Conception and nidation

1. Big picture

Conception and nidation are the first biological steps of pregnancy:

Ovulation → fertilization → embryo transport → blastocyst formation → implantation/nidation → trophoblast invasion → hCG production → corpus luteum rescue

For the OBGYN final exam, this topic is important because it explains:

• When pregnancy can occur
• Where fertilization normally happens
• How implantation creates the early placenta
• Why β-hCG becomes positive
• Why ectopic pregnancy is dangerous
• Why early pregnancy loss can occur before a woman even knows she is pregnant
• How to interpret early pregnancy bleeding, pain, β-hCG, and ultrasound

Core clinical idea:

Fertilization happens in the ampulla of the fallopian tube.
Implantation happens in the secretory endometrium of the uterine cavity.
Anything outside the uterine cavity = ectopic pregnancy until proven otherwise.

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