USMLE Step 2 CK Lecture Notes 2019: Obstetrics/Gynecology (Kaplan Test Prep)

OB TRIAD

Post-Conception Week    3

Another significant event is the invasion of the maternal sinusoids by the
syncytiotrophoblast. Because β-human chorionic gonadotropin (β-hCG) is
produced in the syncytiotrophoblast, this now allows β-hCG to enter the
maternal bloodstream. β-hCG pregnancy test now can be positive for the first
time.

Postconception week 3: most significant event is the migration of cells through
the primitive streak between the epiblast and hypoblast to form the trilaminar
germ disk with ectoderm, mesoderm, and endoderm layers. These layers will
give rise to the major organs and organ systems.

Postconception weeks 4–8 (period of major teratogenic risk): during this time,
the major organs and organ systems are being formed.

Starts  with    2-layer embryo

Ends    with    3-layer embryo

Yields  tri-laminar germ    disk

Ectoderm:   central and peripheral  nervous systems;    sensory organs  of  seeing

and hearing;    integument  layers  (skin,  hair,   and nails)

Mesoderm:   muscles,    cartilage,  cardiovascular  system, urogenital  system

Endoderm:   lining  of  the gastrointestinal    and respiratory tracts