Early fetal development

Understanding early fetal development is essential for medical students preparing for the USMLE Step 1. This guide covers each milestone in early embryonic development, from fertilization through the initial stages of organ formation.

Key Stages of Early Embryonic Development

Stage	Day/Week	Description
Ovulation	Day 0	Release of the secondary oocyte (1N, 2C) from the ovary.
Fertilization	Day 0	Fusion of sperm and oocyte, forming a zygote (2N, 2C).
Zygote	Day 1	Zygote undergoes mitotic divisions to increase cell count.
Morula	Days 2-3	16-cell stage formed through continued cleavage.
Blastocyst Formation	Day 5	Fluid-filled blastocyst forms; differentiation into embryoblast and trophoblast begins.
Implantation	Days 6-10	Blastocyst implants in the endometrium, initiating hCG secretion.

Week-by-Week Breakdown

Week	Major Developmental Event
Week 1	hCG secretion begins around implantation. “Blastocyst sticks at day 6.”
Week 2	Formation of the bilaminar disc: Epiblast and Hypoblast. “2 weeks = 2 layers.”
Week 3	Gastrulation creates the trilaminar disc: Endoderm, Mesoderm, and Ectoderm. “3 weeks = 3 layers.”
Weeks 3-8	Organogenesis occurs; neural tube formation by neuroectoderm (closes by week 4). Susceptible to teratogens.
Week 4	Heart begins to beat; upper and lower limb buds appear. “4 weeks = 4 limbs and 4 heart chambers.”
Week 6	Fetal cardiac activity is detectable by transvaginal ultrasound.
Week 8	Fetal movements begin. “Gait at week 8.”
Week 10	Genital differentiation; identifiable male/female characteristics (“Tenitalia”).

Important Points to Remember

hCG Secretion: Begins around the time of blastocyst implantation in Week 1.
Bilaminar Disc: Comprises the epiblast and hypoblast in Week 2.
Gastrulation: Forms three primary germ layers in Week 3.
Neural Tube Formation: Critical in Weeks 3-4; neural tube closes by the end of Week 4.
Teratogen Susceptibility: Highest during Weeks 3-8, as organogenesis is active.
Heartbeat and Limb Buds: Both begin by Week 4, marking key development milestones.
Fetal Movements: First observed at Week 8, with significant motor development ongoing.

Summary Table of Key Milestones

Week	Blastocyst sticks on day 6	Mnemonic or Key Note
1	hCG secretion starts	Blastocyst sticks at day 6
2	Bilaminar disc (epiblast, hypoblast)	“2 weeks = 2 layers”
3	Gastrulation, Trilaminar disc (endo-, meso-, ecto-)	“3 weeks = 3 layers”
4	Heartbeat and limb buds	“4 limbs and 4 chambers”
6	Fetal cardiac activity detectable	–
8	Initial fetal movements	“Gait at week 8”
10	Genital differentiation	“Tenitalia”

Points to Review

Embryonic Germ Layers: Differentiate into specific organ systems, with endoderm forming internal linings, mesoderm forming connective tissues and muscles, and ectoderm forming skin and neural tissues.
Neural Tube Defects: Prevention with folic acid supplementation is critical during Weeks 3-4 when the neural tube forms.
Organogenesis: Major organs begin development between Weeks 3-8, making this period extremely sensitive to teratogens.

USMLE Step 1 Review Series

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