Second Trimester

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Introduction

Fetal size change

Ultrasound12wk 3D image2.jpg

Ultrasound Image of an early Fetus (12 week)


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| Category:Second Trimester

Some Recent Findings

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<pubmed limit=5>Second Trimester</pubmed>


Neural Development

Timeline of events in Normal Human Neural Development[2]

Three-dimensional reconstruction of the lateral (top row) and medial (bottom row) surface of second trimester (13 to 21 week) brains to reveal the development of the Sylvian fissure or lateral sulcus (green arrow), the calcarine fissure (blue arrow), and the parieto-occipital sulcus (red arrow), respectively.


Brain fissure development 02.jpg


The images below are from a recent MRI study of fixed fetal brains at different weeks of development during the second trimester.[3]


Links: Neural System - Fetal

Second Trimester Timeline

(Clinical Week 14)

Week
Stage
Event
12
Clinical second trimester Fetal head lateral.jpgWeek 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm

Hearing Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph

Genital male and female external genital differences observable

Respire Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant

Tongue Week 12 - first differentiated epithelial cells (Type II and III)

female genital canal (80 days) formed with absorption of the median septum

13
  Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers

 

14
Tongue Week 14 to 15 - taste pores develop, mucous
15
  Pancreas glucagon detectable in fetal plasma
16
14 cm Hearing Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.

Pituitary adenohypophysis fully differentiated

Respire Week 16 to 25 lung histology - canalicular

Skin 4 months - basal cell- proliferation generates folds in basement membrane; neural crest cells- (melanocytes) migrate into epithelium; embryonic connective tissue- differentiates into dermis, a loose ct layer over a dense ct layer. Beneath the dense ct layer is another loose ct layer that will form the subcutaneous layer. Ectoderm contributes to nails, hair follictles and glands. Nails form as thickening of ectoderm epidermis at the tips of fingers and toes. These form germinative cells of nail field. Cords of these cells extend into mesoderm forming epithelial columns. These form hair follocles, sebaceous and sweat glands.

primary follicles begin to form in the ovary and are characterized by an oocyte

glandular urethra forms and skin folds present

17
 
18
Tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia

Skin vernix caseosa covers skin

Spleen SMA-positive reticulum cells increase in number and begin to form a reticular framework. PMID: 1925578 18 week post-fertilization age or 20 week gestational age (GA) developing fetal thymus histology .

19
   
20
  Pituitary week 20 to 24 growth hormone levels peak, then decline

Skin lanugo, skin hair

Skin 5 months - Hair growth initiated at base of cord, lateral outgrowths form associated sebaceous glands; Other cords elongate and coil to form sweat glands; Cords in mammary region branch as they elongate to form mammary glands.

Uterus Development uterine horn fimbrial development begins and continues after birth

21
   
22
  Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present(PMID:11158907

Spleen antigenic reticular framework diversity, T and B lymphocytes segregated in the framework PMID: 1925578

23
   
24
  Respire Week 24 to 40 lung histology - terminal sac

Earliest potential survival expected if born

ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells

25
  Respire end month 6 alveolar cells type 2 appear and begin to secrete surfactant

References

  1. <pubmed></pubmed>
  2. Report of the Workshop on Acute Perinatal Asphyxia in Term Infants, U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Child Health and Human Development, NIH Publication No. 96-3823, March 1996.
  3. <pubmed>19339620</pubmed>

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Cite this page: Hill, M.A. (2024, March 28) Embryology Second Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Second_Trimester

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