The 40 weeks of pregnancy

Fonte: shutterstock

La pregnancy lasts an average of 40 weeks, About 280 days.

A pregnancy is considered full term if the birth occurs between the 37th week and the 41st week.

Read also: Calculate the expected date of delivery and Calculation of the weeks of pregnancy

By convention, doctors mark the first day of the last menstruation as the onset of pregnancy, because it is a more precise reference than the calculation of actual conception.

Il calculation of the weeks of pregnancy it is defined by the gestational age (or epoch), and starts as mentioned from the first day of the last menstruation.
For this reason, the calculation of the weeks of pregnancy begins when the woman is not yet pregnant, but at least two weeks before conception: starting from the first day of the last menstruation it is possible to know, albeit with a certain margin of error, when the baby is born.

Starting from the first week, let's see together the evolution that the embryo, later the fetus, will follow until the moment of birth.


Pictures of pregnancy from conception

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After fertilization and the implantation of the embryo in the uterine cavity, complex transformations take place that start the path that leads from conception to formation ...

How to calculate the weeks of pregnancy? The podcast

How to calculate the weeks of pregnancy? The answer of Dr. from STUDIO INTRECCIO TREVIGLIO, Multidisciplinary study of personal services. 

In this article

  • First quarter
  • Second quarter
  • Third quarter

First trimester of pregnancy

After fertilization and implantation of the embryo in the uterine cavity, complex transformations take place that initiate the path that leads from conception to the formation of the embryo and placenta: the cell begins to multiply through successive subdivisions, and soon all organs and parts of the body will develop. Growing rapidly, the fetus will soon be the size of a bean and move constantly. The heart is beating fast and the intestines are forming. Just as the earlobes, eyelids, mouth and nose are taking shape.

2 week of pregnancy


It's a potentially life-changing week. At this stage the egg has not yet been fertilized, but the path to birth starts here. During ovulation, if the egg meets the sperm, the pregnancy will start.

3 week of pregnancy

Picture of the fetus at 3 weeks

From the meeting between the ovum (female gamete) and the sperm (male gamete) the zygote is formed, the first cell of the new organism, which can start its journey towards the endometrium from here: cell division begins. When the zygote is implanted it will be called a blastocyst and it measures from 0,1 to 0,2 millimeters.

4 week of pregnancy

Picture of the fetus at 4 weeks

The cells of the blastocyst that has nested in the endometrium are continuing the division process: one part of these cells will form the fetus, the other part will form the placenta that will envelop it during the 40 weeks of gestation. During the fourth week the formation of the chorionic villi is completed.

5 week of pregnancy

Picture of the fetus at 5 weeks

The embryo measures 1,5 to 2,5 millimeters. The arms and legs begin to form and the ears and eyes begin to sketch, while the heart beats very quickly and regularly. The spine is also forming.

6 week of pregnancy

Picture of the fetus at 6 weeks

The embryo measures between 4 and 6 millimeters. During this time, the larynx and the inside of the ear begin to form, as well as the arms and legs. The neural tube is then closing, from which the baby's nervous system will originate. The heart beats regularly and internal organs, such as the liver, pancreas, stomach and lungs, are also forming.

7 week of pregnancy

Picture of the fetus at 7 weeks

The embryo has reached 1 cm in length: it is developing a sketch of the gums and the genital system is in the process of forming. The development of the internal organs is now in full swing: pancreas, kidneys, eyes and they are slowly appearing. With an ultrasound it will be possible to recognize the heartbeat.

8 week of pregnancy

Picture of the fetus at 8 weeks

The sex of the unborn child is being determined, but it is not yet possible to know whether it will be male or female. The fetus measures approximately 20 millimeters and can begin to move within the uterus.

9 week of pregnancy

Picture of the fetus at 9 weeks

The sex has been established: the gonads have become testicles if it is a male and ovaries if it is a female. The elbows have formed and the toes are also being defined. The fetus can move through the uterine wall and move. It is about 25 millimeters long and weighs just over 1 gram. The development of the eyes is complete.

10 week of pregnancy

Picture of the fetus at 10 weeks

During this week, the fetus is moving, but it is still early for the movements to be perceived by the mother. Her body is almost entirely developed: her eyes are moving towards the inside of the face, the internal organs have formed, the "tail" has closed and the fingers and toes are now divided.

11 week of pregnancy

Picture of the fetus at 11 weeks

The fetus is about 3 centimeters long and starting from this moment it will begin to double its length. The nails are forming, the iris is developing and the formation of all internal organs is almost complete. The eyelids have almost fully formed and protect the baby's eyes.

