Third Trimester

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. 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.

| Category:Third Trimester

Some Recent Findings

Local tissue growth patterns underlying normal fetal human brain gyrification quantified in utero "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

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 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 System Development

Genital

Testis Descent Data from a study of male human fetal (between 10 and 35 weeks) gonad position 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)

Week	Stage	Event
	Clinical third trimester	Hearing 3rd Trimester - vibration acoustically of maternal abdominal wall induces startle respone in fetus.
27
28		Respire Month 7 - respiratory bronchioles proliferate and end in alveolar ducts and sacs
29
30		Genital male gonad (testes) descending
31
32
33		Neural brain cortical sulcation - primary sulci present PMID:11158907
34		Neural brain cortical sulcation - insular, cingular, and occipital secondary sulci present PMID:11158907
35
36
37
38	Birth	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

References

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Cite this page: Hill, M.A. (2026, September 15) Embryology Third Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Third_Trimester

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© Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G