Difference between revisions of "BGDA Practical 12 - Third Trimester"

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==Additional Information==
==Additional Information==
The following information is a detailed timeline of third trimester development and content does not form part of the current practical class.
{{Med Prac additional Information}}
:'''Links:''' [[Third Trimester]]
:'''Links:''' [[Third Trimester]]

Revision as of 15:33, 2 June 2014

Practical 12: Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities


Historic drawing of the fetus in the uterus at 8 months compared to non-pregnant uterus size.
  • Survival issues for early delivery
  • Fetal Respiratory
  • Fetal Genital
  • Fetal weight
  • Fetal Neural
  • Fetal Cardiovascular
  • Fetal Renal
  • Fetal origins hypothesis

Week 24+

  • Earliest potential survival expected if born.
  • Most of the serious illness and mortality is concentrated in the 1 to 2 percent of infants who are born at less than GA 32 weeks (week 30) and who weigh less than 1500 g.


Week 24 to 40 lung histology - terminal sac, end month 6 alveolar cells type 2 appear and begin to secrete surfactant
  • saccule - a large thin-walled airspace lined by flattened epithelium present from about GA 28 weeks to 2 months after birth


Testis 001 icon.jpg
 ‎‎Testis Descent
Page | Play

Testes Descent

The linked animation shows the descent of the testes (between week 7 to 38, birth).

Descent of the testes into the scrotal sac begins generally during week 26 and may take several days.

  • testis (white) lies in the subserous fascia (spotted)
  • a cavity processus vaginalis evaginates into the scrotum
  • gubernaculum (green) attached to the testis shortens drawing it into the scotal sac
  • as it descends it passes through the inguinal canal extends
    • from the deep ring (transversalis fascia)
    • to the superficial ring (external oblique muscle)

Incomplete or failed descent can occur unilaterally or bilaterally, is more common in premature births, and can be completed postnatally. (see also cryptorchidism).

Testis-descent start.jpg Testis-descent end.jpg
Start of testis descent End of testis descent

Fetal Weight

Fetal weight change.jpg


Dev anat 01.jpg Comparison of brain growth through the third trimester.

Note the increasing cortex surface area forming visible gyri (folds) and fissures (grooves) on the surface. (see also Additional Information below)

Links: Neural System Development


Adult nephron structure
Nephron histology

The functional unit of the kidney is the nephron and the process of their initial formation is called nephrogenesis. During development nephron number increases from about 15,000 at 15 weeks (GA 17 weeks) increasing to about the adult number by 36 weeks. Each adult kidney typically contains about 750,000 nephrons, though the total number can vary significantly from as few as 250,000 to as many as 2,000,000.

In humans, nephrogenesis only occurs before birth, though nephron maturation continues postnatally.

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)

Infant Drug Clearance

Post-conceptual Age (weeks) Clearance of Drug (percentage of adults)
24-28 5%
28-34 10%
34-40 33%
40-44 50%
44-68 66%
> 68 100%
Renal drug clearance data only approximate calculated rates for the fetus and infant. Graph version

Based on: NZ Drug Safety in Lactation

Drug clearance rates

Teratogen (Greek, teraton = monster) Any agent that causes a structural abnormality following exposure during pregnancy. The overall effect depends on dosage and time of exposure.

Practical 12: Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities


Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.
Links: Third Trimester

Neural Development


Third Trimester

(Clinical Week 28) Third Trimester

Clinical 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

Genital male gonad (testes) descending

  Nail Development fingernails reach digit tip
  Neural brain cortical sulcation - primary sulci present[1]
  Neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present[1]
  Frazer006 bw600.jpg Nail Development toenails reach digit tip

Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation[2]

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


  1. 1.0 1.1 <pubmed>11158907</pubmed>
  2. <pubmed>19541779</pubmed>


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Cite this page: Hill, M.A. (2020, October 21) Embryology BGDA Practical 12 - Third Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_12_-_Third_Trimester

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© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G