Week by Week Fetal Development Timeline For Moms-To-Be

Studying and understanding week by week fetal development is surely exiting for every mom-to-be.

For many women, getting pregnant is a big task by itself. And for those women who are fortunate enough to get pregnant, the next nine months will become the most exciting time in their entire life.

There are plenty of resources available over the internet and in books providing you invaluable information about pregnancy and childbirth. But, do you have the time to look and glance over all those pages of information, with your busy pregnancy schedule?

One of the shortcuts is to use a pregnancy calendar. It helps you find the right information and things in one place.

Though the first few weeks of pregnancy seem to be quite torturous and painful, the succeeding weeks will be those of happy memories and good feelings. By the end of the first trimester, you will be proud to show off your bump to your friends and colleagues.

Every woman will be too keen to know what exactly happens and how the baby develops in her womb. When you study the growth and morphological changes of your baby from the first week till the last week, you’ll be amazed to know that you’re acting as a custodian for a tiny little mass of life.

Stages of Fetal Development

“What are some of the most important stages of baby development in the womb?”… parents often want to know.

Let’s start by looking at a bit of background.

A tiny life begins to take form in a woman’s womb, soon after the process of conception, when a highly motile sperms impregnates and fertilizes a highly fertile ovum egg. Biological processes have created a unique mechanism, when at the moment of conception, a unique organism comes into existence.

This new organism is essentially lovely in its nature and has tremendous ability to undergo several anatomical changes, throughout a nine month period till such a day, when it comes out of the mother’s womb.

  • At the moment of conception, a male sperm unites with a female ovum to fertilize it. The single celled organism thus formed, is called a zygote,
  • A zygote is a wonderful repository of biological information of both the parents, and it may include such critical information like 50,000 genes that belong to both parents,
  • Fertilization occurs in the Fallopian tube, and soon after this process, the tiny mass of cells starts migrating to the interior portion of the canal and implants itself on the uterine wall of mother’s womb,
  • Within the next 24 to 48 hours, the tiny zygote starts multiplying at a furious rate and becomes a stage called Blastocyst, which is also called a placenta; a placental structure contains almost 150 cells in the shape of a tiny ball,
  • This life form is also called embryo, and this stage will last from the first cell division stage, until eight week of the unborn baby development,
  • Whereas, the stage of an unborn baby from eight week of development up, until the birth is called “fetus”.

Though these stages are classified according to the cell division and primary developmental stage, you can name your baby in any way you like; call it a tiny tot, baby, infant, toddler, or child, it is entirely up to you.

Along with the most basic anatomical changes, there are several other compulsory changes that occur throughout the pregnancy period.

Here are just a few stages or milestones of baby development in the womb:

  • At 4 weeks from conception: Eyes, ears and respiratory stem begin to develop at a rapid pace,
  • Thumb sucking practice will be first observed during the 7th week,
  • At 8 weeks from conception: Your physician will start feeling the heartbeat of your baby, through the ultrasonic stethoscope,
  • 9 weeks from conception: Your baby can hold a tender object, by bending their finger around it,
  • 11 to 12 weeks from conception: Steady breathing will occur and this continues till the labor,
  • 11 weeks from conception: Your baby will learn to swallow the amniotic fluid
  • 13 and 15 weeks from conception: Baby will also develop the sense of taste by creating a functioning taste bud,
  • At 16 to 20 weeks: Your baby becomes very sensitive to external noises and mother’s own soothing heartbeats,
  • 23 weeks from conception: Baby will develop a definite sleep pattern,
  • Six months from conception: Oil and sweat glands will start forming, as a system to eject the waste from the body,
  • 7 months after conception: Baby will start moving tirelessly by stretching and kicking,
  • 9 months till the birth: the baby will put on a considerable amount of weight of almost half a pound per week. Its skin begins to start thickening, and it also starts the breathing practice, by consuming more than a pint of amniotic fluid.

Learning to know and understand week by week fetal development within your womb is a great lesson that teaches you many principles of nature. Here are some bits of information on how your baby grows and develop from week one to week thirty-six.

