Fetal Development week by week (First trimester)
The first trimester lasts up to the 14th week of pregnancy. The embryo begins to form. Its first organs appear and its heart starts to beat. At the end of the first trimester, even if it is only 9 centimeters long, its head, trunk, arms, and legs are clearly distinguishable. Here are the main stages of its development, week by week.
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| Fetal Development |
We will present to you, my dear pregnant woman, and to your husband as well, in this article the development of the fetus week by week during the first trimester of pregnancy (from week 0 to
week 14).Week 1: Why 40 weeks of pregnancy?
It is difficult to determine the exact day of conception. To estimate the duration of pregnancy, doctors use the number of weeks without menstruation. Therefore, the due date calculation starts from the first day of the last menstrual period. This method of calculation adds 14 days to the length of pregnancy. With this way of calculating, you are therefore not pregnant during the first 2 weeks.
Here is a more detailed explanation:
- The average menstrual cycle is 28 days long.
- Ovulation typically occurs around day 14 of the menstrual cycle.
- If fertilization occurs, the fertilized egg takes about 6 days to travel to the uterus and implant.
- The pregnancy is considered to start from implantation.
So, why are there 40 weeks in a pregnancy?
- The 40 weeks are calculated from the first day of the last menstrual period.
- It is difficult to determine the exact day of ovulation or implantation.
- Using the first day of the last menstrual period as a starting point provides a more accurate estimate of the due date.
It is important to note that:
Not all pregnancies last 40 weeks.
Some pregnancies are shorter (preterm) and some are longer (post-term).
Your doctor will use your due date as an estimate of when your baby will be born.
The actual date of birth may be a few days or even weeks before or after your due date.
Week 2: Ovulation
Ovulation is the process when one of the ovaries (left or right) releases an egg. In general, women ovulate 14 days before their period, but this can vary from 11 to 18 days depending on the length of the menstrual cycle. The released egg travels down the fallopian tube, where fertilization takes place. It then continues on its way to the uterus.
The lifespan of an egg is 12 to 24 hours. Therefore, the fertile period lasts about 5 days each month: from 4 days before ovulation to 24 hours after. As for sperm, they can live for 3 to 5 days in a woman's body.
Signs of ovulation:
Some women experience physical signs and symptoms during ovulation, such as a change in cervical mucus, increased basal body temperature, and breast tenderness.
Tracking ovulation:
There are a number of methods that women can use to track their ovulation, such as using ovulation kits, charting their basal body temperature, and observing cervical mucus changes.
Importance of ovulation: Knowing when you are ovulating can be helpful if you are trying to conceive. It is the time of the month when you are most likely to get pregnant.
Week 3: Fertilization
A sperm enters the egg: they fuse to become a single-celled egg. This is fertilization and the real beginning of pregnancy.
This egg will take several months to develop and become a baby. But already, it contains everything the future child needs: its sex, eye color, height, skin pigmentation, etc. All this, in a tiny sphere the size of a pinhead!
During its first days of life, the egg descends slowly from the fallopian tube towards the uterus. During this journey, the egg will divide to give two cells. These will then divide in turn to give two more. These four cells will do the same to give eight, and so on. This is how, little by little, all the organs, limbs and systems that make up a human being will be formed.
In rare cases, the egg will implant outside the uterus, most often in a fallopian tube. This is called an ectopic or extrauterine pregnancy. The pregnancy must then be terminated to protect the mother's health.
Week 4: Implantation of the Egg
During this remarkable week, the fertilized egg, now measuring a tiny 4-5 millimeters, embarks on a crucial journey, attaching itself to the inner lining of the uterus, a process known as implantation. This delicate event sets the stage for the incredible transformation that will unfold over the coming months.
To facilitate this implantation, the egg releases a special hormone called hCG, which plays a pivotal role in signaling pregnancy. This hormone surge triggers the detection by pregnancy tests and prompts the production of progesterone, another essential hormone. Progesterone's task is paramount: it ensures the thickened uterine lining remains in place, providing a nurturing haven for the developing embryo.
As the egg burrows deeper into the uterine lining, it begins to form intricate connections with the surrounding blood vessels, establishing a lifeline for the nourishment it requires to thrive. These vessels will deliver a steady supply of nutrients and oxygen, fueling the embryo's remarkable growth.
