ASA Meeting 2013 - Placenta

From Embryology

Placenta Embryology and Circulation

Australian Sonographers Association (ASA) Annual Conference 2013

Educate Innovate Celebrate

May 17 to 19 May 2013
ASA meeting2013 logo.jpg

Links: Conference Homepage | Sonographers Association | Placenta Development | Embryology


De formato foetu liber singularis (1626) by Adriaan van den Spiegel (1578-1625)

Sonographic analysis of the placenta and uterus and their associated blood flow are key diagnostic prenatal assessments in human development. This review will provide an overview of the basic biology of the placentation process and key events in the developmental timeline. This talk should be of value to those wanting a better understanding of the process of human haemochorial placentation.

Placentation begins at the implantation site in the second week of development (GA week 4) with conceptus trophoblast cells invading the maternal endometrial epithelium and stroma. From that time on the process of placentation involves complex interactions between maternal uterine and fetal tissues. While there are many animal models of this process, none currently exactly match that seen in humans.

Maternally, these changes include modification of the maternal vascular, endocrine and immune response. Fetally, an entire organ is grown from extra-embryonic tissue that has many functions outside of acting as a simple exchange tissue. The main maternal vascular changes include increased vascularity and trophoblast modification of spiral arteries. The fetal vascular bed consists of large cord vessels and an exponentially growing capillary bed consisting of kilometres of villous capillaries. Identification of cord vessel number, size and blood flow, are important indices of normal fetal development. Furthermore extensive remodeled of the capillary bed occurs throughout development, with some villi morphologies influencing the efficiency of diffusional gas exchange. Clinically, abnormalities of placentation site, placental development, function and blood flow can have both maternal and fetal ramifications.

The full presentation and additional information/resources/links is available online (


Dr Mark Hill

This talk is intended to provide a broad introduction to placentation and its associated vascular development. This is such an interesting, and both clinically and diagnostically relevant topic in sonography.

My talk will go through a time-course of development from earliest implantation through to the term placenta, specifically related to vascular development. Given that this process takes about 9 months, I will pick some key events in my 20 minute talk.

I have designed this online resource for ongoing "self-directed" learning after the conference.

As always, I welcome feedback and questions from my readers and encourage you to contact me with any potential educational materials that you would like to share.

--Mark Hill 09:14, 10 May 2013 (EST)

Embryo GA Week 4 (Week 2)

<html5media height="520" width="360">File:Week2_001.mp4</html5media>
Week2 001 icon.jpg

This animation shows the process of implantation, occurring during week 2 of development in humans.

The beginning of the animation shows adplantation to the the uterus lining (endometrium epithelium). The hatched blastocyst with a flat outer layer of trophoblast cells (green), the inner cell mass which has formed into the bilaminar embryo (epiblast and hypoblast) and the large fluid-filled space (blastocoel).

  • green cells - trophoblast layer of the conceptus
  • blue cells - epiblast layer of the bilaminar embryo
  • yellow cells - hypoblast layer of the bilaminar embryo
  • white cells - uterine endometrium epithelium
  • red - maternal blood vessel

Implantation Movie Links: MP4 version | Week 2 Chorionic Cavity Movie | Implantation | Week 2 | Trophoblast | Human Chorionic Gonadotropin | Placenta Development | Movies
<html5media height="260" width="220">File:Chorion 001.mp4</html5media>
Chorion 001 icon.jpg
Animation shows the events following implantation and focuses on changes in the the spaces surrounding the embryonic disc, the extraembryonic coelom.

The blastoceol cavity is converted into two separate spaces: the yolk sac and the chorionic cavity.

The third space lies above the epiblast layer of the embryonic disc, the amniotic cavity.

