Fetal Development

Relative fetal sizes

Introduction

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This page shows some key events of human development during the fetal period (weeks 9 to 37) following fertilization. The long Fetal period (4x the embryonic period) is a time of extensive growth in size and mass as well as ongoing differentiation of organ systems established in the embryonic period. Clinically this period is generally described as the Second Trimester and Third Trimester. Many of the critical measurements of growth are now carried out by ultrasound and this period ends at birth. Many different systems formed in the embryonic period (organogenesis) grow and differentiate further during the fetal period and do so at different times. For example, the brain continues to grow and develop extensively during this period (and postnatally), the respiratory system differentiates (and completes only just before birth), the urogenital system further differentiates between male/female, endocrine and gastrointestinal tract begins to function. Also consider the systems (respiratory, cardiac, neural) that will still not have their final organization and function determined until after birth.

Changing fetal proportions, not size growth.

Use the links below to get more detailed information about this period of development.

| original Fetal Development page

Some Recent Findings

Size and location of the kidneys during the fetal period "The level of the left kidney was higher than the level of the right kidney in the fetal period. The posterior surface relations to the ribs showed certain ascendance during gestation, corresponding to vertebral levels. However, fetal kidneys do not reach the same level as adults at full term. The kidneys move farther apart from the midline of the body during the fetal period. The dimensions, weight, and volume of the kidneys increased with gestational age during the fetal period. The ratio between kidney weights and fetal body weights were determined, and we observed that the ratio decreased during the fetal period. There were no sex or laterality differences in any parameter." Development and Function of the Human Fetal Adrenal Cortex: A Key Component in the Feto-Placental Unit "The steroidogenic activity is characterized by early transient cortisol biosynthesis, followed by its suppressed synthesis until late gestation, and extensive production of dehydroepiandrosterone and its sulfate, precursors of placental estrogen, during most of gestation. The gland rapidly grows through processes including cell proliferation and angiogenesis at the gland periphery, cellular migration, hypertrophy, and apoptosis."

Reading

Fetal length and weight change

• Human Embryology (3rd ed.) Larson Chapter 15: Fetal development and the Fetus as Patient p481-499
• The Developing Human: Clinically Oriented Embryology (8th ed.) Moore and Persaud Chapter 6: The Fetal Period: Ninth Week to Birth
• Color Atlas of Clinical Embryology (2nd ed.) Moore, Persaud and Shiota Chapter 3: 9th to 38th weeks of human development p50-68

Second Trimester

Fetus - second trimester
(ultrasound)

• Second Trimester
• Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm.

Begin by working through the features present in the early 10 week female fetus. Then look in detail at the head development in a 12 week fetus.

10 Week Fetus head images

Then look in detail at the head development in a 12 week fetus showing both forms of ossification in the skull.

12 Week Fetus head images

Third Trimester

Fetus - third trimester
(historic image)

• Third Trimester
• Vibration acoustically of maternal abdominal wall induces startle respone in fetus.
• Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs.
• Week 37 to 38 Birth.

References

Journals

• Seminars in Fetal & Neonatal Medicine

Reviews

• Fetal assessment during pregnancy. Farley D, Dudley DJ. Pediatr Clin North Am. 2009 Jun;56(3):489-504, Table of Contents. Review. PMID: 19501688
• Regimens of fetal surveillance for impaired fetal growth. Grivell RM, Wong L, Bhatia V. Cochrane Database Syst Rev. 2009 Jan 21;(1):CD007113. Review. PMID: 19160321

Search PubMed

Search Pubmed: human fetal development | fetal development | Second Trimester | Third Trimester

Glossary Links

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Cite this page: Hill, M.A. (2024, April 27) Embryology Fetal Development. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G