12 week of pregnancy

Picture of the fetus at 12 weeks

In this period the fetus measures about 5 centimeters and weighs about 14 grams. The developing brain of the fetus is taking place, the cells of which multiply rapidly. His internal organs are almost fully formed and begin to function, while his heart is always beating very fast.

13 week of pregnancy

Picture of the fetus at 13 weeks

The fetus is undergoing major transformations: its intestinal system has fully formed and its pancreas is already producing insulin. The vocal cords are also forming, as are the tooth buds.

Second trimester of pregnancy

The second trimester is a golden period: gestation has now begun, the belly does not weigh excessively and the most annoying symptoms, such as nausea and vomiting, disappear.

The organs generated in the first trimester develop together with the senses of hearing, touch and taste. From now on the mother begins to feel the baby's movements, movements that could be like “butterfly wings flapping” or like air bubbles in the belly.

14 week of pregnancy

Picture of the fetus at 14 weeks

The fetus produces urine which it releases into the amniotic fluid. He can also practice breathing by ingesting amniotic fluid in and out of the airways.
It is good for the mother to avoid taking foods and drinks that are potentially harmful to the development of the fetus and it is recommended to follow the guidelines for proper nutrition during pregnancy.

15 week of pregnancy

Picture of the fetus at 15 weeks

The fetus may have developed a habit of thumb sucking or yawning. Her skin is very thin, some hair is growing and also eyelashes and eyebrows. The heart pumps its blood vigorously and the ears are functioning: it can begin to hear sounds and noises coming from outside.

16 week of pregnancy

Picture of the fetus at 16 weeks

The fetus weighs around 100 grams and its movements become more intense: at this stage they could begin to be felt even by the mother.
Arms and legs are formed, bones are getting harder and lips are developing. The eyelids are formed but the eyes are not yet open. She starts opening and closing her little hands.

17 week of pregnancy

Picture of the fetus at 17 weeks

The fetus continues to grow and weighs about 140 grams, more than the placenta that surrounds it. It is also starting to produce meconium, the feces that it will excrete once it is born. His hearing is able to perceive sounds from the outside: you can try to make him hear the music chosen by his mother and the voice of his father.

18 week of pregnancy

Picture of the fetus at 18 weeks

The fetus weighs approximately 200 grams, its limbs have almost fully developed. His bones are getting stronger and his fingerprints are also forming.

19 week of pregnancy

Picture of the fetus at 19 weeks

The fetus weighs just under 250 grams on average and fluff is growing on the body, a fine hair that will protect the skin from prolonged contact with amniotic fluid. During this period, the fetus spends most of its time sleeping and only stays awake for 4 to 6 hours a day.

20 week of pregnancy

Picture of the fetus at 20 weeks

Pregnancy is halfway there! This week the fetus's brain continues to grow, and will increase by about 90 grams each month. Eyelashes, nails and hands are fully formed and the eyes, under the lids, continue to move and rotate. The heart beats very fast, about 120 to 10 beats per minute.

21 week of pregnancy

Picture of the fetus at 21 weeks

The fetus weighs just under 400 grams and is about 30 centimeters long. It "breathes" by opening and closing the lungs about 40-100 times per hour.
The space that surrounds it within the amniotic fluid is still large and the movements are so free that it could do somersaults.

22 week of pregnancy

Picture of the fetus at 22 weeks

Hair, eyelashes and eyebrows are formed. The fetus gains weight faster and stretches. Keep sleeping for many hours a day, and stay awake for about 4 hours - during this time you may feel small kicks.

23 week of pregnancy

Picture of the fetus at 23 weeks

The nails have now fully formed and the fluff has become darker. The fetus now possesses the traits of a tiny human being, even though it weighs only 500 grams. During this period its growth will begin to double. She continues to sleep most of the time and starts dreaming.

24 week of pregnancy

Picture of the fetus at 24 weeks

The fetus is almost fully formed. Although the surrounding space is increasingly reduced, it continues to move and indeed will make itself felt more and more. Her lungs are developing and her heart beats at about 160 beats per minute.

25 week of pregnancy

Picture of the fetus at 25 weeks

The fetus weighs about 600 grams and its bone structure is hardening more and more. Her face and hair are getting lighter, her hearing has developed and she recognizes her mother's voice. Her brain continues the growth process and begins to develop an early core of what will be memory.

26 week of pregnancy

Picture of the fetus at 26 weeks

The fetus weighs about 700 grams and measures just over 30 centimeters. He still sleeps for a long time but when he is awake he hears the noises inside his mother's body as well as those outside. Particularly loud noises may cause him to flinch. Light could pass from the uterus, which the fetus is able to perceive and thus distinguishes light and dark.

