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During the first week of pregnancy the symptoms are still very subtle and few women can really understand that something is changing in their body.
However, it is during the first days after fertilization that the greatest hormonal changes occur, as the body is no longer in a constant menstrual cycle. Thus, some women may refer symptoms such as abdominal cramps, increased sensitivity of the breasts, excessive tiredness, mood swings or repulsion towards stronger smells, for example.
See also which symptoms may appear during the 1st month.

1. Abdominal cramps
This is a very common symptom during a woman's life, which usually happens during periods of greater hormonal change, such as during pregnancy, or simply during menstruation. However, and unlike the menstrual cycle, in pregnancy this symptom is not accompanied by bleeding.
In addition to abdominal cramping, a woman may also notice that her belly is a little more swollen than usual. This is not due to the fetus, which is still in a microscopic embryonic stage, but due to the action of hormones on the tissues of the uterus and the entire female reproductive system.
2. Breast tenderness
Shortly after fertilization, a woman's body enters a phase of major hormonal changes and one of the first signs that can be identified is increased sensitivity in the breasts. This is because breast tissue is very sensitive to hormonal changes, being one of the first places in the body to prepare for pregnancy.
Although sensitivity can be noticed in the first week, many women only report this discomfort after 3 or 4 weeks, along with changes in the nipples and areola, which can become darker.
3. Excessive tiredness
Most pregnant women report the onset of fatigue, or excessive tiredness, only after 3 or 4 weeks, but there are also some reports of women who had unexplained tiredness soon after fertilization.
Usually, this tiredness is related to the increase in the hormone progesterone in the body, which has as a side effect an increase in sleepiness and a decrease in energy during the day.

4. Mood Variations
Mood swings are another symptom that can appear during the first week and that are often not even understood by the woman herself as a sign of pregnancy, being only confirmed when the woman obtains a positive drug test.
These variations happen due to the oscillation of hormones, which can lead the woman to have feelings of joy and, in an immediate moment, to feel sadness and even irritability.
5. Repulsion for strong smells
With the intense variations in hormone levels, women also tend to be more sensitive to smells, and may be repelled by more intense smells, such as perfumes, cigarettes, spicy foods or gasoline, for example.
Like mood swings, these revulsions for strong smells usually go unnoticed, at least until the woman takes the pregnancy test.
How to confirm if you are pregnant
Since many of the symptoms of the first week of pregnancy are similar to those that occur at other times in a woman's life, due to hormonal changes, they should not be taken as a surefire way to confirm pregnancy.
Thus, the ideal thing is for a woman to take a drug test within the first 7 days after her period is late, or to consult an obstetrician to perform a blood test to identify the levels of beta HCG hormones, which is a type of hormone that is only produced during pregnancy.
Better understand when to take pregnancy tests and how they work.
What is the first week of pregnancy?
The first week of pregnancy is considered by the obstetrician as the week from the first day of the last menstrual period. This means that during this week the woman is not actually pregnant, as the new egg has not yet been released and, therefore, cannot yet have been fertilized by the sperm, to generate a pregnancy.
However, what the woman considers to be the first week of pregnancy is the 7 days immediately after the egg is fertilized, which only happens after 2 weeks of the gestational age considered by the doctor. Thus, the week that is popularly considered the first week of pregnancy actually happens around the third week of pregnancy according to the doctor's calculations, or the 3rd week after menstruation.