  • white cells - (left) endometrial gland (right) endometrial epithelium
  • blue - epiblast layer (forming ectoderm and amniotic cavity lining)
  • yellow - hypoblast layer (replaced with endoderm during gastrulation)
  • red cells - extraembryonic mesoderm layer
  • green - trophoblast layers (cytotrophoblast and syncitiotrophoblast)
  • red spaces - blood-filled spaces, maternal lacunae

Links: MP4 version | Implantation Movie | Week 2 | Movies

Early embryo membrane development cartoons: Image 24 | Image 25 | Image 26 | Image 27 | Image 28

Gray0032.jpg Stage7 bf53.jpg

Embryo GA Week 5 (Week 3)

Week 2 (GA 4) Week 3 (GA 5) Week 4 (GA 6)
Gray0036.jpg Gray0037.jpg Gray0031.jpg
Primary villi Secondary villi Tertiary villi
first stage of chorionic villi development, trophoblastic shell cells (syncitiotrophoblasts and cytotrophoblasts) form finger-like extensions into maternal decidua. second stage of chorionic villi development, extraembryonic mesoderm grows into villi, covers entire surface of chorionic sac. Basal region will form chorionic plate. third stage of chorionic villi development, mesenchyme differentiates into blood vessels and cells, forms arteriocapillary network, fuse with placental vessels, developing in connecting stalk.
Week3 folding icon.jpg
 ‎‎Week 3
Page | Play
Amnion 001 icon.jpg
 ‎‎Amniotic Cavity
Page | Play

Embryo GA Week 7 (Week 5)

Stage13 bf4.jpg

Embryo Links: Embryo and Placenta | Embryo | Embryo (label) | Embryo (animated) | Embryo (animated large) | Head region | Head region (label) | Body region | Body region (label) | Carnegie stage 13
Stage 13 Links: Week 4 | Week 5 | Somitogenesis | Placodes | Head | Movies | Lecture - Early Vascular | Lecture - Gastrointestinal | Lecture - Head Development | Science Practical - Gastrointestinal | Science Practical - Head | Carnegie Embryos | Category:Carnegie Stage 13 | Next Stage 14
  Historic Papers: 1905 Embryo 148 | 1926 Precervical Sinus | 1945 | 1955

Embryo GA Week 9 (Week 7)

Placental membranes.jpg

Stage18 bf10.jpg Stage18 bf11.jpg
Embryo in gestational sac Embryo open sac
Stage18 bf12.jpg Stage18 bf13.jpg
Embryo with placentation (ectopic) Embryo in amniotic sac

Villi Development

First Trimester Third Trimester
Placental villi 2.jpg Placental villi 4.jpg
Secondary Villi Tertiary Villi
Placenta anchoring villi.jpg Placental villi 6.jpg

Human placental villi cartoon 01.jpg

Maternal Blood Vessels

Uterine and Placental Vasculature in Non-pregnant, Pregnant and immediate Post-partum State

Uterine and placental vasculature.jpg

Diagrammatic representation of uterine and placental vasculature (red shading = arterial; blue shading = venous) in the non-pregnant, pregnant and immediate post-partum state.

Placental Cord


Cord cross-section

Cord Length

The following are lengths and classifications at term.

  • Normal range - 50 to 60 cm.
  • Short cord - less than 35 cm.
  • Long cords - over 70 cm can be associated with wrapping around the fetus and other abnormalities.


Placental cord ultrasound 03.jpg

Ultrasound image of transverse scan through the cord show the method of estimation of the cross-sectional area.


Placenta oxygen exchange levels.jpg Placenta spiral artery conversion.jpg

Term Placenta

Maternal and Fetal Side

Placenta term anatomy 01.jpg

Placenta Vasculature - MRI and CT

Term placenta viewed from the fetal side.[1]

Human placenta vascular MRI 02.jpg Human placenta vascular CT 01.jpg
Magnetic Resonance Angiography (MRA) Computed Tomography Angiography (CTA)
  • CA - chorionic artery
  • PSA - primary stem artery
  • SSA - secondary stem artery
  • TSA - tertiary stem artery

Placenta Measurements

Term Placenta Measurements

Morphometric indices at term of placental composition, villous capillarization and the mean cross-sectional areas of peripheral villi and capillaries.

