BGDA Practical 12 - Third Trimester
Practical 12: Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities |
Introduction
- 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.
Respiratory
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
Testes Descent
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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.
Incomplete or failed descent can occur unilaterally or bilaterally, is more common in premature births, and can be completed postnatally. (see also cryptorchidism). |
Start of testis descent | End of testis descent |
Fetal Weight
Neural
Comparison of brain growth through the third trimester.
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Renal
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:
- Vesicle (V) stage (13-19 weeks, second trimester)
- S-shaped body (S) stage ( 20-24 weeks, second trimester)
- Capillary loop (C) stage (25-29 weeks, third trimester)
- Maturation (M) stage (infants aged 1-6 months, neonatal and postnatal)
Teratogens - Infant Drug Clearance
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% | |
Renal drug clearance data only approximate calculated rates for the fetus and infant. Graph version
Based on: NZ Drug Safety in Lactation |
Practical 12: Embryo to Fetus | Second Trimester | Third Trimester | Birth | Neonatal | Abnormalities |
Additional Information
The following information is a detailed timeline of third trimester development and content does not form part of the current practical class.
- Links: Third Trimester
Neural Development
Third Trimester
(Clinical Week 28) Third Trimester
Event | ||
Clinical third trimester | 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] | ||
Nail Development toenails reach digit tip
Lens Development - lens growth and interocular distance plateaus after 36 weeks of gestation[2] | ||
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
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References
BGDA: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12 | Lecture Neural | Practical 14 | Histology Support - Female | Male | Tutorial
Glossary Links
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2024, June 8) Embryology BGDA Practical 12 - Third Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Practical_12_-_Third_Trimester
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G