Fetal Development Overview

Fundamental Information

• Average human pregnancy lasts 40 weeks, beginning from the first day of the last menstrual period (LMP).
• Trimesters:
  • 1st trimester: 0-13 weeks - foundation of major body systems.
  • 2nd trimester: 14-26 weeks - growth and maturation of body systems.
  • 3rd trimester: 27-40 weeks - primarily focused on weight gain.
• Definitions:
  • Term: 37-42 weeks gestation.
  • Pre-term: less than 37 weeks gestation.
  • Post-term: greater than 42 weeks gestation.
  • Estimated Gestational Age (EGA) refers to the estimated duration of the pregnancy.

Stages of Fetal Development

Stage 1: Preembryonic Stage (0-2 Weeks)

• Begins with fertilization, forming a zygote with 46 chromosomes.
• Zygote travels to the uterine cavity, entering about 72 hours post-fertilization.
• Implantation occurs 7-10 days after fertilization, typically at the uterine fundus.
• Cellular differentiation results in three germ layers:
  • Ectoderm: Central nervous system, skin, glands.
  • Mesoderm: Musculoskeletal, urinary, circulatory, and reproductive systems.
  • Endoderm: Respiratory, liver, pancreas, and digestive systems.

Stage 2: Embryonic Stage (2-8 Weeks)

• Development of the umbilical cord and placenta begins.
• Umbilical cord contains one vein (carrying oxygenated blood to fetus) and two arteries (carrying deoxygenated blood to mother).
• Amniotic fluid serves multiple purposes:
  • Provides cushioning, maintains stable temperature, and promotes symmetric development.
• Oligohydramnios (low amniotic fluid) linked to poor placental flow and fetal malformations.
• Hydramnios (excess amniotic fluid) may occur under certain conditions.
• Placenta functions:
  • Acts as a barrier and filter, facilitating nutrient and gas exchange.
  • Transfers antibodies from mother to fetus, providing immune protection.
• Teratogens negatively impact fetal development (e.g. syphilis, cytomegalovirus, drugs).
• Endocrine functions of the placenta include hormone production (e.g. hCG, estrogen, progesterone).

Stage 3: Fetal Development (9-40 Weeks)

• Focus on further growth and body system development; prenatal supplements like folic acid are essential.
• Key developmental milestones:
  • Week 3: Formation of all body systems.
  • Week 4: Detectable heart rhythm; neural tube closes.
  • Week 6: Brain waves detectable; lungs begin to develop.
  • Week 8: Fully formed heart; fetal structures start resembling humans.
  • Week 12: Genital differentiation can be recognized.
  • Week 20: Rapid brain growth; fetal heart tones detectable.
  • Week 24: Surfactant production starts in the lungs.

Fetal Circulation

• The placenta acts as the "lungs" for the fetus, where blood remains separate.
• Structure of the umbilical cord: one vein for oxygenated blood and two arteries for deoxygenated blood.
• Three important shunts facilitate oxygen-rich blood to the fetal heart and brain:
  • Ductus venosus: Shunts blood from the umbilical vein to the inferior vena cava.
  • Foramen ovale: Allows blood to flow between the right and left atrium, bypassing the lungs.
  • Ductus arteriosus: Connects pulmonary artery to descending aorta, redirecting blood away from the lungs.

Transition to Neonatal Circulation

• Upon cutting the umbilical cord, several changes occur:
  • Ductus venosus closes.
  • Fluid in the lungs releases, decreasing pressure and facilitating closure of foramen ovale.
  • Removal of the placenta leads to closure of the ductus arteriosus due to changes in prostaglandin levels.