Third Trimester

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Introduction

The fetus in the 8th month

The third trimester is a period of development when the fetus slows in growth in length and begins to increase substantially in weight, this is also the time where Individual variations increase. Some fetal systems, such as the respiratory system, only now begin to mature just prior to birth.

Immune system, IgG to specific maternal antigens transferred passively across the placenta during the last trimester of pregnancy.


Fetal Links: fetal | Week 10 | Week 12 | second trimester | third trimester | fetal neural | Fetal Blood Sampling | fetal growth restriction | birth | birth weight | preterm birth | Developmental Origins of Health and Disease | macrosomia | BGD Practical | Medicine Lecture | Science Lecture | Lecture Movie | Category:Human Fetus | Category:Fetal
Historic Embryology  
1940 Fetus Physiology
Carnegie Fetal: 95 | 96 | 142 | 145 | 184 | 211 | 217 | 300 | 362 | 448 | 449 | 538 | 590 | 607 | 625 | 662 | 693 | 847 | 858 | 922 | 928 | 948 | 972 | 1318 | 1388 | 1455 | 1591 | 1597b | 1656 | 1686 | 2250a | 2250b | 3990 | 5652 | 6581 | 7218

Category:Third Trimester

Some Recent Findings

  • Review - Second- and third-trimester biochemical and ultrasound markers predictive of ischemic placental disease.[1] "Ischemic placental disease is a recently coined term that describes the vascular insufficiency now believed to be an important etiologic factor in preeclampsia, intrauterine fetal growth restriction, and placental abruption. Given the increased risk for poor maternal and fetal outcomes, early prediction and prevention of this disorder is of significant clinical interest for many. In this article, we review the second- and third-trimester serum and ultrasound markers predictive of ischemic placental disease. Limited first-trimester data is also presented. While current studies report a statistical association between marker levels and various adverse perinatal outcomes, the observed diagnostic accuracy is below the threshold required for clinical utility. An exception to this generalization is uterine artery Doppler for the prediction of early-onset preeclampsia. Metabolomics is a relatively new analytic platform that holds promise as a first-trimester marker for the prediction of both early- and late-onset preeclampsia." ultrasound
More recent papers  
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Older papers  
These papers originally appeared in the Some Recent Findings table, but as that list grew in length have now been shuffled down to this collapsible table.

See also the Discussion Page for other references listed by year and References on this current page.

  • Local tissue growth patterns underlying normal fetal human brain gyrification quantified in utero[2] "we applied recent advances in fetal MRI motion correction and computational image analysis techniques to 40 normal fetal human brains covering a period of primary sulcal formation (20-28 gestational weeks). Growth patterns were mapped by quantifying tissue locations that were expanding more or less quickly than the overall cerebral growth rate, which reveal increasing structural complexity. We detected increased local relative growth rates in the formation of the precentral and postcentral gyri, right superior temporal gyrus, and opercula, which differentiated between the constant growth rate in underlying cerebral mantle and the accelerating rate in the cortical plate undergoing folding. Analysis focused on the cortical plate revealed greater volume increases in parietal and occipital regions compared to the frontal lobe. Cortical plate growth patterns constrained to narrower age ranges showed that gyrification, reflected by greater growth rates, was more pronounced after 24 gestational weeks. Local hemispheric volume asymmetry was located in the posterior peri-Sylvian area associated with structural lateralization in the mature brain." neural

Neural

Human Fetus (CRL 240 mm) Brain (scale bar 1 cm)
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.


Links: Neural System Development

Respiratory

Alveolar-sac-01.jpg

Alveolar sac structure

Saccular stage

  • week 24 to near term.
  • most peripheral airways form widened airspaces, termed saccules.
  • saccules widen and lengthen the airspace (by the addition of new generations).
  • future gas exchange region expands significantly.
  • Fibroblastic cells also undergo differentiation, they produce extracellular matrix, collagen, and elastin. May have a role in epithelial differentiation and control of surfactant secretion
  • The vascular tree also grows in length and diameter during this time.


  • in late fetal development respiratory motions and amniotic fluid are thought to have a role in lung maturation.
  • Development of this system is not completed until late fetal just before birth.
    • Therefore premature babies have difficulties associated with insufficient surfactant (end month 6 alveolar cells type 2 appear and begin to secrete surfactant).


Links: respiratory

Genital

Testis-descent start.jpg

Testis-descent end.jpg

Testis Descent

Data from a study of male human fetal (between 10 and 35 weeks) gonad position[3]

  • 10 to 23 weeks - (9.45%) had migrated from the abdomen and were situated in the inguinal canal.
  • 24 to 26 weeks - (57.9%) had migrated from the abdomen.
  • 27 to 29 weeks - (16.7%) had not descended to the scrotum.


Links: Testis Development

Third Trimester Timeline

(Clinical Week 28-29)

Third Trimester Timeline
Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline
Week
Stage
Event
Clinical third trimester Fetal size change.jpg hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
27
 
28
  respiratory Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs
29
 

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

30
   

Genital male gonad (testes) descending

31
 
32
  nail fingernails reach digit tip
33
  neural brain cortical sulcation - primary sulci present[4]
34
  neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present[4]
35
   
36
  Frazer006 bw600.jpg Nail Development toenails reach digit tip

Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation[5]

37
   
38
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

Systems  
Systems: bone timeline | eye neural crest timeline | heart abnormality timeline | hearing EAM timeline | muscle timeline | ovary timeline | placental villi timeline | shoulder timeline | smell timeline | spleen timeline | ventricular timeline

References

  1. Savasan ZA, Goncalves LF & Bahado-Singh RO. (2014). Second- and third-trimester biochemical and ultrasound markers predictive of ischemic placental disease. Semin. Perinatol. , 38, 167-76. PMID: 24836829 DOI.
  2. Rajagopalan V, Scott J, Habas PA, Kim K, Corbett-Detig J, Rousseau F, Barkovich AJ, Glenn OA & Studholme C. (2011). Local tissue growth patterns underlying normal fetal human brain gyrification quantified in utero. J. Neurosci. , 31, 2878-87. PMID: 21414909 DOI.
  3. Sampaio FJ & Favorito LA. (1998). Analysis of testicular migration during the fetal period in humans. J. Urol. , 159, 540-2. PMID: 9649288
  4. 4.0 4.1 Garel C, Chantrel E, Brisse H, Elmaleh M, Luton D, Oury JF, Sebag G & Hassan M. (2001). Fetal cerebral cortex: normal gestational landmarks identified using prenatal MR imaging. AJNR Am J Neuroradiol , 22, 184-9. PMID: 11158907
  5. Paquette LB, Jackson HA, Tavaré CJ, Miller DA & Panigrahy A. (2009). In utero eye development documented by fetal MR imaging. AJNR Am J Neuroradiol , 30, 1787-91. PMID: 19541779 DOI.

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Cite this page: Hill, M.A. (2019, June 18) Embryology Third Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Third_Trimester

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G