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All You Need to Know About Fetal Development

A baby brews for nine months in the mother’s womb. There are many stages of growth and development, all of which are important. Each stage has its own unique needs and growth patterns.

prenatal development stages

But, if you do a general search for how a fetus develops, you’ll come across many different and differing explanations.

One of the hardest to answer questions is when does life begin? As we will show, a baby can survive after about week 22 at the earliest, and that is with extensive medical intervention and luck. Less than 25% of babies born between weeks 22 and 26 survive.

Yet, if we truly look at survival and the ability to be on its own, a baby can’t survive on its own until it is several years old.

We are not debating when life begins because we can’t answer that. We do know when a fetus develops gender, when fetal lungs develop, when a fetus starts to kick, a when a fetus starts to feel pain.

We’re going to go through the fetal development week by week in order to help you understand what is happening. Most weeks are combined, as little new development is happening. Day by day development of the fetus is highlighted in the first couple of weeks because that is the time for the newest features to form.

One of the most important things to remember is that while all stages of fetal development are important, the first trimester is given slightly more importance because of the organ formation.

when human organs start to develop infographic

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Your doctor will be able to tell you if your fetus is healthy or if problems are occurring. While growth should be continuous and steady, there are many who go through spurts. If your baby isn’t growing, your doctor should check it out. But, many times, it’s just a delay and a growth spurt will happen soon.

Basic Terminology


the unfertilized egg


lining of the uterus


the fertilized egg


group of undifferentiated cells in the fallopian tubes


first differentiation of cells


when the blastocyst bonds with the endometrium


entrance/exit of the uterus

Mucus plug

biological block to the uterus/stops the release of amniotic fluid (occurs shortly after implantation)


growth of undifferentiated cells prior to week 8 of implantation

Amniotic sac

biological membrane that surrounds the embryo and fetus and contains amniotic fluid


 a round, flat organ that transfers nutrients from the mother to the embryo/fetus, and transfers wastes from the embryo/fetus


growth of differentiated cells after week 8, now vaguely taking on shape of an animal


growth outside of the womb (newborn)

Early Stages of Pregnancy

pregnancy duration in weeks and months infographic

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The different stages of prenatal development are divided into three trimesters of 13 weeks each. This time is a bit flexible since most women don’t know the exact date their egg was fertilized. Early fetal development is considered one of the most important times and also one of the most mysterious.

Weeks 1 & 2

You are not actually pregnant yet. Doctors start the count of pregnancy at the time of your last period, or about 1 to 2 weeks prior to fertilization of the egg. This is a time when the egg is finishing its development and preparing for release. The uterus is preparing for a fertilized egg to implant.

Your doctor will calculate the stage of pregnancy based on these weeks.

first two weeks of pregnancy signs and symptoms infographic

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Week 3

This is when fertilization happens. We recommend you read this article on ovulation and this article on fertility to understand the process of ovulation and how conception occurs.

When the sperm meets the egg in the fallopian tubes, they combine to form the zygote. Then, a rush of cell divisions and expansions take place forming the morula, a little ball of cells. This whole process takes a few days and exhausts the reserves that were stored in the original egg.

Week 4

pregnancy week 4 baby size

Just before the morula reaches the uterus, it suddenly folds in on itself creating 2 layers of cells. It is now a blastocyst, that kind of looks like a tube.

The outer layer of cells forms the placenta, which is used to connect to the mother and nourish the fetus. The inner cells continue to divide and will eventually form the baby/fetus.

The ability to implant and form a bond with the mother is not guaranteed. There is still about a 25% chance implantation will not occur. This can be due to many factors, such as the endometrium being too thick or thin, damage to the uterus, or insufficient hormones.

When the blastocyst implants into the uterus, it triggers luteinizing hormone (LH) to be released. This hormone is responsible for the maintenance of the pregnancy. You can read more about this hormone and how we detect pregnancy here.

There is still another 25% chance the hormone will not be sufficient to maintain a pregnancy and a menstrual cycle will occur. It is completely natural for this to happen, as well as implantation not occurring. For the women this happens to, they never knew there was a fertilized egg. It is also one of the biggest arguments against life occurring at conception; how could it happen if nearly half of all fertilized eggs never develop through completely natural and normal means?

Week 5

week 5 pregnancy facts

This is the big week for prenatal development. As the blastocyst turns into an embryo, it begins another series of twists and folds to create organs and body systems.

First, the ball of cells turns in on itself again, resembling a tube of cells once more. This will eventually turn into the whole digestive system. Yes, technically everything from the mouth to the anus is considered outside the body. We are a big tube.

