3.1 Stages of Prenatal Development

Prenatal development unfolds in three stages: germinal, embryonic, and fetal. Each stage marks crucial milestones in a baby's growth, from conception to birth. Understanding these stages helps us grasp the incredible journey of human development.

The first trimester encompasses the germinal and embryonic stages, where major organs form. As pregnancy progresses through the second and third trimesters, the fetus grows rapidly, preparing for life outside the womb.

Stages of Prenatal Development

Germinal, Embryonic, and Fetal Stages

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• Germinal stage begins at conception and lasts about 2 weeks
  • Involves rapid cell division and the beginning of cell differentiation
  • Zygote travels down the fallopian tube and implants in the uterine wall (implantation)
• Embryonic stage lasts from week 3 to week 8
  • Involves the development of major organs and systems (organogenesis)
  • Embryo is particularly vulnerable to teratogens during this stage
• Fetal stage lasts from week 9 until birth
  • Involves rapid growth and development of the fetus
  • Organs and systems continue to mature and become functional

Trimesters of Pregnancy

• First trimester (weeks 1-12)
  • Encompasses the germinal and embryonic stages
  • Morning sickness and fatigue are common symptoms
• Second trimester (weeks 13-27)
  • Fetal movement becomes noticeable (quickening)
  • Ultrasound can reveal the sex of the fetus
• Third trimester (weeks 28-40)
  • Fetus gains weight rapidly and prepares for birth
  • Braxton Hicks contractions may occur as the uterus prepares for labor

Early Prenatal Structures

Zygote and Blastocyst Formation

• Zygote is formed when a sperm fertilizes an egg
  • Contains genetic material from both parents
  • Undergoes rapid cell division (cleavage) as it travels down the fallopian tube
• Blastocyst is a hollow ball of cells that forms about 5 days after fertilization
  • Inner cell mass will become the embryo
  • Outer layer of cells (trophoblast) will become part of the placenta

Implantation and Early Development

• Implantation occurs when the blastocyst attaches to the uterine wall
  • Typically happens about 6-10 days after fertilization
  • Trophoblast cells begin to secrete hormones (hCG) that maintain the uterine lining
• After implantation, the inner cell mass develops into the embryo and supporting structures
  • Amniotic sac forms around the embryo, providing a protective fluid environment
  • Yolk sac provides early nourishment to the embryo

Prenatal Growth and Development

Organogenesis and Placental Development

• Organogenesis is the development of organs and systems during the embryonic stage
  • Ectoderm gives rise to the nervous system and skin
  • Mesoderm forms the circulatory system, bones, and muscles
  • Endoderm develops into the digestive system and lungs
• Placenta is a specialized organ that develops during pregnancy
  • Allows for the exchange of nutrients, oxygen, and waste between the mother and fetus
  • Produces hormones that support pregnancy (progesterone, estrogen)

Umbilical Cord and Amniotic Sac

• Umbilical cord connects the fetus to the placenta
  • Contains two arteries and one vein
  • Provides a conduit for nutrient and waste exchange
• Amniotic sac is a fluid-filled membranous sac that surrounds the fetus
  • Amniotic fluid cushions the fetus and allows for movement
  • Helps maintain a constant temperature for the fetus
  • Protects the fetus from external pressures and impacts