ANAT2341 Lab 12 - Third Trimester

From Embryology
Lab 12: Introduction | Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities | Online Assessment


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

Week 24+

  • Earliest potential survival expected if born


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 28 weeks gestation to 2 months after birth

Testes Descent

Testis 001 icon.jpg 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

Fetal Birth

Most of the serious illness and mortality is concentrated in the 1 to 2 percent of infants who are born at less than 32 weeks of gestation (week 30) and who weigh less than 1500 g.

Teratogens - Infant Drug Clearance

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.

The drug clearance data below are only approximate calculated rates for the fetus and infant from NZ Drug Safety in Lactation

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%

Additional Information

The following information is a detailed timeline of third trimester development and content does not form part of the current practical class.

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>
Lab 12: Introduction | Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities | Online Assessment

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Cite this page: Hill, M.A. (2024, April 23) Embryology ANAT2341 Lab 12 - Third Trimester. Retrieved from

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