Embryonic period, growth and development

Embryonic period, growth and development

The embryonic period is the phase in which the formation of all the apparatus and systems of the embryo occurs, it is a process known as organogenesis. This phase comprises from the fourth to the eighth weeks.

Pregnancy begins with fertilization, which is the union of male and female sex cells: the egg and the sperm. At this time the embryonic period begins until the eighth week of pregnancy .


For fertilization to occur, the woman must be in the ovulation period.

Ovulation occurs when an egg is released from one of the ovaries, about halfway through the menstrual cycle . If at this precise moment, a sperm meets the egg in the fallopian tubes, fertilization can occur. The zygote is the cell resulting from the union of the sperm with the egg. It is a cellular structure that has all the genetic information necessary to develop and become a baby.

At fertilization, the sex of the baby is determined . The egg only has an X chromosome but the sperm has an X or Y chromosome. Depending on the chromosome provided by the sperm, the baby will be a boy or a girl:

  • X Chromosome (ovum) + X Chromosome (sperm) = GIRL
  • Chromosome X (ovum) + Chromosome Y (sperm) = BOY

Each baby is unique, with its own DNA, which is made up of 46 chromosomes that will determine the baby’s genetic heritage.

embryonic period

First week

The fertilized egg divides for the first time 24 hours after conception, it is what we call a zygote . Over the next few days, it continues to divide and forms a ball of cells called a blastocyst . At 5 days, the blastocyst cells separate into two groups : those that will form the placenta and those that will form the embryo (stem cells).

At the end of this first week, the fertilized egg travels down the fallopian tubes to reach the uterus on the fifth day. It anchors in the wall of the uterus (implantation) at the end of the first week and will spend nine months there.

The mother’s uterus has changed and adapted for pregnancy. The inner wall of the womb has become softer and thicker to favor the implantation of the blastocyst, which is necessary for the embryo to receive the necessary nutrients to grow.

Second week

The embryo that measures 0.2 millimeters begins to form . At this stage, the embryo has a rounded shape and is made up of two layers : the endoderm and the ectoderm. The cells multiply very quickly and acquire a specific function.

  • From the endoderm will form: skin, the inner lining of the respiratory and digestive apparatus, the bladder, eardrum, thyroid, pancreas and liver.
  • From the ectoderm : nervous system, skin, sense organs (eyes, ears, nose), nails, tooth enamel, pituitary, hair, mammary and sweat glands.

Third week

A third layer appears in the embryo, the mesoderm , from this tissue it will derive:

  • The musculoskeletal system (bones, muscles and cartilage)
  • The vascular system (heart, veins and arteries)
  • blood cells
  • The genital and urinary system (except the bladder)
  • The spleen and adrenal glands

Chorionic villi are formed that will connect with the mother’s blood vessels. In this way, the embryo obtains the food and oxygen it needs to grow. On the other hand, in the trunk of the embryo, organs such as the intestine, liver, kidneys and lungs are hollowed out.

Fourth week

The embryo grows one millimeter each day (it measures about 5 millimeters) and the development of:

  • Extremities (arms, legs, muscles, and bone)
  • The embryo’s heart (mass of elongated muscle cells) begins to beat at about 22 days. This rudimentary heart will be in charge of distributing food and oxygen to all parts of the embryo so that it can grow. The embryonic heart beats at about 150 beats per minute.
  • The digestive system is differentiated into its different parts (esophagus, stomach and intestine).
  • The sense organs appear, that is, the eye sockets and ears are formed.
  • The neural tube begins to develop , which will become the baby’s nervous system (brain and spinal cord) and spinal column.

fifth and sixth week

The embryo gradually acquires human or baby form. It grows very fast and is now the size of a chickpea (5-6 millimeters). Its head is very large compared to the rest of the body and the extremities are very short:

  • In the face the eyes, nose, mouth and ears are perfectly distinguished .
  • Fingers still not distinguishable
  • The arms and legs can begin to move at the end of the sixth week.

seventh week

The embryo measures about 22 millimeters . The tissues and organs formed in the embryonic phase mature :

  • The heart already has four chambers
  • Palate and tongue begin to form
  • The placenta increases in size to properly nourish the baby
  • The umbilical cord grows long and widens
  • Nipples and hair follicles form
  • Elbows and fingers can already be seen
  • The digestive system and the urinary system of the fetus (which were the same structure) are separated
  • Neurogenesis or production of neurons occurs .

At the end of this week, the embryo already measures 1 centimeter and these are the most delicate weeks of the baby because some medications, alcohol, nicotine, caffeine or radiation (such as X-rays) can affect it.

eighth week

The embryonic period officially ends and the embryo is renamed a fetus . The outline of all the organs of the future baby has been formed and already begins to have a human form.

  • The baby’s face is now more defined and has eyelids, a budding nose and an upper lip.
  • The body becomes longer , more and more human-shaped, and the skin is translucent, so the skeleton can be seen. But the bones are not calcified, they are made of cartilage, that is, they are soft.

The embryo measures between 4 and 5 centimeters and weighs 9 grams. The circulation through the umbilical cord is very well developed.

What you should know:

  • Before the fourth week the embryo has a round shape made up of different layers, endoderm, ectoderm and mesoderm.
  • In the fourth, fifth and sixth weeks the embryo begins to develop limbs and take on the shape of a baby.
  • The embryo is renamed a fetus in the eighth week.
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