In recent weeks we have been reviewing in great depth everything that happens during the first three weeks of pregnancy. Today we finish this Asalvo biology course with the fourth week. But why have we done it this way? We have realized that we actually know much more about the next two quarters than the initial one. For this reason, at Asalvo, we wanted to delve deeper into the unknown first stage of gestation, perhaps the most magical of all. Let's go there!
Approximately seven days after fertilization and at the beginning of week 4 of pregnancy, the fertilized egg nests in the uterus. At this stage it is decided according to the principle of "all or nothing" whether the pregnancy continues.
Just before nesting, external influences can be harmful to the embryo. If the body cannot compensate for this type of damage, its development stops and the blastocyst is expelled during the next menstruation.
Shortly before nesting, the embryo leaves the membrane that surrounds it and penetrates the endometrium until it is completely covered. Between the embryoblasts and the trophoblasts, the gestational sac and the yolk sac are formed, which is responsible for feeding the embryo in the coming weeks. From now on, the placenta forms around the nested egg. At the end of week 4, the embryo measures only half a millimeter: until birth, the baby reaches a length of between 51 and 52 centimeters and an average weight of about 3,200 grams.
Until the fourth week of gestation, embryonic cells are pluripotent: in theory any organ could develop from any cell. Cellular specialization begins this week. From the embryoblasts, the so-called germ disc develops, which has a rounded shape. It is formed from two germ layers and shortly after three:
During embryonic development, most internal organs (including the lungs, liver, pancreas, urinary bladder and urethra, as well as the internal digestive tract) are formed from the inner germ layer (endoderm). The heart and blood vessels, connective tissue and skeleton, smooth muscles of the intestines, spleen, blood cells and lymphatic system, kidneys as well as gonads and internal sexual organs develop from the middle germ layer (mesoderm). From the outer germ layer (ectoderm) the skin, nervous system and sensory organs are formed.
At week 4 of gestation, many women experience the first physical signs of pregnancy. The nesting of the embryo in the uterus can lead to what is known as implantation bleeding, which is a slight loss of blood. Often this week the breasts begin to stretch and tighten painfully, some women have the skin of the areola darkening.
The production of progesterone slows down the course of all the pregnant woman's body processes to protect the embryo, but it also ensures that she often feels tired and that the first symptoms of constipation caused by pregnancy appear.
Some women already begin to experience the first discomforts in the fourth week: Morning sickness, increased salivation and emotional ups and downs that will accompany them throughout the first trimester of pregnancy.
If we manage to bring together everything we learned today, we know that in week 4 the embryo nests in the endometrium. This week is when the cells specialize: the three germ layers with different types of cells will transform into different organs over the next few weeks and months. Many women begin to experience the first symptoms of pregnancy. Have you found all the information we have explained about the most unknown stage of pregnancy interesting? We read you in the comments!