At this early stage, the egg, resembling a delicate raspberry, bears within it the spark of life. Its structure consists of two distinct layers: the inner layer, destined to form the embryo, and the outer layer, which will transform into the placenta, an organ that will serve as the embryo's protective shield and nourishment provider. Remarkably, even at this nascent stage, the beginnings of the nervous system start to take shape, laying the foundation for the complex network that will coordinate the body's functions.
Week 5: The Umbilical Cord
During this week, the embryo changes from a round to an elongated shape. It resembles a small seahorse that measures 2.5 mm from head to coccyx. Helped by the circulatory system that is being put in place, the embryo grows a lot during this week. It is now visible to the naked eye. The heart has begun its first stage of development. Gradually, the face takes shape: the eyes, nose, mouth, and ears.
The cells then organize themselves into three groups. Each will form different parts of the baby's body:
- the skin, eyes, ears and nervous system;
- the muscles, skeleton and blood vessels;
- the digestive tract, pancreas and lungs.
The umbilical cord now connects the embryo to the mother. Through it, the embryo will receive nutrients from the mother's diet as well as oxygen. The cord also allows some of its waste products to be eliminated. Nestled in the uterine cavity, the embryo is bathed in amniotic fluid.
Additional points:
- The umbilical cord is about 30 cm (12 inches) long.
- The umbilical cord is made up of two arteries and one vein.
- The arteries carry oxygen-rich blood from the placenta to the embryo.
- The vein carries waste-filled blood from the embryo to the placenta.
- The umbilical cord is cut shortly after the baby is born.
Week 6: The heart starts to beat
During this week, or 4 weeks after fertilization, an important part of the embryo will be completed: the neural tube. It is important because the entire nervous system develops from this structure: the brain at one end of the tube, as well as the spinal cord and nerves. The ends of the tube will close at the end of this week. Taking folic acid supplements before and during pregnancy helps to prevent malformations during the formation of the neural tube, such as spina bifida.
It is also from the 6th week that the embryo's heart starts to beat! During an ultrasound, it is possible to see the heartbeats. But this little heart is still in the process of being formed: it actually looks like a small bump. It can be seen, very prominent, at the front of the embryo's body.
The embryo looks like a bean with little buds that will later form the arms and legs. The neck is also beginning to take shape. It measures almost 5 mm from head to rump. The lens of the eyes and the inner ear begin to develop, as do some organs: the lungs, stomach, liver and pancreas.
Week 7: Brain development takes center stage
This week, the brain's development takes precedence over everything else! A first outline of the cortex takes shape with the development of the two hemispheres, the cerebral cortex, and gray matter.
The head gradually forms. The back of the brain becomes wider, causing the embryo to tilt its head forward.
On its face, the nostrils can now be distinguished. The gum line, where the baby teeth will grow, is already formed. On each side of the head, 2 dark areas can be seen: these are the eyes! This is the retinal pigment that gives them this color.
All vital organs are under development. However, they are not yet functional. Inside the heart, the 4 cavities can now be distinguished. The embryo then measures 10 mm from head to tailbone.
More information:
- The brain is now generating new brain cells at a rate of 250,000 per minute.
- The neural tube, which will eventually become the brain and spinal cord, has closed.
- The first synapses, or connections between neurons, are forming in the spinal cord.
- The embryo's heart is beating at about 150 times per minute.
- The embryo is now about the size of a sesame seed.
Week 8: Movement Begins
The eyelids and external ears (which look like little pits) begin to develop. However, the first senses to "awaken" in the embryo will be smell and taste. At this stage, the nose and upper lip are already formed. From this week on, receptors that can detect odors will appear.
At the ends of the limbs, small buds can be seen. They will form the fingers and toes. The elbows and knees can also be distinguished. The embryo is no longer static: it moves its trunk and limbs. It moves inside the uterus, but it cannot be felt.
Several vital organs take shape: the stomach, intestines, pancreas and kidneys appear. However, they are not yet functional. At the end of this week, the embryo measures 12 mm.
More information:
- The embryo's heart is beating at a rate of 150 to 190 beats per minute.
- The neural tube, which will eventually become the brain and spinal cord, is closing.
- The embryo's face is becoming more defined, with the eyes, nose and mouth becoming more distinct.