From here, cells begin to differentiate and clusters of cells form the heart, brain, eyes, and connective tissues. By the end of week five, we look like a tube with bumps.

As the tube grows, it develops into 3 layers. The top layer is the ectoderm. It will become the skin, the eyes, central and peripheral nervous systems, inner ears, and many connective tissues.

The mesoderm, or middle layer, will start to form the heart and a primitive circulatory system. It’s not the true circulatory system and is directly connected to the mother. Later on, this layer will form bones, muscles, kidneys and much of the reproductive system.

In inner layer, the endoderm, forms most of the mucous membranes of the body – lungs, digestive system, and bladder.

At this point, the embryo is about the diameter of a dot left by the tip of a pen. It begins to resemble an abstract drawing of an animal.

Week 6

Here is when the heart cells begin to beat. The heart is not formed, it is just a group of cells responding to stimulus from the mother to contract.

Another tube has formed that will eventually form the brain and spinal cord. More small tube-like shapes form, known as the pharyngeal arches, form and are colloquially called gill slits. These arches eventually form bones and other organs.

Small buds that will eventually become limbs begin to form. Now, too, the tube begins to take on more animalistic features. The C shape fetuses take starts to happen. If you squint, the embryo starts to look like a mouse combined with a tadpole and is the size of rice grain.

Week 7

pregnancy week 7 uterus size

This is a big week because the head is finally starting to develop. You cannot see the features on an ultrasound because the embryo is still only the size of a pea. The nose, eyes, ears and mouth have started to develop. The brain starts to develop, but there are no neural impulses, yet.

The arm and leg buds now look like tiny paddles. At this stage, a dolphin and human look identical.

Week 8

Your embryo is now taking on human characteristics. The eyes are forming, as are fingers, ears lobes, and other delicate features.

The tail we originally had is being absorbed and disappears around this time. Now, the embryo is about the size of a kidney bean.

Week 9

Development continues and now toes start to form. It is about this time that you can begin to see a fetal development on the ultrasound, rather than just a rounded spot.

The embryo is about ¾ of an inch long and about as around as a pencil.

Week 10

This week more human features emerge and the neck forms. The head is still about half the size of the fetus. We are now calling it a fetus, rather than the embryo.

Week 11

pregnancy week 11 size

Red blood begins to form in the liver of the fetus, beginning to form the basis of a true circulatory system. The heart cell cluster has begun to form chambers and is still contracting erratically.

The genitalia begins to form, with all fetuses beginning as female. The chromosomes from the father will determine whether the fetus develops a penis, although, with new DNA mapping, we now see there are cases where this isn’t always true.

Week 12

The fetus is now about 1 inch long, or about the size of your nose. All human features have formed, even if they need some maturing.

Middle Stages of Pregnancy

This is considered the second trimester for fetal development. During this time, very little new organs and functions are forming. The ones that have already formed continue development and take on better and better function.

Week 13

In this week, the urinary system begins work and urine is discharged. This is considered the first excretion by the fetus and it goes directly into the amniotic fluid.

Bones begin to develop, especially in the head and extremities. Tooth buds begin to form. The genitals are forming.

Week 14

At this point, the fetus will begin to really look like a baby. The arms are nearly proportionate to the body and there is now a defined neck. In ultrasounds, you might be able to tell the gender of the fetus, whether it is a boy or girl.

The fetus is about 3.5 inches long, but still has no definite structure other than skin holding it together.

Week 15

pregnancy week 15

And here is where bones begin to form. All other organ development is proceeding and the physical features have begun to develop. The basic hair pattern begins to form.

The fetus has reached 4 inches long, or about the size of a peach.

Weeks 16 & 17

Muscle tissues begin to twitch and learn to move together. This is one of the first motions of the fetus. However, since the brain is not yet developed, these motions are involuntary.

At this time, the fetus begins to make sucking motions. Fat begins to accumulate under the skin and around organs, providing insulation and padding.

Weeks 18 & 19

The tympanic membranes have formed and the ears are now functional, although they can’t actually hear because the nerves are not formed yet. Vernix caseosa, a greasy, cheese-like coating, begins to form around the fetus to protect it from the amniotic fluids. These fluids can cause abrasions and injury to the fetus.

The fetus has now reached the size of a grapefruit.

Weeks 20 & 21

pregnancy week 21 baby size

Around this time a mother may feel fluttering in her abdomen fetal movement begins. This is called quickening.

During this time, the fetus begins the swallow reflex. Yes, your baby is swallowing amniotic fluids. During these weeks, and the next couple, the fetus begins to put on significant amounts of weight. We want to define significant as being only about 8 to 16 ounces, or less than a pound. But, it is the equivalent of doubling in weight.

