The 3 phases of intrauterine or prenatal development

During the normal nine months of pregnancy, the fertilized egg develops following a series of phases: the pre-embryonic, 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 of the practical.

In this article we will analyze the process by which the embryo develops into a baby throughout the phases of intrauterine development. Although childbirth 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 utero maternal.

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Main phases of intrauterine development

The chain of biological stages that go from the fertilized ovum to the formation of the fetus are the following.

1. Pre-embryonic period

The pre-embryonic phase of intrauterine development, which

sometimes it is also called the "germ phase"is the shortest of the three: it lasts from fertilization to the second week. Since pregnancy is not usually detected until after about a month, the woman is not yet aware of fertilization.

In this period of time the fertilized ovum (known as zygote) descends through the tube of fallopian until reaching the uterus, where it implants approximately between the eighth and tenth day of gestation. When this happens, the placenta begins to develop.

During this process the zygote replicates itself repeatedly. This division gives rise first to the morula and later to the blastula, names that are given to the set of cells that will give rise to the embryo depending on 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 process of cellular differentiation.

The presence of harmful external agents (teratogens), such as infections, maternal illnesses or certain substances, can cause miscarriages spontaneous or not affect the pre-embryo at all if it occurs during this phase of development prenatal.

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2. Embryonic period

From the third week of gestation, the embryo is known as a gastrula. The cell layers 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, mesoderm, and endoderm.

Throughout intrauterine development the ectoderm will give rise to the nervous system and the epidermis. From the mesoderm the bones, muscles and circulatory system will emerge. On the other hand, the endoderm cells will differentiate as cells of the respiratory and digestive systems.

The embryonic period is considered to last up to eight and a half weeks of gestation; Although there is no specific point that distinguishes when the embryo becomes a fetus, shortly after two months it is already possible to identify a future baby.

During this phase the embryo acquires the basic physical features, both internally and externally. Thus, the head, face, limbs, body 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 continue to grow during the first years of life.

In the embryonic period the future baby is very vulnerable to teratogens; As fundamental organs and systems are developing, harmful agents can cause irreparable damage to them by altering their normal growth.

3. Fetal period

In the fetal stage the development of the fundamental structures of the body continues and is consolidated, which were already present at the end of the embryonic period. It is the longest stage of intrauterine development, encompassing from the ninth week to the time of delivery.

Biological sex manifests itself 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 boy, although there is some variability in this sense.

In this period the organism of the fetus prepares for survival outside the womb. Among other aspects, the immune system is strengthened by obtaining maternal antibodies and a layer of fat appears on the skin with the function of keeping the body at a stable temperature and adequate.

The effects of teratogens are milder in the fetal period than in the embryonic one. The tissues of the body are already formed, so the potential interference in their development is less, although it continues being common that there are delays in growth and chronic defects of variable severity due to teratogens.