The 3 phases of intrauterine or prenatal development: from zygote to fetus

From the fertilization of the ovum to the formation of the fetus there are a series of decisive phases.

During the nine months that pregnancy normally lasts, the fertilized ovum develops in a series of phases: the preembryonic phase, the preembryonic phase, the preterm stage, the pre-embryonic stage and the fetal stage. a series of phases: pre-embryonic, embryonic and fetal.. The concept of "prenatal" or "intrauterine development" is used to refer to these three stages as a whole, although the passage from one to the other is progressive and the distinction is of a practical nature.

In this article, we will discuss the process by which the embryo develops into a baby through the stages 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 that which takes place in the mother's womb.

Main stages of intrauterine development

The chain of Biological stages from the fertilized egg to the formation of the fetus is as follows.

Pre-embryonic period

The pre-embryonic phase of intrauterine development, which is also sometimes sometimes also referred to as the "germinal phase", is the shortest of the three phases.is the shortest of the three: it lasts from fertilization to the second week. Since pregnancy is not usually detected until after about one month, the woman is not yet aware of fertilization.

During this period of time the fertilized egg (known as the zygote) descends through the fallopian tube until it reaches 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 repeatedly self-replicates. This division gives rise first to the morula and later to the blastula.These are the names given to the set of cells that will give rise to the embryo according to their 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, once implanted in the uterus, it will begin to develop rapidly through a process of cell differentiation.

The presence of external harmful agents (teratogens), such as infections, diseases of the mother and the baby's mother.such as infections, maternal diseases or certain substances, may cause spontaneous abortions or may not affect the pre-embryo at all if they occur during this phase of prenatal development.

2. Embryonic period

From the third week of gestation the embryo is known as the gastrula. The blastula cell layers have differentiated to the point of giving rise to the three structures from which the baby's body will form: the ectoderm, mesoderm and endoderm.

Throughout intrauterine development the ectoderm will give rise to the nervous system and the epidermis.. The mesoderm will give rise to bones, muscles and the circulatory system. The cells of the endoderm will differentiate into cells of the respiratory and digestive systems.

The embryonic period is considered to last until 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 possible to identify a future baby.

During this phase the embryo acquires the basic physical features, both internally and externally.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 be maintained 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 still 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, which were already present at the end of the embryonic period, continues and consolidates. This is the longest stage of intrauterine development, spanning from the ninth week to the time of delivery. from the ninth week to the time of delivery..

Biological sex is manifested 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 case a boy, although there is some variability in this regard.

During this period the fetal organism prepares for survival outside the uterus. 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 maintaining the body at a stable and adequate temperature.

The effects of teratogens are milder in the fetal period than in the embryonic period. The body's tissues are already formed, so the potential interference with their development is less, although growth retardation and chronic defects of varying severity due to teratogens are still common.