First Trimester – Weeks 1 to 12

Baby development starts right from conception when a highly motile sperm impregnates a highly fertile egg. At this initial stage, the sperm penetrates the ovum and fertilizes it to create a small node like a cell mass called the zygote. This little zygote takes three to four days to reach the warm and cozy environs of the uterus. All these significant developments will be carried out within the first two weeks and this process lays the foundation for the creation of a beautiful baby.

Throughout the fourth week, there are a series of developments within the uterus. The embryo may float in the uterus for more than 48 hours, before implanting onto the walls of the uterus. Once there is a confirmed connection between the embryo and the uterine wall, a highly sophisticated tool called the placenta, starts developing to connect the embryo and the uterus. This is often called the lifeline for the undeveloped baby, as it is the root through which life support is provided to the unborn baby.

By about the sixth week of pregnancy, the embryo reaches about 1/5th of an inch and a primitive heart develops and starts beating. Several organs like the head, mouth, liver, and embryo will also start developing at this time.

Well into the eighth and tenth week of pregnancy the embryo becomes quite larger in size (1 inch) and several facial and limb features become visible under an imaging machine. At this stage, the baby can perform several activities all related to the nervous system.

Stated differently, first trimester baby development surely is the basis of new life.

couple holding hands over baby bump
Your baby bump isn’t even that noticeable in the first trimester, although symptoms start to emerge.
  • It is the ripe time for that elusive ovulation. You just need a single egg to be fertilized by a sperm. It finally occurs.
  • It is also the time for conception in your Fallopian tube. Healthy sperm will fertilize another healthy egg.
  • The fertilized egg travels down to the uterine area to implant itself on to the wall. This stage is called the implantation stage. The process of implantation may sometimes cause spot bleeding, which is quite harmless and common.
  • Once embedded on to the wall, a primitive neural tube forms, which eventually becomes the nervous system.
  • Next on the line are the primitive circulatory system and a tiny heart. This combination will provide a solid foundation, on which the entire life will be supported and nourished.
    The most significant anatomical development in the fifth week is the beating heart, whose murmurs could be heard with the help of a special stethoscope.
  • You’ll also see the appearance of an umbilical cord, that is designed specially to provide fresh blood, oxygen, and food to the young fetus.
  • The rudimentary heart will function on its own, by pumping blood through its primitive circulatory system.
  • This is also the stage for other organs to develop and sustain; along with the lungs, important organs like hands, feet, and arms also start developing.
  • During the sixth week, the limbs continue to develop at a quick pace, to create a strange sensation in your womb,
  • Brain cells will also multiply at an alarming pace; almost 100 billion cells will form with in the next two or three months.
  • Eyes will also make their debut, and a pair eye buds will form at the edge of the head,
    Internal digestive organs like pancreas, stomach, intestine, and liver will soon appear during the sixth and seventh weeks.
  • During the seventh week, you will also see tiny little feet and hands, jetting out of the body with a well-formed elbow to support the hands. A pair of legs will support those tiny feet as well.
  • At this time, a thin, but firm gum line will also form in the mouth, which soon will be the solid base for plenty of teeth.
  • By week eight, cartilages and bones will occupy their place in the body; you may need plenty of calcium supplement at this time to start feeding your baby with calcium.
  • Eyes and digestive systems are soon evolving as efficient internal organs.
  • Your baby would have also grown bigger and healthier during this month.
  • Your baby will also start moving both in sideways, as well as upside-down. This stage is the most significant stage, as you’ll start experiencing those tingling sensations in your stomach.
  • The sense of mobility will soon be developed by the tiny little baby, as it will start trying to hold things that are within easy reach.
  • The fingers will also start showing distinctly formed fingerprints and the fingers will also have trough-shaped ends, where the future nails will grow.
  • By week ten, your tiny tot will become a fetus, meaning a larger form of life with highly advanced organ systems in its body.
  • Now that all the major organs are firmly in place, a new phase will start when the baby starts growing at a rapid rate.
  • Eyes are still shut down, but the cornea and iris will start forming within the shut eyelids.
  • The placenta will start working during this time, to transport fresh food and oxygen to feed the baby.
  • By week eleven, hairs and nails start to grow and develop.
  • Kidneys also start functioning, as amniotic fluid wastes will be ejected by the tiny pair of kidneys.
  • The genital organs will also develop and take the set gender characters; this is the time when you can check the sex of your baby.
  • The muscle system is still primitive, but this primitive system will start to contract and swell to practice contractions that digest food.
  • To bolster the body’s biological functions, all critical internal organs like the intestine, liver, and stomach will complete their growth.
  • Ears shift to the sides of the head, while the eyes also begin to move close to each other.
  • Very tiny vocal cords will also begin to take shape so that by the end of the pregnancy tenure, the baby will be ready to cry and announce that it has arrived.

