1st -4th week
The date of the first day of your last period is important because your pregnancy will be calculated from this date, even though conception doesn’t usually take place until around two weeks later. This means that although pregnancy is described as 40 weeks, you won’t actually have conceived until around the end of the second week.
After ovulation at the end of the 2nd week of pregnancy, the egg is released into the fallopian tube and is destroyed if it is not fertilized within 12-24 hours. Sperm enter the uterine cavity through the cervix after sexual intercourse and then enter the fallopian tubes, where fertilization takes place.
The genetic material in each cell is organised into particles, called chromosomes. Every body cell has 46 of these chromosomes. The only cells that have 23 chromosomes are the eggs in the female and the sperm in the male, therefore, with fertilisation, the genetic material they carry is united, with the result that the zygote carries 46 chromosomes.
The cell, which results from the union of the egg with the sperm, is called a zygote, the creation of which marks the beginning of the third week of pregnancy. The zygote is surrounded by a membrane (transparent zone) in which it begins to divide into identical cells, for example on the second day it divides into two identical cells, then into four cells and on the third day into about eight such cells.
Around the fifth day, the fetus is transferred to the endometrial cavity, where a fluid-filled cavity, the blastocyst, begins to form. Two groups of cells are located in the blastocyst: the embryoblast inside, from which most of the embryo is formed, and the trophoblast outside, from which the membranes are formed, covering the embryo and the placenta. Towards the end of the third week of pregnancy, the blastocyst enters the endometrial cavity and the implantations is begun.
At the beginning of the 4th week (8-14 days following the fertilisation) the wall of the uterus is covered by a thick layer rich in blood vessels and nutrients, ready for the implantation of the fetus. At this stage, the fetus is partially implanted in the endometrium and about twelve days after the fertilisation of the egg, the fetus is almost completely covered by the endometrium. During the full implantation of the fetus around the 13th day of pregnancy, but there is a possibility at the site of implantation to appear local bleeding and bleeding from the vagina.
5th and 6th week
During the fifth week of pregnancy until the 10th week, the genesis of the various organs of the fetus and the formation of its external characteristics begins. During this period, the fetus shows the greatest sensitivity to the action of different factors that could cause abnormalities in its development, such as drugs, alcohol and various infections, such as rubella.
In the 6th week of pregnancy the fetus is about 4 mm long and has begun to acquire a curvature. The most dramatic change during this week is the beginning of the formation of the fetal heart, which begins to beat and promotes blood circulation. These heartbeats can be detected by ultrasound.
The neural tube on the dorsal surface of the fetus will form both the spinal cord and the brain. It is also marked from the beginning of the formation of the ears and the mouth, but also of various features of the face and neck. At the same time, the formation of internal organs of the fetus begins, such as the lungs, pancreas, liver and spleen, as well as the kidneys.
7th and 8th week
The length of the fetus at this stage is about 9 millimeters. During the seventh week of pregnancy, the fetal head, eyes and nostrils begin to shape, as well as the formation of structures in the brain, stomach and kidneys. Finally, during the 7th week of pregnancy, the blood circulation between the fetus and the structures that will develop in the placenta begins to settle.
During the 8th week of pregnancy, the formation of the lungs begins and the development of the brain continues. Moreover, at this stage we have the formation of the upper lip and the nose of the fetus. Both the upper extremities and the lower extremities become even longer, while we begin to distinguish the hand, the foot and the small toes, which at this stage are glued together through a thin membrane. Finally, during the eighth week of pregnancy, the formation of the genitals of the fetus begins. By the completion of the 8th week of pregnancy the fetus has a length of 11 to 14 mm.
9th and 10th week
By the end of week 9, the fetus is 19 to 20 millimeters long. The elbow joint is formed in the upper extremities and the formation of the fingers begins in the lower extremities. The precursors of all the organs of the fetus have already been formed, so we can identify the basic organs of the fetus at this stage.
At the end of this week the fetus will be about 3 cm long. The head of the fetus acquires a more spherical shape, while the formation of its neck begins.
11th and 12th week
At the beginning of the 11th week of pregnancy, the body of the fetus will begin to grow in length at a faster rate, while towards the end of this week the formation of the external genitalia of the fetus begins.
During the 12th week of pregnancy, the fetus develops reflex movements. The fingers of both hands and feet begin to open and close. The intestine of the fetus protrudes into the umbilical cord, ie out of the abdomen. The kidneys begin to produce urine and it is possible to detect the bladder on ultrasound. The eyes, which were originally located on the sides of his head, have now moved to its face and his ears have taken their final position. At the end of this week the fetus is about 5.4 cm long from the top of its head to the coccyx.
13th week
During the 13th week the intestine enters the abdomen of the fetus. The production of urine has already started on week 12th, but now it is such that the fetus begins to pass urine within the amniotic sac. At the same time, the formation of the bones of both the head and the ends of the fetus begins. From the 13th week of pregnancy, the head has a length equal to one third of the total length of the fetal body. At the end of this week the length of the fetus is about 7.5 cm.
