Fetal Development

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Relative fetal sizes
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.

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

Fetal Links: Introduction | Week 10 | Week 12 | Second Trimester | Third Trimester | Fetal Neural | Fetal Blood Sampling | Birth-Weight | Fetal Origins Hypothesis | Fetal Growth Restriction | BGD Practical | Medicine Lecture | Science Lecture | Preterm Birth | Birth | Category:Human Fetus
Historic Embryology  

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[1] "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[2] "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
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This table shows an automated computer PubMed search using the listed sub-heading term.

  • Therefore the list of references do not reflect any editorial selection of material based on content or relevance.
  • References appear in 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.

Links: References | Discussion Page | Pubmed Most Recent | Journal Searches

Search term: Fetal Development

Amber L Guidry, Zachary E Tibbs, Melissa Runge-Morris, Charles N Falany Expression, purification and characterization of human cytosolic sulfotransferase (SULT) 1C4. Horm Mol Biol Clin Investig: 2017; PubMed 28222028

Xavier Bonnet, Guy Naulleau, Richard Shine The Evolutionary Economics of Embryonic-Sac Fluids in Squamate Reptiles. Am. Nat.: 2017, 189(3);333-344 PubMed 28221829

Hiroyuki Koike, Ran-Ran Zhang, Yasuharu Ueno, Keisuke Sekine, Yun-Wen Zheng, Takanori Takebe, Hideki Taniguchi Nutritional modulation of mouse and human liver bud growth through a branched-amino acid metabolism. Development: 2017; PubMed 28219950

Gerelchimeg Bou, Shichao Liu, Mingju Sun, Jiang Zhu, Binghua Xue, Jia Guo, Yueming Zhao, Bo Qu, Xiaogang Weng, Yanchang Wei, Lei Lei, Zhonghua Liu CDX2 is essential for cell proliferation and polarity in porcine blastocysts. Development: 2017; PubMed 28219949

Makoto Suzuki, Masanao Sato, Hiroshi Koyama, Yusuke Hara, Kentaro Hayashi, Naoko Yasue, Hiromi Imamura, Toshihiko Fujimori, Takeharu Nagai, Robert E Campbell, Naoto Ueno Distinct intracellular Ca(2+) dynamics regulate apical constriction and differentially contribute to neural tube closure. Development: 2017; PubMed 28219946


  • 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 growth icon.jpg
 ‎‎Fetal Development
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Fetal length and weight change

Fetal length change

Human Fetal Length - Weight Fetal Development
Gestational age Fertilization age Length Mass
(LMP) (weeks) (weeks) (cm) (g)
8 (embryonic) 6 1.6 (crown to rump) 1
9 7 2.3 2
10 8 3.1 4
11 (fetal) 9 4.1 7
12 10 5.4 14
13 (second trimester) 11 7.4 23
14 12 8.7 43
15 13 10.1 70
16 14 11.6 100
17 15 13 140
18 16 14.2 190
19 17 15.3 240
20 18 16.4
25.6 (crown to heel)
21 19 26.7 360
22 20 27.8 430
23 21 28.9 501
24 22 30 600
25 23 34.6 660
26 24 35.6 760
27 25 36.6 875
28 (third trimester) 26 37.6 1005
29 27 38.6 1153
30 28 39.9 1319
31 29 41.1 1502
32 30 42.4 1702
33 31 43.7 1918
34 32 45 2146
35 33 46.2 2383
36 34 47.4 2622
37 35 48.6 2859
38 36 49.8 3083
39 37 50.7 3288
40 38 51.2 3462
41 39 51.7 3597
42 40 51.5 3685

L Cussen, J Scurry, G Mitropoulos, C McTigue, J Gross Mean organ weights of an Australian population of fetuses and infants. J Paediatr Child Health: 1990, 26(2);101-3 PubMed 2361065

Katrine Hansen, C James Sung, Carol Huang, Halit Pinar, Don B Singer, Calvin E Oyer Reference values for second trimester fetal and neonatal organ weights and measurements. Pediatr. Dev. Pathol.: 2003, 6(2);160-7 PubMed 12548377

John G Archie, Julianne S Collins, Robert Roger Lebel Quantitative standards for fetal and neonatal autopsy. Am. J. Clin. Pathol.: 2006, 126(2);256-65 PubMed 16891202

Second Trimester

Fetus - second trimester
  • 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
Human- fetal week 10 head A.jpg Human- fetal week 10 head B.jpg Human- fetal week 10 head C.jpg Human- fetal week 10 head D.jpg

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 lateral.jpg Fetal head medial.jpg Fetal head section.jpg


Second Trimester Timeline 
Timeline human development | First Trimester Timeline | Second Trimester Timeline | Third Trimester Timeline
Clinical second trimester (GA Clinical Week 14) Second Trimester Fetal head lateral.jpg Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm

Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph

Genital male and female external genital differences observable

Respiratory Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Tongue Week 12 - first differentiated epithelial cells (Type II and III)

Genital female genital canal (80 days) formed with absorption of the median septum

  Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers

Hearing - Outer Ear Development Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.  

