How does the fetus develop during the second trimester of pregnancy?
During the second trimester, the baby's growth accelerates dramatically. The pregnancy becomes more tangible as the baby's movements become increasingly noticeable. The senses begin to develop, and the mother's abdomen enlarges.
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| Développement du fœtus : 2e trimestre |
By the end of the second trimester, the fetus will measure approximately 25 cm crown-rump length and weigh around 1 kilogram. The following are the primary developmental milestones, week by week.
Week 15: Small Movements
Thirteen weeks post-conception, the fetus enters a period of rapid growth. It measures 9.5 cm crown-rump length and weighs 80 grams.
All organ systems are now in place and begin to assume their respective functions. A notable change occurs as the legs start to grow longer than the arms.
The vascular network becomes more complex. To supply this growing body, the heart works tirelessly, pumping approximately 24 liters of blood per day at a rate of 140-150 beats per minute.
Fetal movements become more frequent. For example, the fetus can now turn its head, which measures 3.5 cm. As there is still ample room for movement and the movements are quite delicate, it is normal not to feel them yet. This will change soon!
A tip: introduce your favorite foods to your unborn baby as the taste buds are now functional. Your diet, through the amniotic fluid, gradually exposes the fetus to various flavors!
Week 16: More Vigorous and Agile
The fetus is gaining strength, resulting in more forceful kicks!
Their movements are also becoming more coordinated. When they touch the umbilical cord, their initial reaction is to move away. However, the cord will soon become a "toy" for them: they will grasp, pull, and push it. Their small hands are becoming more dexterous.
Under the fetal eyelids, the eyes are beginning to move. As the fetus drinks a large amount of amniotic fluid (400 ml per day), they often hiccup. This is normal. Additionally, their digestive system is starting to produce a black substance called meconium, which accumulates in their intestines.
By the end of this week, the fetus measures 11.5 cm from head to tailbone and weighs 110 grams.
Week 17: Fine Hair Begins to Appear
Intrauterine fat deposition commences, a crucial developmental milestone. This specialized brown adipose tissue is essential for thermoregulation in the newborn, as neonates lack the ability to shiver. This fat reserve also provides energy during the initial postnatal period. Concurrently, the fetus begins to synthesize its own antibodies.
Post-week 17, no new structures emerge; however, ongoing developmental processes continue. The skeleton, primarily composed of cartilaginous tissue, continues to grow, reaching approximately 13 cm crown-rump length and weighing around 150 grams.
The initial appearance of lanugo, a fine downy hair, marks another significant developmental event.
Week 18: Boy or Girl?
At this stage of pregnancy, a noticeable shift occurs in the fetal body proportions: the head is now smaller relative to the rest of the body.
Blood circulation has now reached the extremities, resulting in significantly increased fetal movement. The fetus is actively moving within the uterus, turning, kicking, and touching its toes, face, and other body parts. You may begin to perceive these movements as gentle waves.
Moreover, a fatty sheath (myelin) is forming around the nerves, providing insulation for nerve fibers, similar to the plastic coating around an electrical wire. This sheath facilitates the conduction of nerve impulses, or the transmission of information between the central nervous system (brain and spinal cord) and the peripheral nervous system.
Soon, the cells of the central nervous system, particularly those in the brain, will also be myelinated. This signifies the establishment of neural connections and the consolidation of neuronal networks.
The retina of the eyes is now sensitive to light. If a sufficiently bright light is shone on the mother's abdomen, the fetus will turn away and cover its eyes.
An ultrasound is typically performed between weeks 18 and 20. The primary purpose of this examination is to assess the proper development of the fetal organs, including the heart, kidneys, and central nervous system. Additionally, the sex of the baby can be determined at this stage, as the penis is visible in male fetuses, while the vulva and uterus are visible in female fetuses.
By the end of the 18th week (or the 16th week post-conception), the fetus measures approximately 14 cm from crown to rump and weighs around 200 grams.
Key medical terms used:
- Fetus: An unborn offspring in the process of development.
- Myelin: A fatty substance that insulates nerve fibers.
- Neural: Relating to nerves.
- Neuronal: Relating to neurons (nerve cells).
- Retina: The light-sensitive inner layer of the eye.
- Ultrasound: A diagnostic imaging technique that uses high-frequency sound waves.
