how does a fetus grow in the womb?

Practically invisible during fertilization, a newborn baby measures, in general rules at birth, between 45 and 55 cm and weighs between 2,500 and 4,300 g. Our growth chart gives an overview of how the length and weight of the baby in the womb change over the 40 weeks (approximately) of gestation.
As a reminder: The baby is first called an embryo and later, from the 12th week of pregnancy, it is called a fetus.

Do not worry if the measurements or information in your maternity record are different from those mentioned in this growth table. It’s normal! Whether in the mother ‘s womb or after childbirth: every baby is unique and no child develops like another.
While embryos are largely the same in size and weight at the start of pregnancy, they develop more and more individually as the pregnancy progresses. This is why we can only display average values ​​in our growth tables.

Video by Clara Poudevigne

First semester :

• Week n ° 1: it corresponds to the 3rd week of amenorrhea (SA). At this time, fertilization has taken place in the tubes of the uterus, the egg then moves slowly through the uterus to implant at the end of the week. Then the egg will develop, cell division begins very quickly: 2, 4, 8, 64 … cells are formed.

• Week n ° 2 or 4th WA: after two or three days of freedom in the uterus and seven days after fertilization, the egg then formed of 150 cells, will make its nest in the muscle and the lining of the uterus. It is implantation. The cells then continue to multiply and begin to specialize.

• Week n ° 3 or 5th WA: the egg grows day after day and now measures nearly 3 mm. The villi on the surface of the egg come into contact with the blood vessels in the uterus to supply the egg with nutrients essential for its development. The cells of the egg organize and differentiate more and more to form the future embryo.

• Week # 4 or 6th WA: The embryo gets bigger and begins to form. It takes on certain volumes, such as the trunk and the head. The outline of some organs begins to appear. The heart forms and then begins to beat. The umbilical cord begins to form as well.

• Week 5 or 7th WA: The embryo continues to develop to reach almost 1 cm at the end of the week. The eyes, the arms and the legs are taking shape little by little. The umbilical cord is now clearly visible and the placenta begins to form.

• Week 6 or 8th WA: The embryo measures approximately 15 mm. The outline of the spine appears. The sex glands begin to appear but at this stage it is still impossible to distinguish the sex of the embryo.

• Week n ° 7 or 9th WA: At this stage, the embryo measures 2 cm. The organs of your unborn baby continue to develop. At the level of the head (“cephalic end”), the different parts of the brain begin to become individual, just like the main nerves.

• Week n ° 8 or 10th SA : The embryo weighs about 10 grams and measures between 3 and 4 cm. His cells have multiplied enormously, there are now several million. The embryo bathes in an aqueous universe, in the amniotic fluid which will protect it until the end of the pregnancy. Organogenesis begins in turn, creating the different organs.

• Week n ° 9 or 11th WA: The embryo measures 3 cm for a weight of about 12 g. The brain and nervous system continue to develop. The two cerebral hemispheres are starting to stand out. All the organs continue to develop.

• Week n ° 10 or 12th WA: The embryo measures 5 cm and weighs approximately 15 g. By the third month, the embryo is fully seated in the uterus. Its development continues, the arms and legs take shape and the fingers and toes become more and more individualized.

• Week 11 or 13th WA: The embryo now weighs 18 g and measures 6 cm. His limbs have also developed well, and the fingers are already showing! His face begins to form, his eyes, his nose, his mouth, his ears are already emerging quite clearly.

• Week 12 or 14th WA: The fetus weighs approximately 45 grams and measures 10 cm. His arms and legs, along with his hands and feet, are well individualized. As the nervous system has developed and muscles are also forming, the fetus begins to move.

• Week 13 or 15th WA: The fetus measures a little less than 12 cm. Almost all the organs (heart, lungs, digestive tract, etc.) are in place. Each of them will develop both anatomically and physiologically, to become functional on the day of birth.

Second trimester :

• Week # 14 or 16th WA: The development of the fetus continues. During the fourth month, his body begins to be covered with a fine down: this is the lanugo. The epidermis, the superficial tissue of the skin, is about to be formed.

• Week n ° 15 or 17th WA: the fetus measures 16 cm, its foot 2 cm and its skull 4 cm in diameter. It weighs 135 g. The various organs of the fetus continue to develop. The heart of the future baby beats about twice as fast as yours.

