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Biology 2nd Baccalaureate

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The pregnancy

The pregnancy is the period between when fertilization occurs until the time of delivery.

The first symptom of pregnancy is the disappearance of menstruationPregnancy, in humans, lasts about 40 weeks from the first day of the last menstrual period or 38 weeks from fertilization (about 9 months).

The changes that occur in pregnancy can be divided into several stages:

Nidation or implantation

The beginning of pregnancy occurs with the implantation or nesting of the blastocyst in the endometrium, about seven days after fertilization. The blastocyst will develop and originate a new structure in which it is distinguished:

  • The embryo: First cells from which a new organism will emerge .
  • The amnion or amniotic sac: The bag that surrounds the embryo and is filled with amniotic fluid, which protects the embryo during pregnancy.
  • The placenta: its name comes from the Latin, flat cake, due to its appearance in humans. The placenta is an organ with many blood vessels that is created in the wall of the uterus and that allows the exchange of substances between the mother and the embryo, being united by the umbilical cord, through which two arteries and a vein pass. Through the placenta, the mother will provide nutrients and oxygen to the embryo, and this will return waste substances from excretion and carbon dioxide.

Embarazo. Placenta y bolsa amniótica.

By BruceBlaus. When using this image in external sources it can be cited staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Commons File:Blausen 0747 Pregnancy.png) [CC BY 3.0], via Wikimedia Commons

First trimester of pregnancy

Internal tissues and organs begin to form. In the fourth week, the head is already visible and the heart begins to beat. The embryo weighs 1 gram.

From the eighth week, it already receives the name of fetus. The face, eyes and extremities are distinguished.

At the end of the trimester, the fetus already measures almost 10 cm and weighs about 15 grams.

During this trimester, the pregnant woman usually has nausea and vomiting, mood swings and alterations in the sense of smell and taste, but they begin to disappear at the end of the trimester as hormonal changes stabilize.

Second trimester of pregnancy

The fetus's body is covered by fine hair called lanugo. Its organs are quite developed, although still immature. Nervous reflexes already appear, and the mother feels the movements of the arms and legs. The sex of the fetus can be recognized. The embryo measures about 30 cm and weighs almost 1 kilo.

The mother's womb grows remarkably. The breasts also increase in size.

Third trimester of pregnancy

The fetus grows rapidly and the uterus reaches its maximum development. The organs of the fetus mature, especially the lungs, which will allow it to breathe when it is born. He could already live if he was born from the seventh month.

The fetus changes position and is almost always lying face down, fitting its head into the mother's pelvis. At the end of the pregnancy, the fetus measures about 50 cm and weighs about 3 kilos. The growth of the fetus presses and displaces the organs of the mother's abdomen from their natural position.

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Mujer embarazada de más de 8 meses

Pregnant woman of more than 8 months of the youngest model that appears in other images of the course.

Activity: Pregnancy.

Activity: Crossword about pregnancy.

Activity: Pregnancy.

Game: The great sperm race.

Answer in your notebook

11.9.- What is the function of the placenta? What about the amniotic sac?


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