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primer trimestre del embarazo

What happens in the first trimester of pregnancy?

What happens in the first trimester of pregnancy?

Let's know what happens in the first trimester of pregnancy: 0 to 12 weeks

Once fertilization occurs, the embryo is formed and begins to grow thanks to the genetic information from its parents.

The mother with the egg provides 23 chromosomes and the father with the sperm another 23 chromosomes, to generate a cell with 46 chromosomes that will form the new being.

The embryo is located in the mother's uterus or womb, where it will develop and nourish for the next 9 months.

The first period of this new being is in the form of an embryo with characteristics similar to those of a fish before being born, after 9 weeks it becomes a fetus and at 12 weeks of life the human characteristics become evident, measuring approximately 5 to 5.5 cm.

All of this occurs in a water-filled balloon, called the amniotic sac, which widens to protect the developing fetus. While protected in the dark, watery environment of the maternal capsule, the fetus will be completely dependent on the oxygen and nutrients circulating through the mother's bloodstream.

The fetus is connected to this vital source through ducts that communicate with its own blood vessels and form the umbilical cord. These blood ducts leave and enter the body of the fetus at a point that will later be the navel.

At the end of the umbilical cord the blood vessels penetrate the wall of the mother's uterus, where they branch to form a disc-shaped system called the placenta where the exchange of oxygen and nutrients with the mother takes place.

Although the blood of the fetus does not mix with the blood of the mother, they are two currents that flow very closely to each other to achieve this exchange.

It is extremely important to avoid toxins that can alter the formation of the embryo.

In the case of medications, for example, you should consult with your doctor, since the vast majority can cause potential harm.

Intense exercise should be avoided, especially impact or vibration exercises that can cause placental abruption or significant vaginal bleeding.

Common symptoms at this stage

During the first trimester, some symptoms are also common, such as nausea,
Vomiting, dizziness, sleepiness, as well as some colicky pain, are usually temporary and do not put the health of the mother or fetus at risk. Your doctor can help you improve them if necessary.


Gastrointestinal symptoms are caused by the exponential increase in some hormones such as Human Chorionic Gonadotropin and Progesterone.

Fractional diets are recommended, that is, consuming foods every 2 to 3 hours, especially salty ones. There are also very safe medications that can be used when symptoms are moderate to severe. These should always be
formulated by the doctor.


The first trimester of pregnancy requires some care, in order to avoid alterations in the developing embryo and possible complications such as abortion.

Development of the central nervous system

The development of the central nervous system begins very early during pregnancy (around the fourth week). Initially, primitive structures are formed that will develop little by little. By the end of the first trimester the cerebral hemispheres will differentiate.

During this period, neurons also begin to produce in a process called neuronal proliferation. These neurons also begin to migrate to where they belong according to their predetermined functionality (a process called migration).

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