Lecture Notes: Human Development and Pregnancy

Introduction

• Initial size of human as a zygote: 2 millimeters
• Growth from zygote to average adult size (~1.7 meters)
• Key Question: How does a single cell grow into a fully-formed human?
• Emphasis on the role of hormones in pregnancy

Zygote to Embryo Stages

• Initial Stage: Sperm and egg form a zygote
• Cleavage Phase:
  • Begins 24 hours after fertilization
  • Zygote divides into 16 cells (blastomeres) without growth between divisions
  • Increasing surface area for nutrient absorption
• Morula Stage:
  • Formed about 3 days post-fertilization
  • Resembles a mulberry (hence the name)
• Blastocyst Stage:
  • Hollow sphere formed from the morula
  • Trophoblast cells form the placenta; inner cell mass develops into the embryo
• Implantation:
  • Occurs around 7 days post-ovulation
  • Endometrium receives blastocyst with help from hormones estrogen and progesterone

Hormonal Regulation

• Human Chorionic Gonadotropin (hCG):
  • Secreted by trophoblasts to maintain estrogen and progesterone levels
  • Prevents menstruation by maintaining the corpus luteum
• Placenta Formation:
  • Temporary organ for nutrient exchange and hormone control

Embryonic Development

• Embryonic Stage:
  • Blastocyst differentiates into various cell types
  • Embryo surrounded by amniotic sac and connected to placenta
• Transition to Fetus:
  • Occurs around the 8th week
  • Rapid development of organs and systems

Maternal Changes During Pregnancy

• Anatomical Changes:
  • Increased size of uterus and breasts
  • Displacement of organs
• Hormonal Changes:
  • Secretion of relaxin and human placental lactogen
  • Increased metabolism and glucose hoarding
• Cardiovascular Effects:
  • Increase in blood volume by 40%
  • Effects on gums, vision, and varicose veins

Labor and Delivery

• Labor Initiation:
  • Shift from progesterone to estrogen dominance
  • Fetal hormones trigger labor preparations
• Stages of Labor:
  • Dilation: Cervix dilates to 10 cm
  • Expulsion: Infant is delivered through the cervix and out
  • Placental Stage: Expulsion of placenta (afterbirth)

Conclusion

• Summary of pregnancy stages and maternal changes
• Thanks to contributors and supporters of Crash Course

Credits

• Filmed at Doctor Cheryl C. Kinney Crash Course Studio
• Written by Kathleen Yale, consulted by Dr. Brandon Jackson
• Directed and edited by Nicholas Jenkins and Nicole Sweeney respectively
• Sound design by Michael Aranda, graphics by Thought Cafe

• Support: Encouragement to support Crash Course via Patreon for continued educational content.