Second Trimester – Weeks 13 to 24

The second pregnancy trimester is a phase of consolidation when all the external and internal organs start stabilizing into a well organized and cohesive unit. Throughout the weeks fourteen and twenty-two, the fetus develops into a strong and virile body and performs several actions and functions like beating hands, moving limbs, and clenching the fist.

It is also easy to notice the development of other critical organs like fingernails, toenails, and eyelids. The tiny tot can move and shift its body, limbs, and hands with considerable ease.

fetal development second trimester
Your baby bump starts to get really noticeable.
  • Week thirteen is a month of continuous growth; all organs begin to finalize their shape and morphology, while the heart is increasingly getting rhythmic and resonant,
  • Baby’s neck is getting longer and stiffer,
  • The placenta is also quite busy in nourishing the tender baby,
  • By the fourteenth week, thyroid and other hormone-secreting glands will get to work to produce minute quantities of hormones,
  • The skin is still transparent and thin fine hairs (lanugo) starts appearing on the body,
  • Bones will also get stronger day by day all the way, till the last week,
  • Muscle and brain coordination starts to show up,
  • 20 milky white teeth buds are also developed,
  • Rapid skeletal movement is observed throughout the trimester,
  • First signs of fluttering movement,
  • Fetus gains more weight and becomes mature,
  • The fetus also becomes very sensitive to sounds and light,
  • Some hint of fat deposition over the body.

Many parents search for more information on week 16 and week 17 fetal development, so we’ve decided to cover them in a bit more detail.

Week 16 Fetus Development

The 16 week baby development stage is an important point in the development of your baby.

This is when your baby starts building up fatty cells underneath their beautiful skin. Roughly at the end of 4th month and soon after 16th week, your baby just starts to show a remarkable growth spurt, both in height and weight. At this stage, it is just around 5 inches long and about 4 ounces in weight.

And that’s just the beginning…

The 16-week baby development coincides with a stronger heart capable of circulating around 5 gallons of blood per day. Heart muscles are getting stronger and tougher to work at a very high rate soon after birth. A high rate of heartbeat means that it is working without end to promote stronger circulation. And also to make available plenty of food to the cells.

At this point, you’ll also notice a small twist like formation on their head, which eventually leads to growth of hair.

Surprisingly, your baby also starts to show some reflex actions, including minor twitching movements, sucking, swallowing, and slight blinking of eyes. Eyes will also be able to recognize some light – but quite dim in nature.

It is also another remarkable, busy week, as your baby gulps a lot of amniotic fluid. This is a test run before it can breathe normally after birth. Its small lungs are powerful and develop to breathe in lots of air.

Her urinary and kidney system starts functioning very slowly, but steadily.

By the end of the 16th week, your baby would have become one playful creature. The first game that your baby will most surely involve herself in, is clutching at the umbilical cord. If you watch any video on baby development, you’re more than likely to see a baby pulling and playing with the umbilical cord. This playful event continues from a few minutes to a few hours, depending on your baby’s development.

At around the 16-week baby development stage, the circulatory system becomes more efficient and independent. This simply means your child can inhale and exhale amniotic fluid through their lungs.

Definite reflex actions develop as a means to defend herself from unknown disturbances. Your baby develops jerky movements and loud noises make your baby extremely agile and alert.