Variable Unit Placenta
Intervillous space mL 213
Stem villi mL 71.4
Peripheral villi mL 326
Trophoblast mL 95.5
Stroma mL 184
Fetal capillaries mL 46.9
Non-parenchyma mL 41.5
Peripheral villi km 89.2
Fetal capillaries km 310
TS area villi µm2 3700
TS area capillary µm2 150
Capillaries mL mL-1 0.147
Length ratio km km-1 3.6

Data from a study sample of 15 normal placenta, mean placental volume, 652 ml.[2][3]


Hydatidiform Mole

Hydatidiform mole.jpg

Hydatidiform Mole

When only the conceptus trophoblast layers proliferates and not the embryoblast, no embryo develops, this is called a "hydatidiform mole" (HM), which is due to the continuing presence of the trophoblastic layer, this abnormal conceptus can also implant in the uterus.

Velamentous Cord Insertion of the Placenta
Placenta velamentous cord 02.jpg The placental cord inserts into the chorion laeve (placental membranes) away from the edge of the placenta.[4]

The placental vessels are therefore unprotected by Wharton's jelly where they traverse the membranes before they come together into the umbilical cord.

This can cause hemorrhage if the vessels are damaged when the membranes are ruptured prior to birth. The condition is more common in monozygotic twins (15%) and triplets.

Implantation Abnormalities

Placenta abnormalities.jpg

Placenta previa and increta 02.jpg

Placenta previa and increta

Online Placenta Links

Placenta Links: placenta | Lecture - Placenta | Lecture Movie | Practical - Placenta | implantation | placental villi | trophoblast | maternal decidua | uterus | endocrine placenta | placental cord | placental membranes | placenta abnormalities | ectopic pregnancy | Stage 13 | Stage 22 | placenta histology | placenta vascular | blood vessel | cord stem cells | 2013 Meeting Presentation | Placenta Terms | Category:Placenta
Historic Embryology - Placenta 
1883 Embryonic Membranes | 1907 Development Atlas | 1909 | 1910 Textbook | 1917 Textbook | 1921 Textbook | 1921 Foetal Membranes |1921 human | 1921 Pig implantation | 1922 Single placental artery | 1923 Placenta Review | 1939 umbilical cord | 1943 human and monkey | 1944 chorionic villus and decidua parietalis | 1946 placenta ageing | 1960 first trimester placenta | 1960 monkey | 1972 Placental circulation | Historic Disclaimer

Historic Embryology - Placenta 
1883 Embryonic Membranes | 1907 Development Atlas | 1909 | 1910 Textbook | 1917 Textbook | 1921 Textbook | 1921 Foetal Membranes |1921 human | 1921 Pig implantation | 1922 Single placental artery | 1923 Placenta Review | 1939 umbilical cord | 1943 human and monkey | 1944 chorionic villus and decidua parietalis | 1946 placenta ageing | 1960 monkey | 1972 Placental circulation | Historic Disclaimer


  1. Rasmussen AS, Lauridsen H, Laustsen C, Jensen BG, Pedersen SF, Uhrenholt L, Boel LW, Uldbjerg N, Wang T & Pedersen M. (2010). High-resolution ex vivo magnetic resonance angiography: a feasibility study on biological and medical tissues. BMC Physiol. , 10, 3. PMID: 20226038 DOI.
  2. Mayhew TM, Jenkins H, Todd B & Clifton VL. (2008). Maternal asthma and placental morphometry: effects of severity, treatment and fetal sex. Placenta , 29, 366-73. PMID: 18328557 DOI.
  3. Mayhew TM. (2009). A stereological perspective on placental morphology in normal and complicated pregnancies. J. Anat. , 215, 77-90. PMID: 19141109 DOI.
  4. Rocha J, Carvalho J, Costa F, Meireles I & do Carmo O. (2012). Velamentous cord insertion in a singleton pregnancy: an obscure cause of emergency cesarean-a case report. Case Rep Obstet Gynecol , 2012, 308206. PMID: 23243528 DOI.


Glossary Links

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Cite this page: Hill, M.A. (2024, June 21) Embryology ASA Meeting 2013 - Placenta. Retrieved from

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