Weeks 22, 23 & 24

The first hairs, the lanugo, begin to grow over the entire body of the fetus. It helps the Vernix caseosa stick to the fetus. Footprints, fingerprints, and the hair on the head begins to grow. The genitalia is now fully formed. Females will now have all the eggs in their ovaries they will ever have, although they are exceptionally underdeveloped.

The skin moves from a translucent, thin coating to actual skin during this time. The layers develop, as well as color. The nerves in the skin begin to develop and reflex reactions begin to happen. The fetus is now about 8” long and weighs about a pound and a half.

Late Stages of Pregnancy

The final trimester is where we really start to see a woman as pregnant. This is also a time of strange food cravings and real motion of the fetus.

Weeks 25 to 27

pregnancy week 25 vegetable

Motion becomes voluntary as the nerves are developed and the brain is able to respond. When the mother places her hand on her stomach, the fetus is able to feel the pressure and push back.

Air sacks form in the lungs, although the lungs are not ready to actually breathe. If special, intensive care is given to a premature birth and luck is on your side, this is the earliest a baby has the possibility to survive. The chances of neurological impairment are very high, even with the best of care. The fetal lung development is not yet complete.

A fetus can start feeling pain at week 27. Prior to this, the nerves that sense pain have not formed. This is important because the formation of organs and the contortions of natural development during the first and second trimester would be highly painful.

Since the special nerves that feel pain have not developed, the fetus does not feel pain prior to this. The nerves for reflex are responsible for movement, not pain.

Weeks 28 to 30

The fetus will reach its birth length around this time, although it still needs to put on significant weight. The fetus moves around, stretching limbs and the womb.

The eyes begin to open and move around and may be able to respond to the little light that may pass through the skin. The bones become fully formed, although many of them are not fully articulated, such as the skull.

Weeks 31 to 33

pregnancy week 31 baby size

The nerves begin to work. Some doctors believe these are the weeks the fetus is actually able to truly feel and respond to outside stimulus. Prior, they believe that any motion or response is simply reflex and instinct. But, at this time, motion and feeling become controlled. The punches and kick by the fetus are the strongest at this point.

The pupil is able to contract and dilate with passing light. The fetus responds to sound with more than just a shudder and jerk. Playing music at this time creates a reaction, and research has found classical and jazz music promotes happiness, while hard rock, rap, and R&B generate fear.

The digestive system begins to absorb nutrients from the amniotic fluid the fetus swallows. And this means the fetus is defecating in the amniotic fluid as well. Remember, this is a sterile environment, so we don’t have to worry about infection.

Weeks 34 to 37

The fetus begins to sleep and wake. The heart and circulatory system have now fully formed and begin moving away from the mothers. Soon, it will be a totally independent system.

Weight gain becomes even more rapid and the skin fills out as fat is deposited beneath the skin. By now, the fetus has taken up nearly all the room in the uterus. There is no more room to punch or kick.

The mother can be in discomfort by now as the uterus pushes the bladder lower into the hips, making her need to urinate more often. Also, as the fetus stretches and moves, her skin and organs are forced to move, sometimes to the point of pain.

Weeks 37 to 40

pregnancy week 39 baby size

The fetus is considered to term and birth at this point is considered normal. All of the fetus’ organs and structures are fully formed and are ready for the outside world. At birth, fetal development comes to an end and your baby is now developing. The terms infant, toddler, and child development come at different stages.

The placenta is now providing antibodies to help the fetus survive the world outside. The mother needs to take extra precautions to not use anti-bacterial soaps or harsh chemicals, as it could reduce the fetus’ immune system.

The fetus will turn upside down in preparation for birth. Otherwise, it is known as a breech birth.

Of course, you may carry several weeks beyond 40. Your doctor could have been wrong about your fertilization date. Information from the Mayo clinic tells us that if you are beyond the 40 weeks and experience no pain or undue distress, and that the fetus is still in an upright position, there is no need to worry.

Some doctors will induce labor because it becomes harder to push a larger fetus out. Much beyond 41 weeks becomes a danger for the mother more than the fetus.

Common Questions:

Why do fetuses get hiccups?

The hiccup is not like our hiccup. It is the diaphragm mimicking the action of breathing. The inhale sucks in amniotic fluid, which is much thicker than air. The resulting motion looks and feels like a hiccup to us. Since the diaphragm starts working at different times in the fetus, this can start at week 6, when the diaphragm starts to form or go as long as week 22 when the nerves to control the diaphragm form.

