Pregnancy and Embryonic Development 🍼

1. Implantation 🌱

After a series of divisions, the blastocyst reaches the uterus. Its outer layer—the trophoblast—attaches to the thick uterine lining (endometrium). Rapid growth of nearby uterine cells then buries the blastocyst inside the lining. This embedding step is called implantation and marks the start of pregnancy. :contentReference[oaicite:0]{index=0}

2. Placenta & Umbilical Cord 👶🏻🩸

  • The trophoblast sprouts tiny chorionic villi that interlock with maternal tissues to create the placenta. :contentReference[oaicite:1]{index=1}
  • The placenta:
    • Delivers oxygen + nutrients to the embryo.
    • Removes CO2 and wastes.
    • Acts as a hormone-making gland.
  • An umbilical cord links placenta and embryo, serving as a two-way “supply cable.” :contentReference[oaicite:2]{index=2}

Key Placental Hormones 🧪

hCG, hPL, estrogens, progestogens, and later relaxin (from the ovary) surge during pregnancy. Levels of cortisol, prolactin, and thyroxine also rise to support fetal growth and maternal metabolism. :contentReference[oaicite:3]{index=3}

3. Germ Layers 🎨

Right after implantation, the inner cell mass organizes into three primary layers:

  1. Ectoderm (outer)
  2. Mesoderm (middle)
  3. Endoderm (inner)

These layers build every adult tissue. Some cells remain as powerful stem cells capable of forming any tissue type. :contentReference[oaicite:4]{index=4}

4. Month-by-Month Milestones 📆

Time PointWhat’s Happening?
End of 1st monthHeart forms ❤️; heartbeat detectable with a stethoscope.
End of 2nd monthLimbs and digits appear 🤲.
End of 12 weeks (≈1st trimester)Major organs complete; limbs + external genitalia well-shaped.
5th monthFirst fetal kicks 👣 and hair growth on head.
≈24 weeks (end 2nd trimester)Fine body hair covers skin, eyelids open, eyelashes form.
End of \(9\ \text{months}\)Fetus fully developed—ready for birth 🎉.

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Important Concepts for NEET 🎯

  • Placental hormones (hCG, hPL, relaxin) and their roles in maintaining pregnancy.
  • Three germ layers and their significance in forming all body organs.
  • Timeline of fetal development—especially the formation of the heart, limbs, and organ systems.
  • Implantation process and trophoblast functions.
  • Placenta’s dual role in nutrient exchange and endocrine support.