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Month 9 and Birth

Week 5

The emergence of a body plan

Human Prenatal Age

Post-conception week 3
Days 14-20
Gestational Week 5

Highlights

If a woman has regular menstrual cycles, she likely realizes that her period is late.
The inner cells of the embryo now form three layers: The top layer becomes the skin, nervous system, eyes and ears. The middle layer contributes to the muscles, bones, kidneys and reproductive system. The inner layer contributes to the baby’s lungs and intestines.
Brain tissue and heart tissue start to develop.

This is no ball of cells! The complexity of the embryo after the fifth week gestation gives the developing human a well-defined blueprint. Cells form distinct layers and the entire body plan begins to emerge. Each cell’s position will give it a specific destiny based on the chemical messages surrounding it.

Cell layers

Remember how the zygote and the early embryonic cells could become almost any cell in the human body? Now the cells start to specialize based on their position in the embryo. As can be seen in the picture, the embryo becomes wide and flat. A cross section of the embryo reveals three different layers: endoderm, mesoderm, and ectoderm.¹

Different body systems form from each layer. Cells from the ectoderm will become the skin, hair, and fingernails. A specialized layer of ectoderm, called neuroectoderm, becomes the nerves and brain. Cells from the middle layer, called mesoderm, will become the heart, kidneys, bones, blood vessels and muscles. Cells from the endoderm, near the yolk sac, become major parts of the intestines, pancreas, liver, and lungs.²

(Image Credit: Nik Spencer/Nature)

The early nervous system

At 18 days after conception, scientists observe the first sign of the developing brain. Neuroectoderm tissue thickens and forms the neural plate. The neural plate will fold together to become the neural tube, which later becomes the brain and spinal cord.³

The early circulatory system

The circulatory system also starts developing remarkably early. Early blood cells start forming in the yolk sac, early blood vessels start to form a network throughout the embryo, and the embryo’s earliest heart cells appear by 18 days after conception.⁴

Dive Deeper

The Placenta

The placenta and umbilical cord form as maternal and embryonic tissues interact...

Previous Stage

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Sources

Sadler, Thomas W., Medical Embyrology. 14th ed., 2019; Mark Hill, “UNSW Embryology, Week 3,” 2020, https://embryology.med.unsw.edu.au/embryology/index.php/Week_3.
Moore, Keith L, TVN Persaud, and Mark G. Torchia. The Developing Human, Clinically Oriented Embryology. 10th ed. Philadelphia: Elsevier, 2016; EHD: 2 to 3 weeks, “2 to 3 Weeks | Prenatal Overview,” accessed June 19, 2020, https://www.ehd.org/dev_article_unit3.php.
Sadler, Thomas W. Medical Embyrology. 14th ed., 2019.
Moore, Keith L, TVN Persaud, and Mark G. Torchia. The Developing Human, Clinically Oriented Embryology. 10th ed. Philadelphia: Elsevier, 2016.

See Full Bibliography

All images and illustrations on The Voyage of Life were reviewed by credentialed scientists for accuracy.

Medical art animation was used to provide a schematic representation of fetal development, with some features, including timing, not to scale. The animation was chosen as an ethically uncompromised guide to fetal development. There are many real human images at each developmental age showing the most accurate fetal forms throughout development. A concerted effort was made to use images and material in which the human embryo or fetus did not undergo any known harm.

Page last updated on August 22, 2023

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Week 5