How does a fetus get oxygen?

how does fetus breathe in womb

While in the womb, the fetus is attached to the mother via the umbilical cord. Through this cord, the fetus gets nutrients and oxygen. As the mother eats and breathes, so does the fetus eat and breathes. The mother feels hungry when the fetus needs specific nourishment. The combined hormones create cravings for certain foods, which the mother eats. This is not hunger in the fetus, but a normal biological function.

That’s why they recommend women do not smoke or eat poorly. Can you image a baby smoking? That’s what happens when a woman smokes when pregnant. Then, the baby, once born, must go through sudden withdrawal, a painful process.

Does a fetus dream?

does a fetus dream in the womb

Honestly, we don’t know. We are certain that a fetus spends most of its time asleep and in that sleep the brain cycles through REM and non-REM sleep. It does spend most of the time in non-REM sleep. But, we don’t know if the fetus is dreaming or not. Psychologists debate whether the fetus has had enough stimulation to trigger dreams.

Some believe that when a fetus sleeps when the mother sleeps, it can pick up on the mother’s dreams, thus dream itself. But, most women will tell you that a fetus sleeps on its own schedule and rarely at the same time the mother wants to.

Why is a fetus’ heart beat so high?

Several factors affect this. First, in the beginning stages, the fetal heart is not controlled by the brain, it is just learning to contract. Then, as the metabolism is kicking in to build new tissues, the heart beats fast to keep the blood and nutrients move around the body fast enough to build the tissues quickly.

normal baby heart rate during pregnancy infographic

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How connected is the fetus to the mother?

Some people speculate the fetus can feel the pain and stress of the mother. Others deny this connection. Most women will agree the fetus responds to our condition, even if they don’t feel what we feel.

We fully believe the fetus can feel our emotions. Emotion is based on the movement of hormones through the body, so the fetus is experiencing them as well. Occasional bouts of stress and pain do not cause any problems for the fetus.

Long-term stress can cause adverse effects on a fetus. Examinations done on children of abused women found they are more prone to anxiety and depression.

how to bond with your baby before birth infographic

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Does a fetus feel pain during abortions?

Despite religious dogma, no, a fetus cannot feel anything during an abortion. First, the nerves to sense pain are not formed until week 27. Secondly, the portions of the brain that are responsible for responding to pain are not developed until week 20. Finally, most nerves and the majority of the brain are not developed at all at week 16, which is considered the cutoff point for most abortions.

How can I help my fetus gain weight?

Your fetus will gain weight regardless of what you do. It’s the type of weight and the quality of their health you have control over.

Focus on a diet that is rich in fruits and vegetables and avoid all processed foods. This way, you will get lots of nutrients and avoid harmful chemicals, most of which are linked to cancer and miscarriage.

foods to eat during pregnancy for healthy baby infographic

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Does every fetus start as a female?

Yes, all fetuses begin as the foundations of a female. However, they do not develop a vagina and uterus, then turn male.

The gonads and penis/clitoris begin at the same stage, just in the positions of a female. Then, depending on what the DNA dictates, they will either continue female development or descend into position for a man.

Does a fetus have gills?

No, a fetus never has gills like a fish. What they do have is pharyngeal arches, which are erroneously referred to as gill slits. In fish development, these will turn into gills. In humans, during the 4th week of development, these arches are pouches of tissues, not slits, which will turn into other structures. Some of these structures are the lower jaw bone, hyoid bone, and endocrine organs.

development of pharyngeal arches infographic

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Bad Habits:

Does alcohol affect fetal development?

This is a well-known and emphatic YES! It is such as definite yes that we can say that for many women, alcohol can affect development to the point of creating a miscarriage. Fetal alcohol syndrome is a painful and debilitating condition caused solely by the mother drinking.

Babies born to women who drink, even lightly, have impaired brain development and are on average 20 to 30 points lower in IQ than their peers. Many are considered underweight and do not have the normal layers of fat for protection.

During certain stages of development, a single alcoholic drink can cause liver failure, brain damage, and heart complications.

alcohol during pregnancy causes infographic

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How does weed affect fetal development?

Like alcohol, marijuana and all other drugs (including prescription medications) are bad. Brain development is depressed and the immune system can be compromised. Later on, this can lead to a baby and child needing extra care through illnesses and disease. The medications that prevent diseases in adults can actually cause the problem in a fetus.

How does caffeine affect fetal development?

caffeine during pregnancy infographic

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Caffeine is a stimulant. As such, it can cause the heart rate of a fetus to rise into the dangerous zone. Normally, a fetal heartbeat rarely goes over 170. But, with caffeine, it can reach 250 beats per minute, or push into defibrillation, a fatal condition. Excessive caffeine intake has also been linked to miscarriage.

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