The pregnancy lasts on average 38 weeks from the date of fertilization, or 40 weeks if we consider the date of the first day of the last menstruation.
The gestation period is usually divided into three phases .
The first trimester of pregnancy
The first trimester of pregnancy goes from the beginning until the twelfth week. This phase begins with the fertilization of the egg by a spermatozoon.
The egg nestles at the level of the uterine mucosa and undergoes a series of important modifications. The cells multiply and differentiate, forming the amniotic sac and the placenta: the amniotic sac allows the embryo that is forming to feed, waiting for the placenta to develop; once the placenta has developed, most of the nourishment will be supplied to the embryo via the umbilical cord and maternal blood.
At the end of the sixth week, during the ultrasound phase it is already possible to appreciate the heartbeat because the heart and blood vessels have already formed. Subsequently the other organs are also formed; at the end of the first quarter most of them are already fully developed.
In the first phase of pregnancy, it is normal for the woman to slightly increase in weight and for the abdomen to appear slightly dilated. In the first trimester the increase in weight should still be limited (1-2 kilos) and is mainly due to the greater retention of liquids.
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The second trimester of pregnancy
During the second phase of pregnancy, that is between the thirteenth and twenty-fourth week of gestation, the fetus grows in size and the organism is almost completely matured.
The child moves energetically and the mother feels its presence more and more. In addition, the subcutaneous fat begins to accumulate and hair appears on the head and skin of the fetus. The placenta is fully formed and ensures sufficient blood and oxygen flow to complete the development of the child.
At the end of the second trimester, the expectant mother begins to increase more rapidly in weight and to acquire the characteristic physical form of pregnant women.
The third trimester of pregnancy
The last stage of pregnancy is the one that goes from the twenty-fifth week of gestation to delivery . The fetus is now very active and the organs are almost entirely formed, missing only the complete maturation of the lungs that will take place precisely in the third trimester.
In this final phase, the fetus should position itself correctly for the birth. At the time of birth, which should normally occur between the 37th and 42nd week, the fetus is on average 50 cm long and weighs on average 3.2 / 3.4 kilos.
In the last trimester of pregnancy, the belly of the pregnant woman takes on the characteristic shape that we all know and increases considerably in volume; the uterus, in fact, at the end of pregnancy will have undergone an increase of almost five times its normal size, passing from about 7 cm in length before the pregnancy, to about 33 cm in the ninth month.
