Weekly Fetal Development in the Third Trimester: What to Expect Week by Week
 |
| Fetal Development: Third Trimester |
The baby continues to grow. It is quite alert and responds to sounds and touches on the abdomen. The baby is preparing for birth. Its lungs continue to develop to be ready for their first breath at birth.
At term, the baby will weigh between 3 to 4 kilograms and measure approximately 50 centimeters. Here are the final stages of its development.
You can also check out: Fetal Development in the First Trimester of Pregnancy, Fetal Development in the Second Trimester of Pregnancy.
Fetal Development -Week 29:
At the beginning of the third trimester, the fetus's digestive system is functional. It continues to ingest amniotic fluid and experiences a range of flavors based on the mother's diet. Additionally, odoriferous substances carried by the amniotic fluid provide the fetus with an initial experience of smells that may influence its preferences after birth.
At this stage of development, the fetus's bones are fully formed but still soft. Its muscles are increasing in mass. The eyes are partially open, and the eyelashes have formed. The skin is becoming smoother.
Due to the significant exertion, the fetus has high nutritional needs. It requires a substantial amount of protein, vitamins, and minerals (especially calcium and iron), which it derives from the mother's diet.
Inside the uterus, space is limited, so fetal movements become more restricted, though the fetus continues to push and stretch, and this is observable.
The fetus now measures approximately 26 cm from crown to coccyx and weighs 1.2 kg.
Fetal Development - Week 30: Eyes Open
At this stage, the fetus's eyes are open most of the time. It is even capable of seeing, although its vision remains limited. The fetus can see objects up to 20 to 30 cm in front of it and can also perceive sounds and voices.
The organs are nearly fully formed, with the exception of the kidneys and lungs, which still require more time to mature. This is the fetus's third pair of kidneys; during embryonic development, it had two previous pairs of kidneys that have been replaced by this final pair, which will be retained for life.
The structures of the brain are strengthening. The myelin sheath surrounding the nerve fibers continues to develop, and this process will intensify during the early years of life. This development will be noticeable in the acquisition of skills such as walking, language, and potty training.
The bone marrow has taken over from the liver and spleen in the production of red blood cells. Growth has slowed somewhat, but the fetus continues to gain weight.
At twenty-eight weeks after fertilization, the fetus measures approximately 27 cm from crown to coccyx and weighs about 1.3 kg. It is beginning to feel somewhat cramped in the uterus.
Week 31: Fetal Development
Another step toward your future baby's independence: the central nervous system is now capable of regulating its body temperature, thanks to its body fat reserves. Additionally, the fetus's appearance has changed—it is rounder!
Inside the lungs, blood vessels have developed. Respiratory movements are becoming increasingly regular.
The testicles have now descended into the scrotum. The oocytes, which will become ova, have completed their development. They will remain in a dormant state until puberty, at the onset of menstruation.
Some fetuses are already in a head-down position. Often, they maintain this position until delivery.
Hair and nails continue to grow, and their length at birth may surprise you. The baby now measures approximately 27.5 cm from crown to coccyx and weighs 1.5 kg.
Week 32: The fetus moves a lot!
You feel it squirming and making large movements with its arms or legs. From time to time, a wave distorts your entire belly: this is the fetus changing position. Soon, it will be too cramped to move as much.
At this late stage of pregnancy, the fetus's weight gain is rapid. It gains half of its birth weight during the last 7 weeks of pregnancy.
Its heart beats at a rate of 135 to 140 beats per minute, though it can reach up to 160 beats per minute. This is about twice the heart rate of an adult.
It measures 28 cm from head to tailbone and weighs 1.7 kilograms.
Week 33: Less Room to Move
Even though the fetus has much less room to move, you will continue to feel its movements until delivery. Since it is curled up (legs folded, arms crossed, and chin on its knees), you might feel these movements differently.
Its body now has the proportions of a newborn (head, body, limbs), but it will use the next few weeks to grow a bit more and, most importantly, to gain weight.
If it hasn't already, the fetus will turn head down in preparation for birth. Its head will press on the bones of your pelvis, gradually descending. Until delivery, your medical visits will ensure that the fetus has not turned back to a head-up position.
If the baby were born today, it would be considered moderately premature (the most common type). It measures 29 cm from head to tailbone and weighs 1.9 kilograms.
Week 34: From Red to Pink
During the last weeks of pregnancy, the fetus swallows a lot of amniotic fluid and releases a significant amount of urine (about 2 tablespoons every hour) into the amniotic sac. The placenta is working at full capacity to filter and eliminate waste, and so are your kidneys!
