Fetal Development of Body Systems

This is a high-yield chart for nursing exams. Focus on when major systems develop and become functional.

Gestational Age

Development of Systems

3 weeks

Neural tube begins forming (brain & spinal cord)

4 weeks

Primitive heart tube forms and begins beating

5–6 weeks

Cardiac activity visible on ultrasound

8 weeks

Major organs formed (end of embryonic period)

10–12 weeks

Kidneys begin producing urine; external genitalia identifiable

12 weeks

Fetal heartbeat heard by Doppler

16–20 weeks

Mother feels quickening (fetal movement)

20 weeks

Vernix caseosa and lanugo present

24 weeks

Lungs begin producing surfactant; age of viability approaches

24–28 weeks

Eyes open; fetus may survive if born prematurely

28–32 weeks

CNS can regulate breathing and body temperature better

34–36 weeks

Lungs become more mature; increased fat stores

37–42 weeks

Term pregnancy

System-by-System Development

🧠 Nervous System

  • Neural tube closes by week 4

  • Brain and spinal cord continue developing throughout pregnancy

  • CNS maturity improves significantly after 28 weeks

NCLEX Pearl:
Folic acid prevents neural tube defects (spina bifida, anencephaly).

❤ Cardiovascular System

  • Heart begins beating around 3–4 weeks

  • Cardiac activity seen on ultrasound by 5–6 weeks

  • Doppler detects FHR around 10–12 weeks

Normal FHR: 110–160 bpm

🫁 Respiratory System

  • Lungs are among the last organs to mature

  • Surfactant production begins around 24 weeks

  • Adequate surfactant usually present by 34–36 weeks

NCLEX Pearl:
Lack of surfactant → Respiratory Distress Syndrome (RDS) in preterm infants.

🩸 Hematologic/Immune System

  • Fetal liver initially produces blood cells

  • Bone marrow gradually takes over

  • Maternal antibodies (IgG) cross placenta in later pregnancy

🚽 Renal System

  • Kidneys begin producing urine around 10–12 weeks

  • Fetal urine contributes to amniotic fluid volume

🍽 Gastrointestinal System

  • Swallowing begins during fetal life

  • Meconium forms in intestines

Meconium: first newborn stool (black, tarry)

👶 Reproductive System

  • External genitalia visible by ultrasound around 12 weeks

  • Sex often identified on anatomy scan at 18–20 weeks

High-Yield Milestones to Memorize

Milestone

Weeks

Heart starts beating

3–4

Cardiac activity on US

5–6

Major organs formed

8

Doppler FHR

10–12

Kidneys produce urine

10–12

Quickening

16–20

Anatomy scan

18–20

Viability

24

Surfactant begins

24

Better lung maturity

34–36

Term pregnancy

37–42

Memory Tricks

  • "4 = Heart at the door" → Heart starts around 4 weeks

  • "8 = Organs complete" → Organogenesis done by 8 weeks

  • "12 = Doppler and pee" → FHR by Doppler and kidneys produce urine

  • "20 = Mom feels movement" → Quickening

  • "24 = Viability" → May survive outside uterus

  • "34–36 = Lungs mature" → Surfactant adequate

These fetal development milestones are commonly tested in OB because they affect prenatal care, viability, and complication management.