Fetal Development

Embryology - 19 Apr 2024        Expand to Translate
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Introduction

Relative fetal sizes

<mediaplayer width='285' height='320' image="http://php.med.unsw.edu.au/embryology/images/f/ff/Fetal_growth_icon.jpg">File:fetal growth.mp4</mediaplayer> Fetal development movie page

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.

Fetal Graphs: Crown-Rump Length (CRL) | Third trimester CRL | Head Circumference | Head Circumference 2nd Trimester | Liver Weight | Pancreas Weight | Thymus Weight | Small Intestine Length | Large Intestine Length | Length and Weight Changes | Fetal Development

Some Recent Findings

Fetus (week 10)

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." (See also Renal System Development) 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." (See also Endocrine - Adrenal Development)

More recent papers

This table allows an automated computer search of the external PubMed database using the listed "Search term" text link. This search now requires a manual link as the original PubMed extension has been disabled. The displayed list of references do not reflect any editorial selection of material based on content or relevance. References also appear on this list based upon the date of the actual page viewing. References listed on the rest of the content page and the associated discussion page (listed under the publication year sub-headings) do include some editorial selection based upon both relevance and availability. More? References | Discussion Page | Journal Searches | 2019 References | 2020 References Search term: Fetal Development <pubmed limit=5>Fetal+Development</pubmed>

Reading

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

‎‎Fetal Development Page | Play

Fetal length and weight change

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

Fetal Head Growth

• 

  Second and third trimesters

• 

  Second trimester

Third Trimester

Fetus - third trimester
(historic image)

Third trimester Crown-Rump Length

• 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.

Links: Third Trimester

Fetal Endocrine

Fetal thymus growth

Pituitary Hormones

• HPA axis established by week 20
• Pituitary functional throughout fetal development

Thyroid Hormone

• required for metabolic activity, also in the newborn
• important for neural development

Parathyroid Hormone

• newborn has total calcium levels (approx 20 grams) accumulated mainly in the 3rd trimester (weeks 28–40)
• fetal parathyroid hormone (PTH) potentially available from 10–12 weeks and PTH does not cross the placenta
• fetus relatively hypercalcemic, active transplacental transport of Ca²⁺ to fetus
• maternal serum - calcium ions (Ca²⁺), inorganic phosphate (Pi) and PTH concentrations are within the non-pregnant normal range throughout pregnancy.
• maternal bone turnover increases in the 3rd trimester.

(Based on Endocrinology - Materno—fetal calcium balance)

Pancreatic Hormones

• maternal diabetes can affect fetal pancreas development (increase in fetal islet beta cells).

Gonadal Hormones

• testosterone - required during fetal development for external genital development and internal genital tract in male.
• estrogens - secreted inactive precursor converted to active form by placenta.

Links: Endocrine System Development | Endocrinology - Control of steroid production in the fetal gonads | Neuroscience - The Effect of Sex Hormones on Neural Circuitry

Gastrointestinal Tract

Fetal small Intestine length growth	Fetal large Intestine length growth

Fetal developmental features include: the growth and rotation of intestines initially herniated outside the ventral body wall; changes in mesenteries; development of the blood supply and tract wall.

The initial functions of the tract with amionic fluid swallowing and the accumulation of both secretions and swallowed components within the large intestine as meconium.

Links: Intestine Development | Gastrointestinal Tract Development

Fetal Surgical Procedures

There are a range of fatal abnormalities that are potentially amenable to surgical intervention, see recent review link Maternal-Fetal Surgical Procedures.

Some examples include:

• Cardiac Malformations
• Congenital Diaphragmatic Hernia
• Myelomeningocele/Spina Bifida
• Obstructive Uropathy
• Sacrococcygeal Teratoma
• Thoracic Lesions
• Twin-Twin Transfusion Syndrome

Links: Maternal-Fetal Surgical Procedures. Walsh WF, Chescheir NC, Gillam-Krakauer M, et al. Rockville (MD): Agency for Healthcare Research and Quality (US); 2011 Apr. (Comparative Effectiveness Technical Briefs, No. 5.) Report | Comparative Effectiveness Research, Health Care

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 19) 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