Development of the embryo: the beginning of life

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Development of the embryo

Before the implantation of the egg in the uterine wall, it is called a fertilized egg or zigote, blastula. The next stage is that ofembryo (up to 8 or 10 weeks after fertilization). Further on, we talk about the fetus. Let's see, then, how it happens development of the embryo during pregnancy.

From embryo to fetus

We talk about the embryo until the end of the 10 week of development and of the fetus beyond this date, but we know that this distinction is only indicative because the development process is continuous and progressive and does not foresee interruptions.


The fetus in the nine months of pregnancy

Photos of the fetus during the nine months of pregnancy

Fertilization and embryonic development

As we read on gynecolink the beginning of pregnancy is made to coincide with the first day of the last menstruation and this is the gestational age but the embryo actually begins its existence on the fourteenth day of the menstrual cycle and therefore the third week of pregnancy (i.e. the third week after the last menstruation) actually corresponds to the first week of the embryo's life (in this case we speak of conceptional age).

In practice, if we want to try to calculate theprecise age of the embryo and therefore the conceptional age will suffice to remove two weeks from the calculation that gynecologists generally do.

Read also: Pregnancy week by week with pictures

Formation of the embryo

The beginning of the life of an individual (understood as a living being with a unique and unrepeatable genetic code (DNA)), is the meeting between a sperm and an oocyte, which takes place in the uterine tube around the 14 day of the menstrual cycle, or in the days of ovulation.

After sexual intercourse, the spermatozoa, ejaculated into the vagina, go up from the vagina into the cervical canal (canal that connects the vagina with the uterus), from here into the uterus and from here into the tubes (in one of which, if ovulation, they will meet the oocyte). From what has been said about their survival, it is clear that after sexual intercourse the woman will have viable and fertilizing spermatozoa inside for about 4-5 days. As already mentioned, in fact, the spermatozoa localized in the cervix survive for 4-5 days and for this period they tend to go up in the direction of the uterus and tubes.

Oocytes are the female germ cells. They are found in the ovary from intrauterine life and every month only one of them he manages to reach maturity. The simultaneous maturation of two or more oocytes is rare. The maturation of an oocyte occurs in the first 13-14 days approximately of the menstrual cycle.

Development of the embryo week by week

The embryo is measured in the first ultrasound in order to evaluate:

  • if it is well implanted in the uterine walls;
  • if there is a heartbeat;
  • la vertex-heel length (as if the fetus were put "on its feet");
  • if the pregnancy started well.

Now let's see how the embryo develops in the first two and a half months of gestation, that is, in the first ten weeks, until it becomes a fetus.

  • Embryo development at 3 weeks: fertilization of the oocyte, formation of the embryo and implantation in the uterine cavity
  • Embryo development at 4 weeks: inside the uterus the gestational sac is formed which will host the embryo, then the embryo itself will gradually become visible, which in this period has the shape of a small cylindrical body. In this first phase of development, specialized areas are formed in the embryo, called somiti which will give rise to the skeleton and muscles. The external part of the embryo, which is in close contact with the uterine walls, is reached by the maternal blood which provides nourishment, thus begins the utero-placental circulation.
  • Development of the embryo at 5 weeks: in the embryo the heart, eyes, first brain structures, liver, sketches of the outer ear, esophagus, stomach, external genitalia and thyroid begin to form. And the first heartbeats occur.
  • Development of the embryo at 6 weeks: the sketches of the upper and lower limbs, the pancreas and the kidneys are formed and the development of the cerebral cortex begins. The umbilical cord is already perfectly formed
  • Development of the embryo at 7 weeks: the process of ossification of the skeleton begins and the neck and head are placed directly on the chest while the hands are on the belly. Hands and feet are more defined and less webbed.
  • Development of the embryo at 8 weeks: the embryo weighs about 1 gram and is about 1,6 cm long, the heart beats rhythmically and the fingers appear. The male or female genitals are morphologically defined.
  • Development of the embryo at 9 weeks: bones, muscles, nerves and large vessels begin to develop. The head is huge, equal to one third of the whole body, and the forehead is prominent, the eyes are covered by the lids which remain closed.
  • Development of the embryo at 10 weeks: the cephalic end of the embryo six slowly separates from the thorax and at this point we can distinguish the sketches of the nose, ears and jaws, so his face will begin to have the human aspect. The first dental buds are formed and the "corpus callosum" is visible, that is the nervous structure that connects the two cerebral hemispheres.
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