The 3 phases of intrauterine or prenatal development: from the zygote to the fetus
During the nine months that pregnancy normally lasts, the fertilized egg develops following a series of phases: the preembryonic, the embryonic and the fetal . The concept of "prenatal development" or "intrauterine" is used to refer to these three stages as a whole, although the passage from one to another is progressive and the distinction is practical.
In this article we will analyze the process by which the embryo becomes a baby along the phases of intrauterine development . Although birth is understood by many people as the milestone that marks the beginning of growth, postnatal development is largely a natural continuation of what takes place in the womb.
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Main phases of intrauterine development
The chain of biological stages that go from the fertilized ovule to the formation of the fetus are the following.
1. Pre-embryonic period
The preembryonic phase of intrauterine development, which sometimes it is also called "germinal phase" , is the shortest of the three: it extends from fertilization to the second week. Since the pregnancy is not usually detected until after about a month, the woman is not yet aware of the fertilization.
In this period of time the fertilized egg (known as zygote) descends through the fallopian tube until it reaches the uterus, where it is implanted approximately between the eighth and the tenth day of gestation. When this happens, the placenta begins to develop.
During this process the zygote self-replicates repeatedly. This division first place the morula and later the blastula , names that are given to the set of cells that will give rise to the embryo according to its degree of development.
During the first week the future embryo does not grow because it is contained within the zona pellucida, a layer of glycoproteins. Later, already implanted in the uterus, it will begin to develop rapidly from a cell differentiation process.
The presence of external harmful agents (teratogens) , such as infections, diseases of the mother or certain substances, can cause spontaneous abortions or not at all affect pre-embryo if it occurs during this phase of prenatal development.
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2. Embryonic period
From the third week of gestation the embryo is known as gastrula. The layers of cells of the blastula have differentiated to the point of giving rise to the three structures from which the baby's body will be formed: the ectoderm, the mesoderm and the endoderm.
Throughout the intrauterine development the ectoderm will give rise to the nervous system and the epidermis . From the mesoderm, bones, muscles and the circulatory system will arise. On the other hand, the cells of the endoderm will differentiate as cells of the respiratory and digestive systems.
It is considered that the embryonic period lasts until eight and a half weeks of gestation; Although there is no specific point that distinguishes when the embryo becomes a fetus, soon after two months it is already possible to identify a future baby.
During this phase the embryo acquires the basic physical traits , both internally and externally. Thus, the head, face, limbs, bodily systems and internal organs begin to develop, and the first movements also appear.
Intrauterine development follows the cephalo-caudal and proximal-distal principles; This means that the upper parts of the body mature first, as well as those closest to the spinal cord. Broadly speaking, this pattern will remain in growth during the first years of life.
In the embryonic period the future baby is very vulnerable to teratogens ; Since the fundamental organs and systems are being developed, harmful agents can cause irreparable damage to them by altering their normal growth.
3. Fetal period
In the fetal stage continues and consolidates the development of the fundamental structures of the body, which were already present at the end of the embryonic period. It is the longest stage of intrauterine development, encompassing from the ninth week until the moment of delivery .
The biological sex manifests during the fetal period through the progressive differentiation of the sexual organs. However, it is determined from fertilization, since it depends on whether the successful sperm carries an X or Y chromosome; in the first case the baby will be a girl and in the second a child, although there is some variability in this sense.
In this period the organism of the fetus is prepared for survival outside the uterus . Among other aspects, the immune system is reinforced by obtaining maternal antibodies and a layer of fat appears on the skin with the function of maintaining the body at a stable and adequate temperature.
The effects of teratogens are milder in the fetal period than in the embryo. The tissues of the body are already formed, so the potential interference in their development is less, although it is still usual that there are delays in growth and chronic defects of varying severity due to the teratogens.