The absorption of food through the umbilical cord is high, and your baby grows at a very fast rate. The fetus starts looking more solid and well defined now, and the whole body starts putting on muscles and skin.

The 16 week baby development stage is also a critical time for you as mother.

You need to undergo several medical checkups and diagnostic tests. Fortunately, if you’re feeling a bit wheezy and not up to it, your doctor will most probably prescribe several simple drugs, vitamins, and mineral supplements to promote the healthy growth of your baby.

But in short, this 16 week baby development interval is one full of excitement and thrill.

Week 17 Fetus Development

During the 17 week baby development stage you undergo a few remarkable changes.

Also, as a would-be mom, you’ll undergo several prenatal tests to confirm some crucial parameters. Two of these diagnostic tests are important: the triple test and amniocentesis.

These are two very important genetic tests used to check for the incidence of neural tube defects and also Down Syndrome. In certain circumstances, you’re not required to undergo these tests. But, if you’re a woman of over 35 years, it’s advisable to take the amniocentesis test. This test verifies the presence of Down Syndrome – which gets more likely the older you as mother-to-be becomes.

Two things can happen.

One of the remarkable highlights of this time period is the distinct growth of your belly. This is when the weight of the fetus starts putting pressure on your abdomen.

Another significant change is the feeling of breathlessness caused by the enlarged uterus. Fortunately, this problem is very temporary, as the stomach muscles soon start stretching to accommodate the growing uterus. Your uterus will finally settle down into your pelvis, before the end of the 18th week thus making breathing slightly easier.

Nature gives a pregnant woman a wonderful way to feel the baby in her womb.

Get ready to experience the exciting sensation of your tiny baby, kicking very slowly but with certainty. This kicking sensation starts right around the 16th week and continues throughout pregnancy.

Also, start experiencing the thrilling conversation between your baby and you, in the form of gentle pushes and kicks. These are considered Nature’s signals to tell you that your baby is alive and energetic.

9 Marked Changes

  • At 17 weeks, your baby may weigh about 6 ounces and measure only 6 inches from head to toe. The head starts gaining some real shape and size is proportional to the body shape.
  • The muscles strengthen and the baby can hold their head more erect.
  • The skeletal system starts to add more bone to it, and the whole structure looks more solid and assured.
  • Primary limbs start to form at this stage, while fingers are getting formed into nice little pads.
  • The eyes are also slowly shifting towards the front, although the eyelids are still closed.
  • Your baby now develops a sense of hearing and can listen to your own heartbeats.
  • As with any growing and developing being, your baby needs constant nourishment given by the umbilical cord which starts thickening and day by day gets stronger to support the baby.
  • Another curious event is your baby’s urge to consume meconium, soon after swallowing the amniotic fluid.
  • All major systems like urinary, excretory, respiratory, and circulatory systems are slowly developing into a full-fledged machine.

Finally, as your baby starts to enter the 18th week of development, you can expect more radical changes in its development calendar.

Third Trimester – Week 27 to Birth

The third trimester is a critical phase when the baby development involves the growth and development of all organs – both internal as well as external. The baby can even cry. At this critical stage, your baby develops the ability to sustain the rigorous environmental conditions of the outside world. This pregnancy trimester also marks the end of the nine months growth period. At the end of this period, your baby is ready to face the outside world and lead a happy life with their mother and father.

baby bump 3rd trimester
Getting ready for delivery in your 3rd trimester is important.
  • Heartbeat is stronger and can be heard by a stethoscope,
  • Eyelids start to open very slowly and clear eyes are easily visible,
  • There is a definite pattern in sleep and wakefulness,
  • Lungs are ready for breathing,
  • Taste buds also form at this stage and the nourishment is stepped by the placenta,
  • By the end of the twenty-sixth week, eyelashes and brows are completely formed,
  • Eyes are focused and sharp,
  • The brain will continue to grow, accompanied by strong sensory feelings,
  • Skin gets clear now with wrinkles slowly disappearing from the body,
  • The fetus really looks like a baby, as the head looks quite proportional to the body,
  • Eyes are very sensitive to the light, while the finger and toenails reach their full length,
  • Each cartilage is quite soft and well-formed,
  • The entire body system is completely functional,
  • The head gets firmer day by day, and the whole body is sustained by a consistent supply of food,
  • The baby moves almost regularly by kicking and moving,
  • The baby starts to put on more weight during this time,
  • The baby also takes a head-down position to facilitate an easy delivery,
  • But, the scalp region is still underdeveloped to accommodate the body through the narrow birth channel.