Week 9: Fingers and toes now visible
During this week, the arms and legs lengthen and the joints (elbows and knees) become more defined. The wrists and ankles can also be seen taking shape. Inside, the muscles and nerves take shape.
The head is still large in proportion to the rest of the body. But it gradually rounds out and the neck straightens. We can distinguish 2 dark areas for the eyes (they are currently far apart on the sides of the head), 2 ear fossae and a single opening for the nose and mouth.
On a sensory level, touch receptors are now found around the embryo's mouth. During this week, the first taste cells will also develop. Depending on the sex of the child, the ovaries or testicles begin to form. However, it is not until the 3rd month of life that the genitals will appear.
The embryo looks more and more like a baby. Its limbs have continued to grow: all parts, including the joints, are clearly visible. The toes and fingers are well defined. The embryo can now make a whole range of movements! It measures 17 mm from head to coccyx.
Week 10: A Tiny Heart Beats Strong!
Your baby is now about the size of a grape, and their development is taking off at an incredible pace. This week, their eyelids have formed, protecting their developing eyes, and their external ears are starting to look more like yours, although they're still positioned a bit low on the head near the neck.
Measuring around 1.4 inches (35 mm) from head to rump and weighing in at about 0.3 ounces (8 grams), your little one's heart is beating at a remarkable 170 beats per minute! This rapid rate will gradually slow down throughout the pregnancy. You may even be able to hear this tiny heartbeat using a Doppler, a device that listens to your baby's heart rate.
A tiny heart, a rhythmic drum,Beats a symphony within,A life unfolds, a soul to come,A precious miracle to begin.
Eyes shielded now from light's embrace,Ears take shape, the world to hear,Tiny hands and feet find grace,As development draws more near.
A fragile form, so small and slight,Yet filled with strength and boundless might,Growing day and through the night,A beacon of love, so pure and bright.
Week 11: Embryo to Fetus
The embryo has become a fetus! Its head is still large compared to the rest of its body, but its growth will now slow down relative to that of the body and it will continue to round out. The fetus now measures 45 mm from head to coccyx and weighs 10 g.
The face is now easy to identify. The eyes have migrated from the sides to the front of the face. The eyelids now cover the eyes. The lips are also well defined and the fetus can open its mouth. It even drinks amniotic fluid. Fortunately, its kidneys and urinary system have started to function. They filter the swallowed liquid and reject it, in the bladder, in the form of urine.
In the mouth and nose, the first cellular structures have transformed to form the taste (papillae) and olfactory buds. The same is true for the salivary glands. And this week, the vocal cords will also begin their development.
Here are some other key developments this week:
- The fingers and toes are now separated and have nails.
- The bones are starting to harden.
- The external genitalia are now developed enough to determine the sex of the baby, although it may still be too early to see on an ultrasound.
- The heart is beating regularly at around 160-180 beats per minute.
- The brain is developing rapidly and the fetus is starting to make some movements, such as clenching its fists and flexing its arms and legs.
- It is important to remember that every baby develops at its own pace, so these are just general guidelines. If you have any concerns about your baby's development, be sure to talk to your doctor.
Week 12: Tiny Fingernails and Developing Nerves
Your baby's remarkable journey continues, with all the essential organs and systems now partially formed. Even the tiniest details are coming into focus, as evidenced by the development of their delicate fingernails. Until recently, a portion of the small intestine (the intestinal loops) resided in the base of the umbilical cord. Now, all of these structures have neatly tucked themselves inside the baby's growing belly.
Within the intricate world of the brain, nerve cells are multiplying at an astonishing rate, forging connections and creating a vast network of synapses. These early connections give rise to fascinating reflexes known as archaic reflexes. For instance, if you gently touch the palm of your baby's hand or the sole of their foot, they'll instinctively curl their fingers or toes. This reflex, along with other archaic reflexes, will accompany your newborn for a while before gradually disappearing, marking the remarkable development of their nervous system.
By this week, your little one measures a remarkable 60 millimeters from head to rump and weighs approximately 18 grams. The entire uterus is now filled with the amniotic sac, providing an ideal environment for your first ultrasound scan. As this week draws to a close, the risk of miscarriage significantly decreases, giving you the confidence to share the wonderful news with your loved ones!
Here are some additional points that you may find helpful:
- The development of fingernails is a sign of the baby's maturing skin and sensory system.
