Chapter 6

• Chromosomes


  46 - 22 pairs of autosomes and 1 pair of sex chromosomes
• Sperm and egg


  23 chromosomes from the sperm and 23 chromosomes from the egg
• Ovum


  Always contributes an X chromosome
• Sperm


  Carry X or Y chromosome
• Chromosomal abnormalities


  XX=female, XY=male
• Chromosomal abnormalities


  • Monosomy - 45 chromosomes in each cell
  • Trisomy - 47 chromosomes in each cell
• Unifactorial inheritance


  Single gene controls trait
• Carrier screening


  Noninvasive, identifies single-gene disorders and detects autosomal recessive gene mutations
• Multifactorial inheritance


  Combination of genetic and environmental factors
• Multifactorial conditions


  • Cleft lip, cleft palate, congenital heart disease, neural tube defects
• Neural tube defects are significantly reduced with folic acid
• Genetic ID and antepartum testing

  • Carrier screening
  • Multiple marker screens
  • Fetal nasal bone on ultrasound
  • Nuchal translucency
  • Maternal blood for biomarkers
  • Chorionic villus sampling
  • Percutaneous umbilical blood sampling
• Alpha-fetoprotein (AFP)


  Low with Down syndrome, high with neural tube defects
• Common trisomies


  • Trisomy 21: Down syndrome
  • Trisomy 18: Edwards syndrome
  • Trisomy 13: Patau syndrome
• Amniocentesis


  Tests amniotic fluid, early pregnancy for congenital anomalies, late pregnancy for lung maturity
• Adequate nutrition, maternal conditions, and teratogens can affect fetal development</b>
• Neural tube defects happen during the first 24 days of pregnancy
• Taking folic acid before and in the first trimester significantly reduces the risk of neural tube defects
• Functions of amniotic fluid

  • Diffused from maternal blood
  • Secreted by fetal respiratory and GI tract
  • Flows in and out of lungs
  • Maintains body temperature
  • Maintains fluid/electrolyte balance
  • Cushions from trauma
  • Allows movement for musculoskeletal development
• Chorion


  Membrane that forms embryonic and fetal portion of placenta
• Amnion


  Second membrane that envelops and protects embryo, forms boundaries of amniotic cavity
• Polyhydramnios


  Greater than 2000 mL, can indicate diabetes and fetal anomalies
• Oligohydramnios


  Less than 400 mL, with postdates, anomalies affecting kidneys and bladder, preeclampsia
• Fetal development


  • Embryonic stage (day 15 to 8 weeks) is most critical
  • All organ systems and external structures present at end of 8th week
  • Fetal period (9 weeks and beyond)
• Fetal circulation


  • Heart begins beating by end of third week
  • Fetal heart can be heard by doppler by 12 weeks
  • Ductus arteriosus diverts blood from fetal lungs
  • Foramen ovale shunts blood from right to left atria
  • Ductus venosus connects umbilical vein to inferior vena cava
  • Umbilical cord - 2 arteries, 1 vein
• Wharton's jelly protects umbilical cord vessels
• Placenta functions


  • Metabolic exchange - respiration, nutrition, excretion, storage
  • Circulatory - dependent on maternal blood pressure
• Other highlights of fetal development

  • ~20 weeks - Mother feels movement, assumes favorite position, head hair/eyebrows/lashes present
  • 24 weeks - Weighs ~1# 10oz, respiratory movements begin
  • 28 weeks - Surfactant needed for breathing at birth is formed
  • 32 weeks - Fingernails/toenails formed, subcutaneous fat appears
  • 38-40 weeks - Baby fills uterus & gets antibodies from mother