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Important Changes in Early Development of an Embryo

By Marie Cheour ; Updated June 13, 2017

The embryonic period begins at implantation, when the fertilized egg attaches itself into the uterine lining, and continues through the first eight weeks of pregnancy. This is when most rapid changes in prenatal development occur. All the major organs and body structures start developing during this short period. The baby is also extremely vulnerable during this time and miscarriages are the most common during the first 13 weeks if pregnancy, says the March of Dimes. Prenatal visits should start as soon as the woman finds out that she is pregnant, to ensure that the embryo is developing normally.


Early on, the embryo changes from a free floating ball of cells into an embryo that is connected to the mother’s bloodstream. The egg arrives in the uterus seven to nine days after fertilization. It implants itself into the uterine lining and from this moment on it is called an embryo. After implantation, the embryo is connected to mother's bloodstream and can receive both nourishment and toxins such as nicotine through it.

Development of Placenta

Since receiving nourishment is vital for the embryo, the structures that provide them are the first to develop. They include the placenta, yolk sac and umbilical cord. The placenta helps to nourish the embryo, carries the embryo’s wastes away, and provides immune support for the developing baby. It is important, that the embryo develops its own blood circulation that is separate from the mother’s blood circulation. A thin membrane forms between the embryo's blood and the mother's blood. The mother’s antibodies are too large to go through this membrane and, thus, the mother's immune system cannot attack the embryo.

Developing Organs

During the first month of pregnancy, the embryonic disk forms. All body parts are formed from this simple structure. It consists of three layers of cell. The ectoderm will become the nervous system and skin. The mesoderm will form the skeleton, muscles, circulatory system and internal organs. The endoderm will form the lungs, digestive system, urinary tract and glands.

The brain begins to form at week three of pregnancy. The first month of pregnancy is the time of rapid development for the nervous system. More than 250,000 neurons per minute are formed at this stage of a pregnancy. The heart begins to pump blood during this month as well.

During the second month, these major organs become more distinct. The heart, for example, develops separate chambers.

Changes in the Posture of the Embryo

During the first month of pregnancy the embryo is curled and about a quarter of an inch long. It looks more like a worm than a human baby. By the end of the second month of pregnancy, the embryo’s body proportions change considerably and the embryo gets much more upright position. The embryo grows rapidly during this time, so that at the end of the second month it is an inch long.


The period of the embryo is the most dangerous one when it comes to teratogens, or environmental agents that can damage the embryo. From the moment the embryo implants and connects itself with mother’s blood stream it becomes vulnerable to teratogens.

The damage that certain teratogens cause depends on many factors. The bigger the dose is, the more serious the damage is. The time of exposure is also important. Earlier exposure to teratogens is more damaging, typically. Thus, during the embryonic period, exposure typically results in miscarriage. The embryo is so vulnerable because all the major organs are developing. The same teratogen can cause different kind of damage during different times of pregnancy depending on what organs are developing when the embryo is exposed. Heredity plays a role as well, since some fetuses are more vulnerable to the teratogens for genetic reasons. Moreover, the effect of teratogens is much greater if, in addition to the environmental toxins, the baby struggles with other negative influences such as poor nutrition or lack of medical care.

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