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Pregnancy Begins at Implantation: Timeline & Biological Definition

Pregnancy is biologically defined as beginning at implantation — when the blastocyst attaches to and invades the endometrial lining.

Timeline (From Fertilization)

  1. Day 0: Ovulation + fertilization (ampulla of fallopian tube).
  2. Day 1–3: Cleavage divisions → morula (16–32 cells).
  3. Day 4–5: Blastocyst formation (inner cell mass + trophectoderm + blastocoel).
  4. Day 5–6: Enter uterine cavity; zona pellucida hatching.
  5. Day 6–7: Implantation begins — apposition, adhesion, invasion.
  6. Day 7–10: Implantation complete; hCG detectable in maternal blood.
  7. Day 9–11: hCG detectable in urine (home test).

    8. Key Distinctions

    • Fertilization ≠ Pregnancy: ~50% of fertilized eggs fail to implant.
    • LMP Dating: Clinical gestational age = LMP + 2 weeks (precedes conception).
    • Emergency Contraception: Prevents ovulation/fertilization/implantation — not abortifacient.

    Clinical Significance

    • Pre-implantation = pre-pregnancy (ethical/legal distinction in many jurisdictions).
    • Ectopic pregnancy = implantation outside uterus (tube, ovary, abdomen) — nonviable, dangerous.

    Implantation is the definitive biological threshold for pregnancy establishment.

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