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| |+ colspan=3|'''Second Trimester Timeline'''
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| | colspan=2|{{human timeline}} | {{first trimester timeline}} | {{second trimester timeline}} | {{third trimester timeline}}
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| |-bgcolor="#AFEEEE"
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| | <center>'''Week'''</center>
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| | <center>'''Stage'''</center>
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| | '''Event'''
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| | <center>12</center>
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| | Clinical second trimester ({{GA}} Clinical Week 14) [[Second Trimester]]
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| | [[File:Fetal_head_lateral.jpg|90px|link=Fetal Development]] Week 12 - CRL 85 mm, femur length 15 mm, biparietal diameter 25 mm
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| [[Sensory_-_Hearing_and_Balance_Development|Hearing]] Week 12-16 - Capsule adjacent to membranous labrynth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph
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| [[Genital_System_Development|Genital]] male and female external genital differences observable
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| [[Respiratory_System_Development|Respiratory]] Month 3-6 - lungs appear glandular, end month 6 alveolar cells type 2 appear and begin to secrete surfactant
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| [[Tongue_Development|Tongue]] Week 12 - first differentiated epithelial cells (Type II and III)
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| [[Genital_System_Development|Genital]] female genital canal (80 days) formed with absorption of the median septum
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| | <center>13</center>
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| | [[Tongue_Development|Tongue]] Week 12 to 13 - maximum synapses between cells and afferent nerve fibers
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| [[Hearing_-_Outer_Ear_Development|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.
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| | <center>14</center>
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| | [[Tongue_Development|Tongue]] Week 14 to 15 - taste pores develop, mucous
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| [[Ovary Development]] 100 days - primary follicles present
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| [[Integumentary_System_-_Nail_Development|Nail Development]] toenails appear
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| [[Head Development]] facial skeleton remodelling begins
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| [[Hearing - Inner Ear Development|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.<ref name="PMID25663387"><pubmed>25663387</pubmed></ref>
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| | <center>15</center>
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| | [[Endocrine_-_Pancreas_Development|Pancreas]] glucagon detectable in fetal plasma.
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| [[Cardiovascular_System_-_Spleen_Development|Spleen]] Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles. <ref name="PMID19255788"><pubmed>19255788</pubmed></ref>
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| | <center>16</center>
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| | 14 cm
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| | [[File:Fetal_size_change.jpg|90px|link=Fetal Development]] [[Sensory_-_Hearing_and_Balance_Development|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.
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| [[Endocrine_-_Pituitary_Development|Pituitary]] adenohypophysis fully differentiated
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| [[Respiratory_System_Development|Respiratory]] Week 16 to 25 lung histology - canalicular
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| [[Hearing_-_Outer_Ear_Development|Hearing - Outer Ear Development]] Week 16.5 - External auditory meatus is fully patent throughout its length, lumen is still narrow and curved.
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| [[Hearing - Inner Ear Development|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.<ref name="PMID25663387"><pubmed>25663387</pubmed></ref> [[:File:Gap junction 01.jpg|gap junction cartoon]]
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| [[Neural_System_Development|Neural]] - [[Neural - Cerebrum Development|Cerebrum]] development of the periinsular sulci (week 16-17, {{GA}} 18-19 weeks)<ref name="PMID17962979"><pubmed>17962979</pubmed></ref>
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| [[Integumentary_System_Development|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.
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| primary follicles begin to form in the ovary and are characterized by an oocyte
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| glandular urethra forms and skin folds present
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| | <center>17</center>
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| | [[File:Brain_week_17_histology.jpg|90px]] [[Neural System Development|Neural]] - Brain development histology week 17
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| [[Integumentary System - Tooth Development|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).
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| | <center>18</center>
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| | [[File:Bailey095.jpg|90px]][[Tongue_Development|Tongue]] Week 18 - substance P detected in dermal papillae, not in taste bud primordia
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| [[Integumentary_System_Development|Skin]] vernix caseosa covers skin
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| [[Cardiovascular_System_-_Spleen_Development|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. <ref name="PMID19255788"><pubmed>19255788</pubmed></ref>
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| [[Hearing_-_Outer_Ear_Development|Hearing - Outer Ear Development]] week 18 - External auditory meatus is already fully expanded to its complete form.
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| [[Neural_System_Development|Neural]] - [[Neural - Cerebrum Development|Cerebrum]] central sulci and opercularization of the insula (week 18-20, {{GA}} 20-22 weeks)<ref name="PMID17962979"><pubmed>17962979</pubmed></ref>
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| | <center>19</center>
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| | [[Neural System Development|Neural]] week 19 neuronal migration ends and the [[Neural_System_-_Glial_Development#Radial_Glia|radial glial cells]] that aided the migration now become transformed into astrocytes and astrocytic precursors.<ref><pubmed>3339373</pubmed></ref>
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| |-bgcolor="#AFEEEE"
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| | <center>20</center>
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| | [[Endocrine_-_Pituitary_Development|Pituitary]] week 20 to 24 growth hormone levels peak, then decline
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| [[Integumentary_System_Development|Skin]] lanugo, skin hair
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| [[Integumentary_System_Development|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.
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| | <center>21</center>
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| | <center>22</center>
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| | [[File:Gray0038.jpg|90px|link=Fetal Development]] [[Neural System Development|Neural]] brain cortical sulcation - sylvian fissure, interhemispheric fissure, callosal sulcus, parietooccipital fissure, and hippocampic fissures present<ref name="PMID11158907"><pubmed>11158907</pubmed></ref>
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| [[Cardiovascular_System_-_Spleen_Development|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. <ref name="PMID19255788" />
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| [[Neural_System_Development|Neural]] - [[Neural - Cerebrum Development|Cerebrum]] covering of the posterior insula (week 22-24, {{GA}} 24-26 weeks)<ref name="PMID17962979"><pubmed>17962979</pubmed></ref>
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| | <center>23</center>
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| | <center>24</center>
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| | [[Respiratory_System_Development|Respiratory]] Week 24 to 40 lung histology - terminal sac
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| [[Cardiovascular_System_-_Spleen_Development|Spleen]] Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp.<ref name="PMID19255788"><pubmed>19255788</pubmed></ref>
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| [[Integumentary System - Tooth Development|Tooth Development]] Week 24 - Permanent incisors and canines appear.
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| Earliest potential survival expected if born
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| ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells
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| | <center>25</center>
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| | [[Respiratory_System_Development|Respiratory]] end month 6 alveolar cells type 2 appear and begin to secrete surfactant
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| [[Neural_System_Development|Neural]] - [[Neural - Cerebrum Development|Cerebrum]] closure of the laeteral sulcus (Sylvian fissure or lateral fissure) (week 25-26, {{GA}} 27-28 weeks)<ref name="PMID17962979"><pubmed>17962979</pubmed></ref>
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| |}<noinclude>[[Category:Template]][[Category:Timeline]][[Category:Second Trimester]][[Category:Fetal]]</noinclude>
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