Week 19: Heightened Senses
In week 19, the fetal brain undergoes significant specialization of sensory regions. The fetus becomes increasingly capable of processing sensory input, initiating a period of sensory exploration. Neuronal proliferation decreases as the brain prioritizes synaptogenesis and neural network formation, essential for learning and development.
The fetal skin develops a protective layer of vernix caseosa, a white, cheesy substance that prevents maceration and irritation. Although growing steadily, the fetus maintains sufficient room for movement, measuring approximately 15 cm crown-rump length and weighing around 260 grams.
Week 20: You Start to Feel the Baby Move
In your womb, the fetus is practicing moving all parts of its body. Its muscles are increasing in size and strength. You may have noticed this as it's around this time that most mothers begin to feel the baby's movements, which become more vigorous. Between periods of activity, the fetus still sleeps a lot: 18 to 20 hours a day.
The fetus's skin is thickening, although it's still quite transparent. In its brain, the first gyri (folds) are forming.
By the end of the 20th week (or 18th week of conception), the fetus measures 16 cm from crown to rump and weighs 320 grams.
Key medical terms used:
- Fetus: The unborn offspring of a mammal, in particular of a human.
- Womb: The organ in female mammals where offspring are carried and gestated before birth.
- Gyri: The convoluted ridges on the cerebral cortex of the brain.
- Crown-rump length: The measurement of a fetus from the top of the head to the bottom of the buttocks.
Note: The term "crown-rump length" is commonly used in obstetrics to describe the size of a fetus during early pregnancy.
Week 21: Fetal Hand and Foot Play
The fetus is now fully formed. It measures approximately 18 cm from crown to rump and weighs around 390 grams. Over the next month, it will gain a significant amount of weight.
The fetus is becoming increasingly tactile. It enjoys grasping its feet and bringing its hands together. It may even suck on its thumb. The umbilical cord has become a fascinating object of play.
The lungs continue to mature. The fetus practices expanding the chest muscles, which will be essential for breathing once it is born.
The eyeballs are fully formed, although the iris is not yet pigmented. The lips are becoming fuller. Hair growth is also more pronounced.
More technical terms that could be used:
- Crown-rump length: This is a more precise measurement used in obstetrics.
- Tactile exploration: This term emphasizes the sensory aspect of the fetus's actions.
- Ocular development: This is a more formal way of referring to the development of the eyes.
- Pigmentation: This is a specific term for the process of coloring.
Week 22: A Sweet Tooth in the Making
The fetus has already developed a preference for sweet tastes. When the amniotic fluid is sweeter, the fetus tends to ingest more of it. However, thanks to the pancreas now secreting insulin, blood sugar levels are well-regulated.
Although the fetus is gaining weight, fat stores remain relatively limited. Additionally, the skin is still wrinkled, a condition that will improve as the fetus continues to grow.
During this second half of pregnancy, brain development is rapidly progressing. In the gums, the permanent tooth buds are starting to develop behind the primary teeth. These teeth will be ready to erupt in the months following birth.
At this stage, the fetus measures approximately 19 cm from crown to rump and weighs around 460 grams.
Week 23: Eyelid Opening Begins
The fetus is now covered in vernix caseosa, a protective, waxy coating that helps to protect the skin from the increasingly concentrated amniotic fluid. This coating will gradually diminish as the baby gets closer to birth.
The fetus has developed a regular movement pattern, which is a good sign of its health. Keep a close eye on these movements.
The eyelids are beginning to separate, and the fetus is becoming more aware of light and dark.
In male fetuses, the testes are descending into the scrotum. In female fetuses, the reproductive system is fully formed, including all of the eggs she will ever have.
The fetus now measures 20 cm from crown to rump and weighs approximately 540 grams.
Key medical terms used:
- Vernix caseosa: A white, cheesy substance that covers the fetus's skin
- Amniotic fluid: The fluid that surrounds the fetus in the uterus
- Crown-rump length: The measurement from the top of the head to the bottom of the buttocks.
Week 24: True hair begins to grow
The immune system is now producing white blood cells. These cells are crucial for the body's ability to fight off diseases and infections.
The fetus's skin is thickening and becoming slightly pigmented, although it is still quite translucent. Blood vessels (capillaries) can still be seen through the skin.
True hair has begun to grow on the fetus's head; it is no longer just lanugo (fine, downy hair). Fingernails are now more distinct.
