Template:Second Trimester Timeline
|Links: human timeline | first trimester timeline | second trimester timeline | third trimester timeline|
|Clinical second trimester|| Week 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
Respiratory 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)
Genital female genital canal (80 days) formed with absorption of the median septum
|Tongue Week 12 to 13 - maximum synapses between cells and afferent nerve fibers
Hearing - Outer Ear Development Week 13 - Meatal plug disc-like, innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.
|Tongue Week 14 to 15 - taste pores develop, mucous
Ovary Development 100 days - primary follicles present
Nail Development toenails appear
Head Development facial skeleton remodelling begins
Hearing - Inner Ear Development Week 14 GA 16 - neural-crest-derived melanocytes, now intermediate cells of the stria vascularis, tightly integrate with Na+ /K+ -ATPase-positive marginal cells, which started to express KCNQ1 in their apical membrane.
|Pancreas glucagon detectable in fetal plasma.|
|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
Respiratory Week 16 to 25 lung histology - canalicular
Hearing - Outer Ear Development Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved.
Hearing - Inner Ear Development Week 16 GA 18 - cells in the outer sulcus express KCNJ10 and gap junction proteins GJB2/CX26 and GJB6/CX30, but these are not expressed in the spiral ligament. gap junction cartoon
primary follicles begin to form in the ovary and are characterized by an oocyte
glandular urethra forms and skin folds present
| Neural - Brain development histology week 17
Tooth Development Week 17 - First papilla of the permanent dentition appear (first molar) immediately behind the second milk molar, milk teeth are well advanced (Fetus 180 mm).
|tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia
integumentary vernix caseosa covers skin
Spleen Week 18 - alpha-SMA-positive reticulum cells increase in number and began to form a reticular framework. An accumulation of T and B lymphocytes occurred within the framework, and a primitive white pulp was observed around the arterioles.
Hearing - Outer Ear Development week 18 - External auditory meatus is already fully expanded to its complete form.
|neural week 19 neuronal migration ends and the radial glial cells that aided the migration now become transformed into astrocytes and astrocytic precursors.|
|pituitary week 20 to 24 growth hormone levels peak, then decline
integumentary lanugo, skin hair
integumentary 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.
| Neural brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present
Spleen - Week 22 - antigenic diversity of the reticular framework was observed, and T and B lymphocytes were segregated in the framework. T lymphocytes were sorted into the alpha-smooth muscle actin-positive reticular framework, and the periarteriolar lymphoid sheath (PALS) was formed around the arteriole. B lymphocytes aggregated in eccentric portions to the PALS and formed the lymph follicle (LF). The reticular framework of the LF was alpha-SMA-negative. 
|Respiratory 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
|Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant|
- Locher H, de Groot JC, van Iperen L, Huisman MA, Frijns JH & Chuva de Sousa Lopes SM. (2015). Development of the stria vascularis and potassium regulation in the human fetal cochlea: Insights into hereditary sensorineural hearing loss. Dev Neurobiol , 75, 1219-40. PMID: 25663387 DOI.
- Satoh T, Sakurai E, Tada H & Masuda T. (2009). Ontogeny of reticular framework of white pulp and marginal zone in human spleen: immunohistochemical studies of fetal spleens from the 17th to 40th week of gestation. Cell Tissue Res. , 336, 287-97. PMID: 19255788 DOI.
- Afif A, Bouvier R, Buenerd A, Trouillas J & Mertens P. (2007). Development of the human fetal insular cortex: study of the gyration from 13 to 28 gestational weeks. Brain Struct Funct , 212, 335-46. PMID: 17962979 DOI.
- Kadhim HJ, Gadisseux JF & Evrard P. (1988). Topographical and cytological evolution of the glial phase during prenatal development of the human brain: histochemical and electron microscopic study. J. Neuropathol. Exp. Neurol. , 47, 166-88. PMID: 3339373
- 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