Fetal development week by week

Fetal development

Pregnancy is the period of time between conception and birth of the baby, i.e. the period in which the future baby grows and develops inside the mother’s womb. Since it is impossible to know exactly when conception occurs, it was decided (not us, but the international scientific medical community) to coincide with day zero; of pregnancy with the first day of the mother’s last menstrual cycle.

As far as the embryo and the fetus are concerned, there are two strategies for periodising development: by making it correspond to the duration of the pregnancy or by postponing its beginning by 14 days with respect to the beginning of gestation.

In this article, the former of the two strategies has been preferred.

The 1st and 2nd week

As we have seen above, the first week of pregnancy begins with the first day of a woman’s menstrual cycle. In this period the woman is not yet pregnant. But it is at the end of the second week that an egg is released from an ovary. This is when the woman is most likely to conceive.

The embryo at the 3rd and 4th week

This is the period in which the egg eventually meets a sperm in the fallopian tube. When the single sperm enters the egg, conception occurs. The cell resulting from the union of the ovum with a sperm is called a zygote. The zygote contains all the genetic information (DNA) needed to become a child. Half of the DNA comes from the mother’s egg and half from the father’s sperm.

The zygote spends the next few days traveling along the fallopian tube. During this time, it divides to form a ball of cells called a blastocyst . A blastocyst consists of an inner group of cells with an outer shell. The inner group of cells will become the embryo while the outer group of cells will form the placenta. Once the blastocyst reaches the uterus it begins to “fuse” with the endometrium, the inner layer of the uterus: it is first implanted and once implanted, it begins to develop in the mother’s uterus.

At the end of this period the embryo, although still appearing formless, will have already begun to develop the first organs and systems: the heart, the spinal cord and the brain.

The embryo at the 5th week

During the 5th week, all the main systems and organs of the embryo develop, multiply and begin to assume specific functions. This is called differentiation. All blood cells, kidney cells and nerve cells develop. The embryo grows rapidly and the external characteristics of your future baby begin to take shape. Your baby’s brain, spinal cord, and heart continue to develop. The gastrointestinal tract of the embryo begins to form.

It is during this period in the first trimester that the fetus is most at risk of harm from medications, drugs, alcohol, and infections.

The embryo at the 6th – 7th week

It is with the 6th week of pregnancy and the 7th week of pregnancy that the arms and legs begin to appear, like small sprouts. Your future baby’s brain is formed in 5 different areas. Some cranial nerves are already present at this time. The eyes and ears are starting to take shape. The tissue that – later – will become the spine of the future child grows. The heart continues to grow and now beats at a regular rate. All of this can finally be observed with a transvaginal ultrasound.

The embryo at the 8th week

The arms and legs of the embryo have become longer. Hands and feet begin to form. The brain continues to grow. The lungs begin to form.

The embryo at the 9th week

During this week, nipples and hair follicles form in the embryo, arms grow, and elbows develop. With an ultrasound you can already see your toes .

The embryo at the 10th week

The eyelids of the embryo are more developed and begin to close. The outer ears begin to take shape. Facial features become more distinct. At the end of the tenth week of pregnancy we no longer speak of an embryo but of a fetus to refer to the future unborn child.

The fetus from the 11th to the 14th week

The eyelids of the unborn baby close and will not reopen until about the 28th week. His face is well formed. The limbs are long and thin, and nails appear on the fingers and toes. The genitals appear. The liver produces red blood cells. The head is very large, about half that of the child. Your little one can now pack a punch. Tooth buds appear for baby teeth.

The fetus from the 15th to the 18th week

During the period between the fifteenth and eighteenth week of pregnancy, the skin of the fetus is almost transparent. Fine hair – called lanugo – grows on the head. Muscle tissue and bones continue to develop and bones become harder. The fetus begins to move and stretch. The liver and pancreas produce secretions. Your little one now makes sucking movements.

The fetus from the 19th to the 21st week

The fetus can now hear noises and voices, is more active in the womb and continues to move and fluctuate. The mother can feel a flutter in the lower abdomen. By the end of this period, the fetus can swallow.

The fetus at the 22nd week

Lanugo hairs cover the baby’s entire body. Meconium, the first bowel movement of the fetus, is produced in the intestinal tract. Eyebrows and eyelashes appear. The fetus is more active with increased muscle development. The mother can feel the baby move. The heartbeat of the fetus can be listened to with a stethoscope. The nails grow to the end of the little fingers.

The fetus from the 23rd to the 25th week

The bone marrow starts making blood cells. The lower airways of the lungs develop and the baby begins to store fat.

The fetus at the 26th week

Eyebrows and eyelashes are well formed. All parts of the baby’s eyes are developed and fingerprints are forming. The development of the lungs continues, but they are not yet ready to function outside the uterus.

The fetus from the 27th to the 30th week

At 26-30 weeks, the fetal brain grows rapidly. The nervous system is developed enough to control some functions of the body. In these weeks he already has eyelids that open and close and has a rapidly developing respiratory system (still non-functional).

The fetus from the 31st to the 34th week

The fetus starts breathing rhythmically, but its lungs are not fully matured yet. The baby’s bones are fully developed, but are still soft. Your baby’s body begins to store iron, calcium and phosphorus.

The fetus from the 35th to the 37th week

The baby weighs about 2.5 kilograms and continues to gain weight day after day. The skin is not as wrinkled as fat forms under the skin. The baby has definite sleep patterns. Your little one’s heart and blood vessels are complete. Muscles and bones are fully developed.

The fetus from the 38th to the 40th week

Lanugo is gone except on his upper arms and shoulders. Nails may extend beyond the fingertips. Small breast buds are present on both sexes. The hair on the head is now coarser and thicker. In your 40th week of pregnancy, it has been 38 weeks since conception and your baby could be born any day now.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.

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