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The embryonic stage begins at about 2 weeks postconception and lasts until 8 weeks. At this point, the conception is called an embryo. The embryo is surrounded by a support system that connects it to the mother. This support system includes two membranes as well as the placenta and umbilical cord. You can think of the membranes as two sacs, one inside the other. The chorion is the outer one, and the connection that it establishes with the uterus gives rise to the placenta. The inner one, called the amnion, surrounds the developing embryo and is filled with amniotic fluid to cushion and protect the embryo (later called the fetus). The placenta performs the essential functions of bringing oxygen and nutrients from the mother to the developing organism throughout the pregnancy through the umbilical cord. Later in the pregnancy, it also will carry away fetal waste products. Both functions are described in more detail in the next section.

Embryonic stage: The prenatal stage that lasts from 2 weeks to 2 months postconception.

Placenta: The organ that supports a pregnancy by bringing oxygen and nutrients to the embryo from the mother through the umbilical cord and carrying away fetal waste products.

During the embryonic stage, all the major organ systems of the body are laid down in a process called organogenesis, meaning the genesis or beginning of the organs. The inner cell mass differentiates into three layers, each of which goes on to become different organs and structures, as shown in Figure 4.3. The outermost layer, the ectoderm (ecto means “outside” or “external”), becomes the skin, the sense organs, and the brain and spinal cord. The innermost layer, the endoderm (endo means “within” or “inner”), goes on to become the respiratory system, the digestive system, the liver, and the pancreas. The layer between these two other layers, the mesoderm (meso means “middle”), becomes the muscles, bones, blood, heart, kidneys, and gonads.

Organogenesis: The process in prenatal development by which all of the major organ systems of the body are laid down.

Description

Figure 4.3 Differentiation of the inner cell mass.

Source: DevelopmentalBiology.net (n.d.). Ectoderm, mesoderm, endoderm. Retrieved from http://www.developmentalbiology.net/images/neurulasm.jpg

At 4 weeks, a primitive heart begins beating, and at about 5 to 6 weeks, spontaneous movement begins, although the mother cannot yet feel this movement. By 8 weeks, all the major organs and structures of the body have been laid down and the brain and nervous system are developing rapidly, although the organs will need quite a bit more time before they are able to do the work they are intended to do. Anything in the prenatal environment that disrupts the process at this point can cause damage that is both severe and irreversible. Unfortunately, at this point the woman may not even realize she is pregnant which is why women of childbearing age are urged to be cautious about decisions they make that could affect the health of a pregnancy even if they do not think they are pregnant.

During the prenatal period, development moves from the head region down through the body. This is called cephalocaudal development (cephalus means “head,” and caudal means “tail”). Consequently, throughout the pregnancy, but especially in the early months, the development of the upper half of the embryo (and later of the fetus) is more advanced than the lower half.

Cephalocaudal development: A principle whereby development proceeds from the head region down through the body.

Embryonic development. At 5 weeks postconception, an embryo is only about one quarter of an inch long, but a primitive heart has already formed. You can see how the cephalocaudal principle affects prenatal development. The head and arms are considerably more developed than the lower parts of the body.

Professor Pietro M. Motta/Science Source

Some women have a prenatal ultrasound between 6 and 10 weeks of the pregnancy. During the test, high-frequency sound waves pass through the woman’s uterus and the returning sound creates a video image that shows the size, shape, and position of the embryo or fetus in the womb. If the test is repeated at about 20 weeks, it can confirm that the fetus is continuing to grow at the expected rate and the physician can check for any physical abnormalities, confirm if there is more than one fetus, determine whether other aspects of the pregnancy appear to be normal, and visually determine the sex of the baby. Because this is a medical procedure, the American Congress of Obstetricians and Gynecologists (ACOG, 2017b) recommends that ultrasounds be performed only when there is a medical reason for them, so women with low-risk pregnancies may not have this test at all.

Ultrasound: A prenatal test that uses high-frequency sound waves to create an image of the developing embryo’s size, shape, and position in the womb.

Prenatal testing. This technician is performing a prenatal ultrasound for this couple. A standard ultrasound produces the top image, but the 3-D ultrasound image at the bottom can show some birth defects that would not be apparent on a standard ultrasound. Both of these images are of a normally developing fetus.

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BSIP / Contributor via Getty Images

BSIP / Contributor via Getty Images