Throughout pregnancy your baby would have added weight and height very slowly. But in the last few weeks the increase in weight is markedly rapid.  

Sometimes things don’t go as planned and premature babies are born… that’s typically before 37 weeks. One of the biggest risks is lung development problems.

Also, parents want more than just an overview of week by week fetal development. Often they look for more on week 33 and week 34 development.

Week 33 Fetus Development

The 33-week baby development stage is almost like being at the 26-mile mark in a marathon… not that I run. But my husband always says it’s so close to the end… you’re hurt and tired, but still, an unbearably long way to go (even though it’s less than half a mile to go).

You may just start to get a bit anxious and nervous. But, do not worry.. everything’s OK.

You’re just three weeks way from delivering that cute little baby.

This is a period of immense hope and anticipation. It’s a significant milestone in your entire pregnancy. Your baby has already witnessed some remarkable events in its lifetime.

Roughly at about the 33 week baby development stage you’d better be careful and cautious in looking after yourself… and what and how you lead your daily life. But don’t be too sluggish and lazy; little bit of exercise and limb movements is great for the upcoming delivery. This is a phase of consolidation and retention – most probably you’ll be asked to get ready for the labor, by applying your mind for the event.

At this stage your baby typically weighs a hefty 2 kg and measure about 18 inches in length. The head too will have grown to a size of around 9 cm. Your baby would have become a big guzzler of more than a pint of amniotic fluid every day.

Surprisingly, you will also start gaining a lot of weight (500 g) every week, of which more than half is exclusively reserved for your baby.

At this critical stage, you may wish to eat and drink more. This is quite normal as you need the nutrients to sustain your baby. Both you and your tiny baby need the energy to sustain bodily growth. It may be advisable to take a calcium supplement. Your baby constantly draws calcium from your body to build their bone structure. Consult your doctor to seek opinions on taking calcium supplements. Your doctor most likely will recommend a good calcium supplement to counter the loss in calcium from your body.

At roughly the 33-week baby development stage your baby starts using their lungs to mimic breathing by inhaling amniotic fluid. The baby does this “practice’ trouble-free air inhalation, soon after delivery.

There is a perfect balance between the intake of amniotic fluid and the excretion of digested amniotic fluid. Hair is also growing rapidly, while the toe and fingernails harden and start growing longer. Sometimes nails grow so rapidly that you’ll have to trim them soon after birth. In fact, there are many cases of unborn babies scratching their faces even before birth.

As you enter the 33 week baby development stage, you may need to be very calm and relaxed. Best advice is to be mentally active and alert to prepare for the approaching labor.

Week 34 Fetus Development

The 34-week baby development stage is yet another important milestone in your entire pregnancy cycle. You’re almost near the end of your pregnancy. Soon you’ll be the proud mother of a beautiful, pretty, fit, and healthy baby.

The last remaining few weeks of your pregnancy could really be tiring and uncomfortable. And you may even start feeling a bit worried about the health of your baby. That’s normal.

Also, the extreme weight of your baby and body really applies a lot of pressure, which is largely responsible to make you feel terribly tired and listless. However, these things are quite common. Despite this, you’ll need to be active and alert and conserve your energy for the big day.

During this time period, you’ll most probably be tested for Group B Streptococcus infection, either in your uterine canal or in your rectal area. This is a fancy word for a bacterial infection. This bacteria may cause problems for your baby.

It is known to cause diseases like meningitisbacterial blood infections, and pneumonia. In the unlikely chance that you prove positive for this test, your doctor will most likely prescribe an antibiotic to treat it.

