USMLE Guide: Fetal Development and Anatomy

Introduction

This USMLE guide aims to provide a comprehensive overview of fetal development and anatomy. Understanding fetal development is crucial for medical professionals, especially those specializing in obstetrics and pediatrics. This guide will cover the major stages of fetal development and highlight key anatomical structures that can be tested on the USMLE.

Table of Contents

1. Embryonic Period
2. Fetal Period
3. Major Organ Systems
   • Central Nervous System
   • Cardiovascular System
   • Respiratory System
   • Digestive System
   • Urinary System
   • Musculoskeletal System
   • Genitourinary System
   • Reproductive System

Embryonic Period

The embryonic period spans from conception until the end of the 8th week of gestation. During this period, the three germ layers (ectoderm, mesoderm, and endoderm) differentiate and form the basis for all major organ systems.

1. Ectoderm gives rise to the nervous system, skin, hair, and nails.
2. Mesoderm forms the musculoskeletal system, cardiovascular system, kidneys, and reproductive system.
3. Endoderm develops into the gastrointestinal tract, respiratory system, liver, and pancreas.

Fetal Period

The fetal period begins at the start of the 9th week and continues until birth. During this phase, the major organs are already formed, and the fetus undergoes rapid growth and maturation.

1. Week 9 to 12: Fetal movements are observed, and the sex of the fetus can be determined through ultrasound.
2. Week 13 to 16: The fetus grows rapidly, and skeletal development progresses.
3. Week 17 to 20: Vernix caseosa (a protective substance) forms on the skin, and lanugo (fine hair) covers the body.
4. Week 21 to 24: The fetus develops a regular sleep-wake cycle, and the lungs continue to mature.
5. Week 25 to 28: Rapid brain development occurs, and the eyes can open.
6. Week 29 to 32: The fetus gains significant weight, and the bones are fully developed.
7. Week 33 to 36: Subcutaneous fat is deposited, and the fetus assumes a head-down position.

Major Organ Systems

Central Nervous System (CNS)

The CNS begins to form during the embryonic period and continues to develop throughout the fetal period.

1. Week 3 to 4: Neural tube formation.
2. Week 8: Brain vesicles (forebrain, midbrain, hindbrain) develop.
3. Week 12: Cerebral hemispheres become distinct.
4. Week 20 to 24: Sulci and gyri formation.
5. Week 25 to 28: Myelination of nerve fibers.

Cardiovascular System

The development of the cardiovascular system is vital for fetal survival.

1. Week 3 to 4: Formation of primitive heart tube.
2. Week 5 to 8: Development of a four-chambered heart.
3. Week 9 to 12: Valves and septa formation.
4. Week 20 to 24: Pulmonary circulation begins.
5. Week 25 to 28: Fetal circulation resembles neonatal pattern.

Respiratory System

Lung development occurs primarily during the late fetal period.

1. Week 4: Lung bud formation.
2. Week 20 to 24: Production of surfactant begins.
3. Week 28 to 32: Surfactant production increases.
4. Week 35 to 36: Terminal sacs form.

Digestive System

The digestive system develops in coordination with other organ systems.

1. Week 4: Foregut, midgut, and hindgut formation.
2. Week 8: Stomach and intestines differentiate