Stages of Development

Germinal Stage

• The zygote is a fertilized egg

• It undergoes rapid cell division

• The cells have XX or XY 23rd chromosomes

  • This determines the sex, which is in turn determined by the father’s genotype

  • Sometimes the 23rd chromosomes are not either of that pair, resulting in an intersex infant

• At this stage, there is the least susceptibility to environmental influences

• This stage lasts from conception to two weeks

Abnormalities

• Phenylketonuria (PKU) is an inability to produce enzymes to digest proteins

  • This is an inherited disorder

  • Causes neurological impairment

• Down syndrome occurs due to a trisomy of chromosome 21

  • Not usually inherited, likely caused by gene copying error

  • Physical growth delays and intellectual disabilities

• Note that genotype relates to genetic characteristics and phenotype relates to physical characteristics

The Placenta

• The sac that encases a child from cellular stage to fetus

• It is also the method of delivery for nutrients from the mother to the fetus

• Whatever the mother breathes, eats, or drinks can pass through the placenta and alter the course of development

• Teratogens are any environmental agent that can reach the embryo or fetus during prenatal development and cause harm

  • Alcohol causes fetal alcohol syndrome, susceptibility to alcohol later in life, neurological and physical impairments

  • Tobacco can cause a cleft palate, and low birth weight

Embryonic Stage

• The embryo is provided nourishment and oxygen by the placenta

• Organs begin to form

• Heart begins to beat

• Central nervous system begins to develop and the neural tube closes

• This lasts two to eight weeks

Fetal Stage

• Sex organs differentiate

• Toes and fingers grow

• Hearing develops

• Lungs develop

• Brain undergoes rapid growth

• Bones and muscles develop