- The formation of synapses in the brain lays the foundation for future learning and development.
- Archaic reflexes provide valuable insights into the baby's early neurological development.
- The first ultrasound scan offers a glimpse into your baby's growth and well-being.
- Sharing the news of your pregnancy with loved ones can bring immense joy and support.
Remember, every baby develops at their own pace, so don't worry if your little one doesn't seem to be following this exact timeline. Embrace the wonders of this extraordinary journey and cherish every precious moment.
Week 13: Vertebral Column
The first bony tissues begin to take shape, particularly those of the pelvis, ribs, head and limbs. However, they are still rather soft. The spine also gradually develops.
At the same time, the buds of the milk teeth appear. Underneath the milk teeth, the adult teeth will grow. These will form later in pregnancy, during the 2nd trimester.
The fetus's blood is formed. The first red blood cells are produced in the yolk sac (umbilical vesicle), a round structure attached to the abdominal wall, and then in the liver. After a few months, the bone marrow will take over.
The fine skin of the fetus becomes covered with very fine hairs: this is the lanugo. It usually disappears before birth. The skin pigments (which give it its color) also develop. The skin, until then, was almost transparent. In addition, touch receptors now cover the entire face, palms of the hands and soles of the feet. The fetus then measures 75 mm from head to coccyx and weighs 30 g.
More information:
The yolk sac is a temporary organ that provides nutrients to the developing embryo. It is no longer needed after the placenta forms, and it is reabsorbed by the body around the 12th week of pregnancy.
The liver is one of the first organs to develop in the embryo. It begins to function around the 5th week of pregnancy and is responsible for producing blood cells, digesting food and storing nutrients.
Bone marrow is the spongy tissue inside bones that produces blood cells. It is found in all bones except the skull and facial bones.
Lanugo is a fine, downy hair that covers the entire body of the fetus from around the 12th week of pregnancy. It usually disappears before birth, but some babies may be born with a small amount of lanugo.
Skin pigmentation is determined by the amount of melanin in the skin. Melanin is a pigment that protects the skin from the sun's ultraviolet rays. The amount of melanin in the skin is determined by genetics.
Week 14: Your Baby's Development
Your baby's face comes alive!
Some features, such as the nose and mouth, become more defined, the ears have taken their place on the side of the head, the fetus is also able to make certain facial expressions such as frowning and grimacing. These are mostly reflex reactions at this stage, another distinctive feature of your future baby appears: fingerprints.
Your baby starts to react to external stimuli:
- If you push on your belly, your baby may respond by pushing back.
- At this stage, they may also have started sucking their thumb.
- Your baby's sex will become apparent this week or next.
In girls, ovarian follicles form; in boys, the prostate becomes visible. At your next ultrasound, you can find out the sex of your future baby if you wish.
At the end of this first trimester, the fetus measures 85 mm (3.3 inches) from head to rump. It weighs about 45 g (1.6 ounces).
Here are some additional details about your baby's development at 14 weeks:
Brain development: The brain is growing rapidly and developing new connections. This is why your baby is starting to make more movements and facial expressions.
Skeletal development: The bones are starting to harden. The fingers and toes are now fully formed.
Muscle development: The muscles are getting stronger and your baby is starting to have more control over their movements.
Digestive system: The digestive system is starting to function. Your baby is now swallowing amniotic fluid and passing it as urine.
Lungs: The lungs are still developing, but your baby is starting to practice breathing movements.
It is important to remember that every baby develops at their own pace. Some babies may be more advanced than others at this stage. If you have any concerns about your baby's development, talk to your doctor.
Here are some things you can do to support your baby's development:
Eat a healthy diet: Make sure you are getting enough of the nutrients your baby needs to grow and develop.
Take a prenatal vitamin: A prenatal vitamin can help ensure you are getting enough of the essential nutrients your baby needs.
Avoid alcohol, tobacco, and drugs: These substances can harm your baby's development.
Get regular exercise: Exercise can help you stay healthy and reduce your risk of complications during pregnancy.
Get enough sleep: When you are pregnant, you need more sleep than usual. Aim for 7-9 hours of sleep per night.
Manage stress: Stress can harm your baby's development. Find healthy ways to manage stress, such as yoga, meditation, or spending time in nature.
By taking care of yourself, you can help your baby develop and grow healthy.