As you get ready for the labor, you may also start thinking about the kind of relationship that you’re going to establish with your baby. Adding a new member to the family could be very thrilling and exciting, as well as a big responsibility. Most probably, you may also start feeling the big sense of responsibility for caring for your baby. You can even strengthen the family ties. A new addition to your family often keeps all other members on their toes.

The 34 week baby development phase is a period of rapid growth and quick development. Your baby will now be about 18 inches in length and about 2.5 kg (5.5 lb) in weight. Should your baby be born now, it will most likely be able to survive outside the womb.

Excessive fat starts appearing on the body, while the eyes are getting very responsive to changing light stimulation. The fingernails are completely formed with longish nails ready for instant clipping soon after delivery.

At about the 34-week baby development stage some women experience the baby turning their head towards a posterior or down position to get ready for the delivery. Of course, the skull is still very tender to help a smooth delivery – should it be necessary.

Your baby has also developed a perfect sense of hearing. That’s one of the reasons why the baby can be calmed by your low and smooth voice.

Also, your baby developed immunity against ordinary infections and illnesses. She’s inhaling amniotic fluid at a furious rate to mimic breathing.

But for now, enjoy the last peace and quiet… before you hear how well the lungs have really developed.

How To Maximize Unborn Baby Development

It’s generally known that unborn baby development and growth start from the moment they are conceived.  In fact, the period in the womb is the highest degree of development a baby has in his or her lifetime.

Development goes from a single cell to many cells that ultimately differentiate into endoderm, mesoderm and ectoderm and further into the parts of the body we recognize as the brain, the spinal cord, the body, the heart and the extremities, which increase in complexity as the baby develops.  

But, what’s not well known is that parents must do whatever they can to maximize growth and development during all aspects of embryonic and fetal development… thus, unborn baby development phases.

Here are things to AVOID during the growth phase

There are things a mother can do that negatively impact the child’s intellectual and physical development. 

  • Avoid smoking. Smoking is particularly hazardous to your baby’s development.  Smoking contain more than 2500 chemicals, some of which are dangerous to baby.  The baby is more likely to be born prematurely or smaller than average if the mother smokes.  The US Public Health Service indicates that if all of the 10 percent of women who currently smoke in pregnancy quit smoking, there would be an 11 percent reduction in stillbirth deaths and a 5 percent reduction in newborn deaths after birth. 
  • Avoid alcohol. Alcohol in pregnancy is similarly dangerous and can negatively affect the development of your baby.  There is a condition called fetal alcohol syndrome that occurs in infants whose mothers drank during pregnancy. This condition affects both the intellect and physical appearance of the child. 

Fortunately there are also a few things you can do to improve your baby’s  

Here are things to DO during the growth phase 

Ensure enough prenatal vitamins. One of the best things a mother can do to enhance unborn baby development is to take adequate prenatal vitamins.  Besides the usual vitamins you’ll find in a regular vitamin, prenatal vitamins are chock full of folic acid, calcium and iron. And all these things are important to the development of your baby. 

Folic acid, specifically, is necessary for the closure and development of the spinal cord and brain.  Not enough folic acid (also called folate) means that the baby can develop an abnormality known as spina bifida or anencephaly. 

Ensure adequate minerals. Calcium is important for the baby’s bony development.  Normally, babies are born with their bones only partly developed and then completes outside the womb.  Nevertheless, calcium is vital to the strengthening of the bones and development of the unseen teeth. 

Iron is important to blood development.  We need iron for all our red blood cells so it is needed from the time the baby has its first blood cell.  Other minerals are important to develop good cellular structure and development of bodily structures.

Stimulate unborn baby development. There are specific things parents can do to stimulate and encourage their baby’s intellectual development while still in the womb.  Research has shown that babies can hear by the eighteenth week. It has also been shown that music enhances your child’s neurons and gives them stimulation in the womb, which is normally a boring place to hang out in. 

So, begin around the 18th week to play music and talk to your baby. Your baby soon learns to recognize and be soothed by your voice, both when still in the womb and once s/he is born. You will definitely give your baby a valuable headstart.

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