27 week of pregnancy

Picture of the fetus at 27 weeks

The baby's skin begins to coat itself more and more with vernix, a fatty substance capable of defending it. The fetus is increasingly assuming the features it will present at birth, weighs about one kilogram and measures just under 35 centimeters.

Third trimester of pregnancy

In this last trimester, the enlargement of the belly is due to the increase in the volume of the uterus, the placenta, the amniotic fluid and the strengthening of the fetus which, in most cases is positioned upside down, the ideal position to be born.
The future mother lives in a whirlwind of emotions: the moment of birth is approaching and she has to face her fears and insecurities.
Fatigue makes itself felt and there are many hassles that it has to endure: heartburn, constant urge to urinate and swelling in the legs and ankles.

28 week of pregnancy

Picture of the fetus at 2 weeks

The fetus's eyelashes and eyebrows are formed and it is starting to open and close its eyes. At this stage, the lungs are completing their formation. In the case of a male, the testicles may already begin to descend, while if it is a female the genitals will be visible with the ultrasound.

29 week of pregnancy

Picture of the fetus at 29 weeks

At this stage, the fetus weighs just over 1 kg and measures just under 40 centimeters. The space available to him inside the mother's womb has shrunk and the movements mainly affect the elbows and knees. His body temperature begins to self-regulate and the bone marrow produces red blood cells by itself. He may begin to get into a cephalic position (i.e., upside down).

30 week of pregnancy

Picture of the fetus at 30 weeks

The fetus has learned to know the environment that surrounds it and is able to distinguish light and dark. Touch is also developing and she uses her hands to explore the environment and manages to touch her face and body or suck her thumb.

31 week of pregnancy

Picture of the fetus at 31 weeks

The placenta provides the fetus with the oxygen it needs, and its pupils dilate and contract in response to light stimuli. Her skin becomes more and more rosy and in the meantime thin layers of fat continue to deposit under the skin.

32 week of pregnancy

Picture of the fetus at 32 weeks

The fetus grew quickly: in just one month it gained more than one kg thanks to fat and muscle tissue, reaching a total of about 1,5 kg in weight and 40 cm in length. The 5 senses are developed: he moves his hands, opens and closes his eyes and hears the sound of his mother's voice and the people around him.

33 week of pregnancy

Picture of the fetus at 33 weeks

The weight of the fetus is about 2 kg. About one kg of amniotic fluid surrounds it, which tends to vary in quantity. His lungs have now reached a good maturity and the baby may have frequent episodes of hiccups: light rhythmic jolts.

34 week of pregnancy

Picture of the fetus at 34 weeks

The lungs have developed and hair begins to form (but not all babies are already born with "hair"), while its bone structure is still quite elastic and its membranes are not yet fully bonded.

35 week of pregnancy

Picture of the fetus at 35 weeks

The baby weighs around 2,5kg, measures around 40-45cm and still grows very fast. His brain develops very rapidly, opens and closes his eyes, following the sleep and wake cycles.

36 week of pregnancy

Picture of the fetus at 36 weeks

The baby now weighs around 2500 grams and is around 46,2 cm long. The accumulation of fat that characterizes it is of help for the development of the immune system. He always kicks according to the same patterns that his mother has come to know over time. Since there is less amniotic fluid, it is possible to see the little foot pushing towards the abdomen.

37 week of pregnancy

Picture of the fetus at 37 weeks

The baby "breathes" continues to inhale the amniotic fluid, and the expectant mother can clearly perceive the rhythmic movements similar to breathing: sometimes more abrupt due to hiccups, which are quite frequent.

38 week of pregnancy

Picture of the fetus at 38 weeks

The baby can weigh 2,5 to 3 kg and more, and gains about 30 grams per day. Its length is around 47/48 cm. The baby ingests amniotic fluid and forms meconium, a dense blackish-green material present at birth in the baby's intestine and which will be excreted in the feces in the first 24-48 hours of life.

39 week of pregnancy

Picture of the fetus at 39 weeks

The baby begins to struggle to move around the uterus, although it is the environment in which it was kept, protected and cared for for nine months.
It weighs between 3,1 and 3,4 and its length is between 50 and 52 cm. He is still accumulating fat and gaining weight more and more.

40 week of pregnancy

Picture of the fetus at 40 weeks

The baby is ready to be born! It weighs an average of 3 to 3,5 kg and has a length of 50-52 centimeters. Her immune system is still immature and she will receive antibodies from her mother, both through the placenta and subsequently through breastfeeding.

Read also: Preconception exams: why it is important to do them before pregnancy

  • weeks of pregnancy
  • development of the fetus
  • embryo
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