Fetal Development and Genetics — Chapter 10

Stages of Fetal Development

  • Preembryonic stage (fertilization through the second week): fertilization; cleavage; morula; blastocyst and trophoblast; implantation.
  • Embryonic stage (end of the second week through the eighth week): basic structures of major body organs and main external features (Table 10.1).
  • Fetal stage (end of the eighth week until birth): ongoing growth and maturation (Table 10.1).

Embryonic Layers

  • Ectoderm: forms CNS, special senses, skin, and glands.
  • Mesoderm: forms skeletal, urinary, circulatory, and reproductive systems.
  • Endoderm: forms respiratory system, liver, pancreas, and digestive system.

Q: Do CNS structures arise from the mesoderm? A: False—the CNS arises from the ectoderm.

Placenta and Functions

  • Interface between the pregnant person and fetus; hormone production to regulate maternal physiology; immune protection for fetus; waste removal; promotes nutrient delivery; hormones support fetal organ maturation.
  • Hormones produced: hCGhCG, hPL (hCS)hPL\,(hCS), EstrogenEstrogen, ProgesteroneProgesterone, RelaxinRelaxin.

Umbilical Cord

  • Formed from the amnion; lifeline from mother to fetus.
  • Contents: one vein (oxygenated) and two arteries (deoxygenated).
  • Wharton jelly surrounds vessels to prevent compression.
  • At term: length ≈ 22 in22\ \text{in}; width ≈ 1 in1\ \text{in}.

Role of Amniotic Fluid

  • Maintains constant fetal temperature; supports symmetric growth; cushions fetus; prevents umbilical cord compression; promotes fetal movement for musculoskeletal development.
  • Conditions: oligohydramnios = low fluid; polyhydramnios = excessive fluid.

Fetal Circulation

  • Blood travels placenta → fetus → placenta.
  • Shunts (close after birth):
    • Ductus venosus: umbilical vein → inferior vena cava
    • Ductus arteriosus: main pulmonary artery → aorta
    • Foramen ovale: right atrium → left atrium

Genetics and Advances

  • Study of heredity and variation; pharmacogenomics; part of perinatal care; genetic testing; gene therapy.
  • Human Genome Project: International 13-year effort started in 1990 to map, sequence, and determine function of all human genes; genome defines genotype and phenotype.

Karyotype and Chromosomes

  • Karyotype: pictorial analysis of chromosome number, form, and size.
  • Commonly uses white blood cells and fetal cells in amniotic fluid.
  • Chromosomes numbered 1–22 by size; sex chromosomes X and Y.
  • Examples: Down syndrome = Trisomy 21; Turner syndrome = monosomy X; Klinefelter = XXY; Fragile X = Fragile X.

Patterns of Inheritance

  • Mendelian or monogenic disorders:
    • Autosomal dominant
    • Autosomal recessive
    • X-linked (recessive and dominant)
  • Multifactorial disorders.
  • Nontraditional inheritance; both parents may be carriers.

Autosomal Inheritance

  • Autosomal Dominant: one normal and one affected parent.
  • Autosomal Recessive: 25% risk of an affected child.

X-linked Inheritance

  • X-linked Recessive: typically more males affected because they have a single X chromosome; affected male pattern shown with female carriers.
  • X-linked Dominant: affected individuals include both sexes; inheritance pattern varies (fathers pass to daughters).

Chromosomal Abnormalities

  • Numerical: monosomies, trisomies, polyploidy.
  • Structural: deletions, inversions, translocations.
  • Sex chromosome abnormalities.

Types of Chromosomal Abnormalities

  • Structural: Cri du Chat syndrome; Fragile X syndrome.
  • Sex chromosome: Turner syndrome (monosomy X); Klinefelter syndrome (XXY).

Genetic Evaluation and Counseling

  • Genetic counseling: process of advising at-risk individuals about disorder, probability, and management/family planning options.
  • Ideal timing: before conception; various indications Box 10.2.

Nursing Roles and Responsibilities

  • Begin preconception counseling; collect family history; refer for genetic testing; explain screening/diagnostic test purposes, risks, and benefits.
  • Address costs, insurance, and potential discrimination; protect privacy; support emotional responses; provide referrals to support groups.

Ethical, Legal, and Social Considerations

  • Potential misuse of genetic information: risk profiling; privacy breaches; workplace discrimination; loss of autonomy; issues with pre-symptomatic risk assessment.