Second Trimester

Embryology - 29 Oct 2026        Expand to Translate
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Introduction

Fetal size change

Ultrasound Image of an early Fetus (12 week)

| Category:Second Trimester

Some Recent Findings

Review - Second trimester sonographic features of fetal chromosomal defects "Advances in ultrasound technology have dramatically improved the detection of fetal chromosomal defects. Each chromosomal defect has its own syndromal pattern of detectable abnormalities prenataly. Most commonly detectable defects are Trisomies & Triploidies. Although only an invasive test can provide a definitive diagnosis, fetuses with major chromosomal abnormalities have either external or internal defects that can be recognized by detailed ultrasonographic examination at second trimester. These are defined as ultrasound markers for fetal aneuploidy. This article provides an overview and discussion on prenatal sonographic features that may suggest the presence of a common fetal chromosomal defect." Ultrasound Review - Second- and third-trimester biochemical and ultrasound markers predictive of ischemic placental disease. "Ischemic placental disease is a recently coined term that describes the vascular insufficiency now believed to be an important etiologic factor in preeclampsia, intrauterine fetal growth restriction, and placental abruption. Given the increased risk for poor maternal and fetal outcomes, early prediction and prevention of this disorder is of significant clinical interest for many. In this article, we review the second- and third-trimester serum and ultrasound markers predictive of ischemic placental disease. Limited first-trimester data is also presented. While current studies report a statistical association between marker levels and various adverse perinatal outcomes, the observed diagnostic accuracy is below the threshold required for clinical utility. An exception to this generalization is uterine artery Doppler for the prediction of early-onset preeclampsia. Metabolomics is a relatively new analytic platform that holds promise as a first-trimester marker for the prediction of both early- and late-onset preeclampsia."

More recent papers

This table allows an automated computer search of the external PubMed database using the listed "Search term" text link. This search now requires a manual link as the original PubMed extension has been disabled. The displayed list of references do not reflect any editorial selection of material based on content or relevance. References also appear on this list based upon the date of the actual page viewing. References listed on the rest of the content page and the associated discussion page (listed under the publication year sub-headings) do include some editorial selection based upon both relevance and availability. More? References | Discussion Page | Journal Searches | 2019 References | 2020 References Search term: Second Trimester <pubmed limit=5>Second Trimester</pubmed>

Neural Development

Timeline of events in Normal Human Neural Development

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.

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

• 

  Three-dimensional reconstruction of the lateral (top row) and medial (bottom row) surface of 13–21 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.

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  3D depiction of developmental white matter fibers. A lateral view of limbic tracts where pink fibers in 13, 15, and 19 week brains are the fornix and stria terminalis and purple fibers in the 19 week brains indicate the cingulum bundle.

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  Three-dimensional reconstruction of the basal ganglia and ganglionic eminence. Different colors represent different brain structures: whole brain (gray), ventricle (pink), ganglionic eminence (red), putamen and globus pallidus together (cyan), thalamus (yellow), and caudate nucleus (green).

• 

  Three-dimensional reconstruction of the lateral (top row) and medial (bottom row) surface of 13–21 week brains to reveal the development of the Sylvian or lateral fissure (green arrow).

Links: Neural System - Fetal

Second Trimester Timeline

Second Trimester (Clinical Week GA 14+)
Age (week)		Event
Fertilisation Age FA	Gestational Age GA
12	14	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 Genital male and female external genital differences observable 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
13	15	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.
14	16	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
15	17	Pancreas glucagon detectable in fetal plasma. Spleen Week 15 -alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells scattered around the arterioles.
16	18	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. 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	19	Neural - Brain development histology week 17
18	20	Tongue Week 18 - substance P detected in dermal papillae, not in taste bud primordia Skin 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.
19	21
20	22	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.
21	23
22	24	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.
23	25
24	26	Respiratory Week 24 to 40 lung histology - terminal sac Spleen Week 24 - marginal zone appeared in the alpha-smooth muscle actin-positive reticular framework around the white pulp. Earliest potential survival expected if born ovarian follicles can consist of growing oocytes surrounded by several layers of granulosa cells
25	27	Respiratory end month 6 alveolar cells type 2 appear and begin to secrete surfactant

References

Search Pubmed: Second Trimester

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Cite this page: Hill, M.A. (2026, October 29) Embryology Second Trimester. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Second_Trimester

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