The development of the foetal organ system
The growth of cells per organ is usually incomplete and requires the remaining 5 months of pregnancy to fully develop. Even at birth, certain structures, especially the nervous system, kidneys, and liver, lack complete development, as described later.
In just one month after fertilization of the egg, the gross characteristics of all the different organs of the foetus begin to develop, and during the next 2-3 months most of the characteristics of the organs. different has been set. After the fourth month, the organs of the foetus are roughly identical to those of the foetus. However, the growth of cells per organ is usually incomplete and requires the remaining 5 months of pregnancy to fully develop. Even at birth, certain structures, especially the nervous system, kidneys, and liver, lack complete development, as described later.
The heart begins to beat from the fourth week after fertilization, contracting at a rate of about 65 beats per minute. This rate steadily increases to about 140 beats/min soon after birth.
Formation of blood cells
Nucleated red blood cells begin to form in the yolk sac and mesothelial layer of the placenta around the third week of foetal development. A week later (fourth to the fifth week) formation of nucleated red blood cells by foetal mesothelial and by mesothelial of blood vessels. At week 6, the liver begins to form blood cells, and by the third month, the spleen and the body's lymphoid tissues begin to form blood cells. Finally, from the third month, the bone marrow gradually becomes the main source of red blood cells, as well as most of the white blood cells, except for the production of lymphocytes and plasma cells in the lymphoid tissue.
Respiration cannot occur during foetal life because there is no air to breathe in the amniotic cavity. However, respiratory motility is attempted beginning at the end of the first trimester of pregnancy. Tactile stimulation and especially asphyxia cause respiratory effort.
During the last 3-4 months of pregnancy, foetal respiratory motility is mainly inhibited, the reason is unknown, and the lungs remain almost completely collapsed. Respiratory depression during the last months of pregnancy prevents the lungs from being flooded with fluid and faecal debris excreted by the foetal alimentary canal into the amniotic fluid. Also, the small amount of fluid secreted into the lungs by the alveolar epithelium increases up to the time of birth, thus keeping only clear fluid in the lungs.
Most of the foetal reflexes, including the spinal cord and including the brain stem, appear between the third and fourth months of gestation. However, nervous system function including the cerebral cortex is still only developed at an early stage even at birth. In fact, the myelination of several important neural regions of the brain is completed only about a year after birth.
In the middle of pregnancy, the foetus begins to digest and absorb large amounts of amniotic fluid, and in the last 2-3 months, the digestive function is like that of a normal newborn. At this time, small amounts of fences are continuously formed in the gastrointestinal tract and excreted from the anus into the amniotic fluid. The stool is composed partly of swallowed amniotic fluid and partly of mucus, epithelial cells, and secretion products remaining from the gastrointestinal epithelium and glands.
Foetal kidneys begin to excrete urine during the second trimester, and the urine produced by the foetus makes up about 70-80% of the amniotic fluid. Abnormal kidney development or severe renal dysfunction significantly reduces the formation of foetal amniotic fluid (oligohydramnios) and can lead to foetal death.
Although the foetal kidney forms urine, the renal control system that regulates extracellular fluid volume and electrolyte balance, and especially acid-base balance, is virtually non-existent until the late foetal stage and is undeveloped. complete until a few months after birth.