Tongue Week 14 to 15 - taste pores develop, mucous

Ovary Development 100 days - primary follicles present

Nail Development toenails appear

Head Development facial skeleton remodelling begins

Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane.[3]

  Pancreas glucagon detectable in fetal plasma.

Spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. [4]

14 cm Fetal size change.jpg Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.

Pituitary adenohypophysis fully differentiated

Respiratory Week 16 to 25 lung histology - canalicular

Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved.

Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament.[3] gap junction cartoon

Neural - Cerebrum development of the periinsular sulci (week 16-17, GA 18-19 weeks)[5]

Skin 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands.

primary follicles begin to form in the ovary and are characterized by an oocyte

glandular urethra forms and skin folds present

Brain week 17 histology.jpg Neural - Brain development histology week 17

Tooth Development Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm).

Bailey095.jpgTongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia

Skin vernix caseosa covers skin

Spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles. [4]

Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form.

Neural - Cerebrum central sulci and opercularization of the insula (week 18-20, GA 20-22 weeks)[5]

  Neural week 19 neuronal migration ends and the radial glial cells that aided the migration now become transformed into astrocytes and astrocytic precursors.[6]
  Pituitary week 20 to 24 growth hormone levels peak, then decline

Skin lanugo, skin hair

Skin 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands.

  Gray0038.jpg Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present[7]

Spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. [4]

Neural - Cerebrum covering of the posterior insula (week 22-24, GA 24-26 weeks)[5]

  Respiratory Week 24 to 40 lung histology - terminal sac

Spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp.[4] Tooth Development Week 24 - Permanent incisors and canines appear. Earliest potential survival expected if born

ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells

  Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Neural - Cerebrum closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, GA 27-28 weeks)[5]

Third Trimester

Fetus - third trimester
(historic image)

Third trimester Crown-Rump Length graph.jpg

Third trimester Crown-Rump Length

Fetal weight change.jpg

  • 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.


Third Trimester Timeline 
Timeline human development | First Trimester Timeline | Second Trimester Timeline | Third Trimester Timeline
Clinical third trimester (GA Clinical Week 28) Third Trimester Fetal size change.jpg Hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
  Respire Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs

Tooth Development Week 29 - Permanent premolars (correspond to the milk molars) appear.


Genital male gonad (testes) descending

  Nail Development fingernails reach digit tip
  Neural brain cortical sulcation - primary sulci present[7]
  Neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present[7]
  Frazer006 bw600.jpg Nail Development toenails reach digit tip

Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation[8]

Birth Newborn.jpg Clinical Week 40

Heart pressure difference closes foramen ovale leaving a fossa ovalis

Thyroid TSH levels increase, thyroxine (T3) and T4 levels increase to 24 h, then 5-7 days postnatal decline to normal levels

Adrenal - zona glomerulosa, zona fasiculata present

Links: Third Trimester


Fetal Head Growth

Second trimester Second and third trimesters
Fetal head growth circumference graph02.jpg Fetal head growth circumference graph01.jpg

Fetal Neural

Brain size embryonic (week 4, 5, 6, and 8) and late fetal (third trimester)

Relative brain size embryonic (week 4, 5, 6, and 8) and late fetal (third trimester)

  • During the fetal period there is ongoing growth in size, weight and surface area of the brain and spinal cord. Microscopically there is ongoing: cell migration, extension of processes, cell death and glial cell development.
  • Brain - folding of the initially smooth surface (Insular cortex, Gyral and Sulcal development)
  • Neural development will continue after birth with substantial growth, death and reorganization occuring during the postnatal period
Fetal Fissure Development Timeline Neural Development
Brain fissure development 03.jpg Neural-development.jpg
Links: Scaled Fissures 13-21 weeks | Fissures 13-21 weeks | Brain Sylvian Fissure | Scaled Brain and Ventricles 13-21 weeks | Scaled Brain, Ventricles and Ganglia 13-21 weeks | Limbic Tract 13-19 weeks | Brain and Ventricles 13-21 weeks | Sylvian Fissure Movie | Neural System Development | Magnetic Resonance Imaging Timeline human development

Links: Neural System Development

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

Fetal Respiratory

  • week 4 - 5 embryonic
  • week 5 - 17 pseudoglandular
  • week 16 - 25 canalicular
  • week 24 - 40 terminal sac
  • late fetal - 8 years alveolar
Links: Respiratory System Development