Remember to drink enough water. A simple tip: drink to quench your thirst and make sure your urine is not too dark in color. If you are well-hydrated, your urine will be pale yellow.
In the fetus's intestines, meconium is accumulating. It will be expelled in the early days after birth as the first stools.
At this stage, most organs and systems are now fully functional. However, some will continue to mature in the last weeks. This is true for the lungs, bones, skin, nails, and hair.
The bones continue to grow and thicken. However, the bones of the skull remain malleable in preparation for birth. They are not yet fused together. The fontanelles (the spaces between the bones) will not close until several months after birth.
The placenta provides a large amount of calcium to the baby. The baby's calcium level is now higher than the mother's.
The fetus's skin has taken on a pinkish hue (instead of red). The fingernails now cover the entire tips of the fingers. The toenails, however, take a bit longer to grow.
This week, the fetus measures about 30 cm from head to tailbone and weighs about 2.3 kilograms.
Week 35: A Mature Brain
This week marks the beginning of your final month of pregnancy. Two important structures have reached maturity: the brain and the immune system. The brain has completed its fetal development, but it will continue to transform throughout life. It retains a plasticity that allows for new learning at any age.
The same is true for your future baby's immune system. During pregnancy, it has integrated your antibodies to build an initial immunity. This system will continue to develop after birth as well.
The last weeks of gestation are beneficial for the fetus. Its digestive system and lungs are almost mature. Additionally, the fetus continues to develop its fat reserves in preparation for birth. It will gain about 230 grams per week over the next month.
Now, its little limbs are becoming plump. It measures 32 cm from head to tailbone and weighs 2.3 kilograms. It is also becoming increasingly cramped. There is only about one liter of amniotic fluid surrounding it.
Week 36: Practicing Breathing
Inhale, exhale... Your future baby is practicing! Hic! Hic! He still has the hiccups. This is normal. He is practicing breathing and developing his lungs. But right now, he is breathing amniotic fluid. He swallows some at the same time, which gives him hiccups. Of all the organs in the body, lung development takes the longest.
Meanwhile, he continues to gain weight. He now has nice, round cheeks! He measures 34 cm from head to tailbone and weighs 2.5 kilograms.
In preparation for birth, he is gradually shedding what served as protection in the uterus: the lanugo (fine hair) and the vernix caseosa (waxy substance).
Week 37: Finally Ready for Childhood
Your future baby is now considered full-term. He can arrive at any moment. All his organs are functional. He has enough strength within himself to survive and start his life as a child.
But it is the fetus who will, in a way, decide the moment of his birth. When he becomes too cramped, he will secrete a hormone that will trigger contractions. In the meantime, you surely feel his head engaging even deeper into the pelvic region (this transition is called lightening).
And from time to time, through the stretched skin of your belly, a little elbow or heel protrudes... You can even recognize them! Your baby now measures 35 cm from head to tailbone and weighs 3 kilograms.
Week 38: Vigorous Movements
The baby is all ready! The layer of vernix (the waxy substance that protects the skin from the amniotic fluid) has now detached and floats in the amniotic fluid, which takes on a milky color.
The heart is fully developed. It beats stronger, and its heart rate is slowing down. However, certain communications between the heart chambers will close after birth as they will no longer be needed.
His movements are vigorous, and the grasp reflex is well developed.
Your baby might now weigh around 3.2 kilograms and measure 35 cm from head to tailbone.
Week 39: Waiting While Sleeping
Your baby sleeps a lot; he is waiting. He no longer has room to move. His limbs are folded and close to the body. His chin rests on his knees.
His growth has now slowed, but he continues to gain weight. He now measures 36 cm from head to tailbone and weighs 3.3 kilograms.
Week 40: He Will Soon Be With You!
All the organs are now fully developed except the lungs, which will complete their development at birth; they will need a few hours to establish a regular breathing pattern. Now, your future baby has all the reflexes needed to start his development in the outside world. The lanugo completely falls off during this week.
Your baby now measures around 50 cm from head to toe. His weight should not change much until birth and will be around 3.5 kilograms. Boys tend to be slightly heavier than girls at birth, but remember these are just averages. Many variations are possible and normal.
Is your baby taking a bit longer than the expected due date? This is very common, especially in a first pregnancy (only 5% of births occur on the due date). The doctor will discuss with you if induction of labor should be considered. One thing is certain, he will be here soon!