Weight gain is significant during this period. The fetus now weighs more than 630 grams and measures 21 centimeters from crown to rump. The fetus is beginning to fill the uterus.
If the fetus were to be born at this time, approximately 22 weeks after fertilization, it could potentially survive but would require extremely specialized care. At this stage, the fetus would be considered extremely premature as the respiratory and digestive systems are not yet fully mature.
Week 25: Your Baby Knows You're There!
The fetus is beginning to recognize certain stimuli and situations: the familiar voice of their mother, the voices of other family members, the patterns of day and night, etc. They may startle in response to sudden stimuli and can even respond to gentle touches from the outside world. In short, they have a growing awareness of your presence.
They also react to the sounds of your body. They hear your heartbeat, your breathing, and your digestive noises. These sounds have a calming and soothing effect. It's no surprise that when you hold your baby after birth, they may feel comforted by these familiar sounds.
You can now feel your baby's movements quite distinctly. During their active periods, they may move 20 to 60 times in half an hour.
The development of the bronchial tubes is nearly complete, but the lungs are not yet fully mature.
The eyes are gradually acquiring their color (pigmentation), although this may change after birth. The enamel that will protect the dentin of the baby teeth is beginning to form.
Depending on the fetus' position, you may be able to hear their heartbeat simply by placing your ear on your abdomen. They measure approximately 22.5 cm from crown to rump and weigh around 720 grams.
Week 26: Facial Features Refine
This week marks another significant milestone in lung development, particularly crucial for a preterm infant. The lungs begin secreting a substance that enhances their ability to expand and provides protection against infection, improving the chances of survival.
Facial features become more defined: eyelashes form, the nose shape becomes more distinct, a hairline may be visible, and the neck becomes more pronounced. In essence, the fetus is developing the facial features you'll soon recognize. Eye-opening episodes become more frequent.
Nerves continue to mature. The brain now exhibits increasingly complex convolutions (folds in the cerebral cortex), which are also deepening. Additionally, until the 32nd week, synapse development accelerates.
Synapses are the connections between brain cells that facilitate information exchange. The nervous system undergoes the most gradual development throughout pregnancy.
Since the beginning of the trimester, fetal size has doubled. It measures approximately 23 cm from crown to rump and weighs around 820 grams. This represents about one-third of its birth weight. Limited space within the uterus is becoming increasingly evident.
Key medical terms used:
- Preterm: Born before 37 weeks of pregnancy.
- Convolutions: Folds in the cerebral cortex.
- Synapses: Connections between brain cells.
- Cerebral cortex: The outer layer of the brain.
Week 27: Is he dreaming?
This week marks a significant milestone in your baby's development. The reflexive crying mechanism, essential for communicating needs post-birth, is now fully formed. Your baby's sleep-wake cycles are becoming more regular, though they may not align perfectly with yours. Intriguingly, some experts suggest that fetuses at this stage may be experiencing REM sleep, characterized by rapid, low-amplitude beta waves—a hallmark of dreaming in adults.
Your baby's tiny hands are becoming increasingly dexterous, as evidenced by thumb-sucking. This soothing behavior also strengthens the muscles in the jaw and cheeks. While blood vessels are now supplying the lungs, these organs are not yet fully mature. Similarly, the bones are becoming stronger, but premature birth at this stage would necessitate intensive medical intervention.
Key medical terms and concepts:
- Reflexive crying: An innate response essential for survival and communication.
- REM sleep: Rapid eye movement sleep, associated with dreaming.
- Beta waves: Brainwave patterns characteristic of active, engaged mental states.
- Premature birth: Birth occurring before 37 weeks of gestation.
Week 28: Increasingly Curious
Your growing baby is becoming more and more adorable: they are plumper and their skin is smoothing out. It's also redder due to the many small blood vessels running through it. Beneath the skin, fat stores are now more substantial, representing approximately 3% of their total weight.
Gradually, the placenta becomes less important as a nutritional source. The amount of amniotic fluid is decreasing as the fetus occupies much more space. They are also improving their swallowing and sucking reflexes.
They are curious: when a light or sound intrigues them, they turn their head towards it.
The liver and spleen are working efficiently, producing a large quantity of blood cells.
The fetus continues to gain a significant amount of weight. They measure approximately 25 cm from head to coccyx and weigh nearly 1 kg.