• Week 16 or 18 SA: A waxy, whitish substance, vernix caseosa, covers your baby’s skin and protects it from the amniotic fluid in which it bathes. The fetus moves more and more and these movements contribute to the increase of its muscle mass and the proper functioning of its joints.

• Week 17 or 19th WA: All the organs of the fetus continue to develop, including its sexual organs. On ultrasound, we can usually begin to identify the sex of the child around the fifth month of pregnancy (provided the baby is well positioned).

• Week n ° 18 or 20th WA: The fetus continues to grow and measures approximately 20 cm. And the nails are starting to form. The bones of the skeleton are growing. They are at this stage still very flexible.

• Week # 19 or 21st WA: The five senses of the fetus are developing more and more and already starting to work for some. Thus, the future baby would even begin to memorize some of these sensations. It happens as a learning of the flavors that are related to what you eat.

• Week # 20 or 22nd WA: In the fifth month of pregnancy, the fetus grows a lot. It now reaches 23 cm, and weighs over 400 g. Its sensory functions are refined, and in particular its taste sensations. Your future baby is in principle able today to distinguish different flavors: sweet, salty, bitter, sour.

• Week n ° 21 or 23rd WA: By the 5th month, it is very likely that the future baby can already perceive noises. In principle, day after day, your voice will be heard by your baby. And gradually, she will become part of his familiar environment.

• Week n ° 22 or 24th WA: The fetus weighs approximately 500g and measures 25 cm. It is more and more mobile. It happens that you are a little “out of step” in relation to its rhythm and its awakening. Sometimes it is in the evening when you are lying down that your unborn baby starts to fuss.

• Week n ° 23 or 25th WA: The fetus continues its development, it now averages 550 g for 28 cm. And the organs gain autonomy.

• Week # 24 or 26th WA: The fetus now weighs almost 700g. His nervous system continues to develop. The cerebellum, which will play an important role in balance, grows towards the back of the brain. Later, when the baby is walking, this organ will play an essential role in regulating the balance.

• Week n ° 25 or 27th WA: Baby continues to grow and now reaches a height of 31 cm. The movements of your future baby are now quite important. He does not control his balance and lets himself be completely carried in your belly.

• Week n ° 26 or 28th SA: Its development continues, its sensory organs also. Its development continues, so do its sensory organs. it distinguishes between your own voice and those of other people.

Third trimester :

• Week 27 or 29th SA: Her weight is now around a kilo, or even a little more. During this period, its organs, its skeleton, its muscles will continue to develop, but it will also build fatty tissue.

• Week n ° 28 or 30th WA: Your baby’s different organs are developing more and more, their functions are becoming more precise every day.

• Week 29 or 31st WA: Your baby’s weight is 1.3 kg. Its size is 36 cm. Subcutaneous fatty tissue grows little by little, stretching its skin and making it look plump.

• Week n ° 30 or 32nd WA: Your future baby weighs about 1.5 kg for a height of 40 cm. His brain continues to develop. It is protected in the cranial box which is not fully closed.

• Week n ° 31 or 33rd WA: It is usually during the seventh or eighth month that the fetus turns around and assumes the position it will have during childbirth. Due to its development, it does not have much space, and therefore stands upside down and “in profile”.

• Week n ° 32 or 34th SA: All its organs are in place. Above all, he still has to gain weight and strength. He will need it when he is born and once outside!

• Week 33 or 35th WA: The baby’s weight is about 2.1 kg and his height is 42 cm. The heart of your unborn baby is practically its final shape.

• Week 34 or 36th SA: Your unborn baby is almost ready to be born. Some of its organs function practically normally, others will have to wait a few weeks to continue their maturation.

• Week 35 or 37th WA: The fetus is approximately 45 cm tall and weighs 2.8 kg. The down (lanugo) that covered him begins to fall off and the fat gradually accumulates under his skin. Baby continues to develop, especially his brain which weighs just over 300 grams.

• Week 36 or 38th WA: The fetus has less and less space to move. It weighs a little less than 3 kg… approximately, because the weight depends on each future baby. He will hardly grow taller, but will above all gain weight.

• Week 37 or 39th SA: Your baby is now also ready for the big day. Measuring about 48 cm and weighing 3 kg, its in utero development is coming to an end. It is time because baby is starting to be tight, moreover, he moves a lot less!

• Week n ° 38 or 40th WA: Your future baby, who weighs about 3 kg or even a little more and measures about 50 cm, is completely folded up on himself. More and more space is lacking! It is generally upside down: we speak of a cephalic presentation.