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Some hours after fecundation, the egg iniciates a multiplication process by successive mitotic divisions. During multiplication, the embryo starts to migrate towards the uterus, due to muscles contractions. In consequence of the celular multiplication, a sferical embryo is formed,designated by morula. This embryo arrives to the uterine cavity 4 days after  fecundation, still with the size of the initial egg.

Celular divisions continue happen and the cells start to organize between internal celular mass (which originate the new being) and the trophoblast (which surround the internal celular mass). The embryo, in this phase, is designated by blastocyst.
About 6 to 7 days after the fecundation the trophoblast attaches itself to the internal wall of the uterus, the endometrium. This process is called nesting. In this stage, the embryo is capable of receiving nutrients and, simultaneously other cell divisions occur.

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It is marked by the formation of the neural tube, which will give rise to the baby's nervous system and brain, and other layers of cells responsible for the development of other organs.

Furthermore, the embryo is fixed in the upper part of the uterus and surrounded by the gestational sac, which protects the baby from infection or trauma and is responsible for forming the placenta and amniotic sac.

In the fourth week of gestation, three layers of cells have already formed that will start all of the baby's organs and which grow lengthwise, giving the embryo a more elongated shape. In addition, the umbilical cord starts to form and the transport of nutrients and oxygen to the embryo begins.

The outermost layer of cells, the ectoderm, forms the baby's neural tube, skin, hair, nails, and teeth, and the intermediate layer, known as the mesoderm, forms the heart, blood vessels, bones, muscles, and reproductive organs. The innermost layer of cells, the endoderm, will form the lungs, liver, bladder and digestive system.

At approximately 27 days of gestation, which is equivalent to the penultimate day of the fourth week, the baby already has its spinal column and spinal cord formed, from the cervical to the coccyx.

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Is marked by the beginning of the development of the embryo's organs such as lungs, liver, intestine and kidneys. In addition, the arms and legs are already more developed and the eyes and small folds of tissue that will form the baby's cheeks, jaw, and chin begin to form.

The embryo's heart beats fast and the features that will give rise to the baby's face begin to develop. The embryo's tiny head shows two dark spots, which will give rise to its eyes, and dimples that will form its ears and nostrils. In addition, small folds of tissue below the developing brain will become the baby's tongue, jaw, chin, and neck.

Muscle and bone tissue are forming covered by a thin layer of skin, arms and legs are developing, appearing as tiny sprouts, and the spinal column extends giving the appearance of a small tail that will disappear in a few weeks.

During this phase, the lungs, kidneys, liver and other organs of the digestive, reproductive and urinary systems also begin to form.

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It's the time when the baby starts to move in the uterus
Your baby's lips, fingers and nose are growing
Your baby makes spontaneous moves
150-170 heartbeats per minute:twice as fast as yours
Your baby's gaining weight
Your level of HCG and estrogen is growing and you can have headaches

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-> At the end of week 28, the bone marrow takes over the production of red blood cells, which previously appeared in the spleen and before that in the liver.
-> The pulmonary system is sufficiently developed to perform gas exchanges.
-> The eyes are open, the hair is undone and his nails are visible.

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Your baby measures about 16 cm from head to bottom and weights about 320 grams
It's covered in vernix-a white, creamy substance that protects baby's skin from some fluids
Your baby's fingerprints are formed
Permanent teeth have grown beneath your baby's first teeth
Your baby can hear sounds, even your heartbeat and your voice.

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-> The fetus doesn’t have more enough space to move.
-> His lungs are fully developed.
-> The rate of fat accumulation in the subcutaneous layer is accelerating.
-> The child is over 50 cm tall and weighs approximately 3 kg.
-> This week, the child can arrive at any time on world.
-> Only 5% of babies are born on their due date pregnancy, the remaining 95% come either may sooner or later.

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https://raisingchildren.net.au/pregnancy/week-by-week/second-trimester/20-weeks

https://femmeboutiquemedical.com/ro/saptamana-8-de-sarcina/

https://www.tuasaude.com/desenvolvimento-do-bebe-4-semanas-de-gestacao/

https://www.pampers.com.br/gravidez/gravidez-semana-a-semana/4-semanas-de-gravidez

DIAS DA SILVA, amparo & al. (2022) Terra, universo de vida, Porto e Porto editora