During the normal nine months of pregnancy, the fertilized egg develops after a series of phases: the pre-embryo, the embryo and the fetus. The concept of “prenatal development” or “intrauterine” is used to refer to all of these three stages, although the transition from one to the other is gradual and the distinction is practical in nature.
In this article, we will look at the process by which the embryo becomes a baby throughout stages of intrauterine development. Although childbirth is viewed by many as the milestone that marks the start of growth, postnatal development is largely a natural continuation of what happens in the mother’s womb.
Main phases of intrauterine development
The chain of biological steps from the fertilized egg to the formation of the fetus is as follows.
1. Pre-embryonic period
The pre-embryonic phase of intrauterine development, which sometimes also it is called “germinal phase”This is the shortest of the three: it lasts from fertilization to the second week. Since pregnancy is usually not detected until after about a month, the woman is not yet aware of fertilization.
During this time, the fertilized egg (known as the zygote) travels down through the fallopian tube to the uterus, where it is implanted around the eighth to tenth day of gestation. When this happens, the placenta begins to develop.
During this process, the zygote replicates repeatedly. this division it gives rise first to the morula and later to the blastula, Names given to all the cells that will give birth to the embryo according to its degree of development.
During the first week, the future embryo does not develop because it is contained in the zona pellucida, a layer of glycoproteins. Later, already implanted in the uterus, it will begin to grow rapidly from a process of cell differentiation.
The presence of harmful external agents (teratogens), Such as infections, maternal illnesses or certain substances, may cause miscarriages or not affect the pre-embryo at all if given during this phase of prenatal development.
2. Embryonic period
From the third week of gestation, the embryo is known as the gastrula. The cellular layers of the blastula have differentiated to the point that they give rise to the three structures from which the baby’s body will be formed: ectoderm, mesoderm and endoderm.
Throughout intrauterine development the ectoderm will give rise to the nervous system and the epidermis. From the mesoderm will emerge bones, muscles and the circulatory system. In turn, the endodermal cells will differentiate into 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 characteristics, 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 cephalocaudal and proximodistal principles; this means that the upper parts of the body mature first, as well as those closest to the spinal cord. Generally speaking, this pattern will continue to develop during the first years of life.
In the embryonic period the unborn baby is very vulnerable to teratogens; As basic organs and systems develop, harmful agents can cause them irreparable damage by impairing their normal growth.
3. Fetal period
In the fetal stage, the development of the fundamental structures of the body, already present at the end of the embryonic period, continues and consolidates. It is the longest stage of intrauterine development, covering from the ninth week until the moment of delivery.
Biological sex manifests itself during the fetal period through the progressive differentiation of the sexual organs. However, it is determined from fertilization, as 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 regard.
In this period, the organism of the fetus preparing to survive outside the womb. Among other things, 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 and adequate temperature.
The effects of teratogens are milder during 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 common for growth delays and chronic defects of varying severity due to teratogens to occur.