Fetal Genital

  • ovary and testis development
  • external genital development
  • testis descent
Tests descent beginning Tests descent end
Testis-descent start.jpg Testis-descent end.jpg
Links: Genital System Development

Fetal Renal

Human fetal kidney
Human fetal kidney (GA week 12)
  • week 32-34 nephron development completed
  • term birth nephron number per kidney about 1 million (300,000 to 2 million)

Nephron development has four identifiable developmental stages:

  1. Vesicle (V) stage (13-19 weeks, second trimester)
  2. S-shaped body (S) stage ( 20-24 weeks, second trimester)
  3. Capillary loop (C) stage (25-29 weeks, third trimester)
  4. Maturation (M) stage (infants aged 1-6 months, neonatal and postnatal)

Links: Fetal Renal | Renal System Development

Fetal Gastrointestinal

Fetal small Intestine length growth graph.jpg Fetal large Intestine length growth graph.jpg
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 Integumentary

4 months
  • basal cell - proliferation generates folds in basement membrane.
  • neural crest cells - (melanocytes) migrate into epithelium. These are the pigment cell of the skin.
  • embryonic connective tissue - differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer.
  • Ectoderm contributes to nails, hair follicles and glands.
    • Cords of these cells extend into mesoderm forming epithelial columns. These form hair follicles, sebaceous and sweat glands.
  • Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field.

5 months

  • Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands.
  • Other cords elongate and coil to form sweat glands.
  • Cords in mammary region branch as they elongate to form mammary glands. These glands will complete development in females at puberty. Functional maturity only occurs in late pregnancy.
Fetal integumentary histology 01.jpg
Links: Integumentary System 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.[9]

Some examples include:


  1. Osman Sulak, Gülnur Ozgüner, Mehmet Ali Malas Size and location of the kidneys during the fetal period. Surg Radiol Anat: 2011, 33(5);381-8 PubMed 21110022
  2. Hitoshi Ishimoto, Robert B Jaffe Development and function of the human fetal adrenal cortex: a key component in the feto-placental unit. Endocr. Rev.: 2011, 32(3);317-55 PubMed 21051591
  3. 3.0 3.1 Heiko Locher, John C M J de Groot, Liesbeth van Iperen, Margriet A Huisman, Johan H M Frijns, Susana M Chuva de Sousa Lopes Development of the stria vascularis and potassium regulation in the human fetal cochlea: Insights into hereditary sensorineural hearing loss. Dev Neurobiol: 2015; PubMed 25663387
  4. 4.0 4.1 4.2 4.3 Takashi Satoh, Eiichi Sakurai, Hiroshi Tada, Tomoyuki Masuda Ontogeny of reticular framework of white pulp and marginal zone in human spleen: immunohistochemical studies of fetal spleens from the 17th to 40th week of gestation. Cell Tissue Res.: 2009, 336(2);287-97 PubMed 19255788
  5. 5.0 5.1 5.2 5.3 A Afif, R Bouvier, A Buenerd, J Trouillas, P Mertens Development of the human fetal insular cortex: study of the gyration from 13 to 28 gestational weeks. Brain Struct Funct: 2007, 212(3-4);335-46 PubMed 17962979
  6. H J Kadhim, J F Gadisseux, P Evrard Topographical and cytological evolution of the glial phase during prenatal development of the human brain: histochemical and electron microscopic study. J. Neuropathol. Exp. Neurol.: 1988, 47(2);166-88 PubMed 3339373
  7. 7.0 7.1 7.2 C Garel, E Chantrel, H Brisse, M Elmaleh, D Luton, J F Oury, G Sebag, M Hassan Fetal cerebral cortex: normal gestational landmarks identified using prenatal MR imaging. AJNR Am J Neuroradiol: 2001, 22(1);184-9 PubMed 11158907
  8. L B Paquette, H A Jackson, C J Tavaré, D A Miller, A Panigrahy In utero eye development documented by fetal MR imaging. AJNR Am J Neuroradiol: 2009, 30(9);1787-91 PubMed 19541779
  9. 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



Darren Farley, Donald J Dudley Fetal assessment during pregnancy. Pediatr. Clin. North Am.: 2009, 56(3);489-504, Table of Contents PubMed 19501688

Rosalie M Grivell, Lufee Wong, Vineesh Bhatia Regimens of fetal surveillance for impaired fetal growth. Cochrane Database Syst Rev: 2009, (1);CD007113 PubMed 19160321


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Cite this page: Hill, M.A. 2017 Embryology Fetal Development. Retrieved February 22, 2017, from https://embryology.med.unsw.edu.au/embryology/index.php/Fetal_Development

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© Dr Mark Hill 2017, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G