions and various birth defects

  
  

• Pollutants

  
  
  • Prenatal exposure to heavy metals (e.g. lead) is linked to premature birth, low birth weight, and impaired intellectual functioning.
  • Prenatal exposure to other pollutants such as polychlorinated biphenyls (PCBs) are also associated with poor reflexes in infancy and later learning difficulties.

Emotional condition

• Chronic maternal stress can result in premature birth or stunted prenatal growth, leading to low birth weight.
• Acute maternal stress during the first trimester is linked to an increased likelihood of developing schizophrenia.
• Maternal depression during pregnancy may contribute to motor delays in new-borns.

Nutritional condition

• Maternal malnutrition affects prenatal development differentially, depending on when it occurs.
• Malnutrition in the 1st trimester can disrupt the formation of the spinal cord, resulting in fewer brain cells or causing stillbirth.
• Malnutrition in the 3rd trimester results in smaller neurons, a smaller brain, and a physically smaller child.

Age

• Typical and safest age range for pregnancy is from about age 20 to 40 years.
• Young mothers (19 years of age or younger) have higher rates of birth complications such as premature deliveries and stillbirths.
• Older women have increased risk of miscarriage, stillbirth and low birth weight babies.