Placental development

Author: Mandolin Ziadie, M.D. (see Authors page)

Revised: 11 October 2017, last major update September 2011

Copyright: (c) 2002-2017,, Inc.

PubMed Search: Placental development[title] "loattrfree full text"[sb]

Cite this page: Ziadie, M. Placental development. website. Accessed November 15th, 2018.
  • Blastocyst implants on postovulation day 6 - 7; by day 10, ovum is implanted in stroma
  • Trophoblasts proliferate and erode the maternal capillaries and venules to form the intervillous space
  • Extraembryonic mesoderm grows into the primary villi and capillary formation occurs
  • By 5 - 6 weeks, the villous vessels are formed
  • At 8 weeks, they contain nucleated red blood cells (nRBC) which diminish to 10% by 10 weeks and are absent at 12 weeks
First trimester
Implantation and arterial plugs:
  • Extravillous intermediate trophoblasts invade the endometrium while endovascular trophoblasts grow into arteries and form cellular plugs

Villous morphology:
  • Early mesenchymal villi are large (170 microns) with scant connective tissue, few Hofbauer cells, no thick walled vessels
  • They have a complete outer layer of syncytiotrophoblast and an inner cytotrophoblast layer
  • During mid trimester, they mature to immature intermediate villi which have loose stroma with many Hofbauer cells and a complete trophoblastic coat
  • Then transform to stem villi, which have denser stroma and thick walled vessels
  • This process continues through the second trimester

Micro images:

Images hosted on other servers:

Chorionic villi covered
by cytotrophoblast and

Second trimester
Implantation and vascular remodeling:
  • Invading trophoblasts extend into the myometrium while endovascular trophoblasts invade the arterial walls and destroy their endothelium and media, replacing them with fibrinoid material, creating a low pressure circulation

Villous morphology:
  • Mesenchymal villi give rise to mature intermediate villi, from which terminal villi sprout
  • Mature intermediate villi are large with loose stroma, capillaries, arterioles and venules
  • Terminal villi appear near the end of the trimester and are much smaller (70 microns) with denser stroma surrounded primarily by syncytiotrophoblasts and a thin cytotrophoblast layer that may have syncytial knots
  • Vessels are numerous (3 - 5 capillaries per villous) and in contact with the trophoblastic coat

Micro images:

Images hosted on other servers:

"Syncytial knots" and pink fibrin

Third trimester
Villous morphology:
  • Mature intermediate and terminal villi are now more prevalent and smaller than second trimester with thin stroma
  • More have syncytial knots (approximately 30%) and vasculosyncytial membranes (fused fetal capillaries with syncytiotrophoblasts)
  • Trophoblastic inclusions are common

Micro images:

Images hosted on other servers:

Vascularized chorionic villi and hCG immunostain

Mean placental weight by gestational age
  • Prior to 28 weeks: 253 grams
  • 28 - 32 weeks: 314 grams
  • 33 - 36 weeks: 391 grams
  • 37 - 40 weeks: 456 grams
  • > 40 weeks: 496 weeks