Pregnancy Week By Week: Fetal Development And Stages

Did you just find out you’re pregnant or are you planning a pregnancy? Then you’re probably curious to learn more about the fetal developmental stages. This pregnancy week by week guide will highlight your baby’s size throughout the nine months and the intrauterine milestones that may concern you.

Doctors calculate the 40 weeks of pregnancy from the first day of the last menstruation. To avoid confusion, this guide uses the same timeframes.

Also, all measurements included in this guide are estimates. Each pregnancy is unique and each baby grows at his own pace, even inside the womb. Some babies weight less than five pounds at birth while others can weight over nine pounds.

As long as your practitioner doesn’t warn you about potential issues, there is no reason for concern.



The first week of pregnancy begins on the first day of your last menstrual period. You’re not pregnant yet but this is a conventional count that allows doctors to have precise references throughout the whole nine months.

Why do they call it the first week of pregnancy then? Well, maybe because now it’s the moment to switch to healthy habits if you’re planning to grow your family.

In fact, in a weird sort of way, the pregnancy begins before conception. You can start taking folic acid to prevent the neural tube defects in the baby. Quit smoking and drinking alcohol, as these substances can harm your child.

In these days, your uterus is also cleaning up, getting ready for a possible pregnancy in the next couple of weeks. The hormones are acting on the maturation of the oocyte to release during ovulation. If you have a regular 28-day cycle, fecundation will happen in the second week after menstruation.


In the second week of pregnancy, you’re still not pregnant, but you’re approaching the most fertile days of your cycle. Fertilization will occur at the end of this week or at the beginning of the next.

During this week, the endometrium – a tissue lining the inner wall of the uterus – is getting ready to welcome the fertilized egg. Its size and thickness increases while waiting for the ovulation.

Your hormone levels also begin to adjust to welcome the fertilized egg. Estrogens act on the endometrium and dictate the timing of the production of cervical mucus, while progesterone levels begin to rise quickly after the ovulation.


Conception occurs in the third week of pregnancy, and with it starts the development of the embryo. In an ideal hypothesis, conception takes place approximately on the 14th day of the luteal phase.

Nesting, or embryo implantation, occurs six to eight days after fertilization, and this is where the endometrium steps in, providing a welcoming bedding to the small bunch of cells. From this moment on and up to about 12 weeks of pregnancy, the initial zygote could also divide into homozygous twins.

Cell division begins immediately after fertilization and continues throughout the journey to implantation. The embryo arrives in the uterus in the stage of morula and the implants take place within 72 hours.


At the end of this week, you’ll get the first sign of pregnancy – your menstruation doesn’t arrive. The embryo has reached the blastocyst stage by now and the implantation can be accompanied by precise symptoms.

At this stage, your baby has a single outer layer of cells that form the trophoblast, or the structure that will develop into the chorion and placenta. An inner cavity also characterizes the embryo at this stage while the internal mass of cells will develop into the fetus.

During this week will take place a succession of cell divisions and migrations with various structural and functional consequences. The bunch of cells will develop the extra-embryonic membranes, the amniotic cavity and the yolk sac which will be temporarily used for the production of blood cells.


By now, more than two weeks have passed since conception. The embryonic and extra-embryonic tissues continue to develop and differentiate from one another. During this week, the embryo gradually takes shape and the most important organs start to develop.

The neural tube and neural crest assume their own functions, while the cardiovascular system starts to develop. The first traces of the eyes and ears also appear and by the end of this week, your baby will have his own bloodstream.

The chorion also continues to grow and develop the chorionic villi, formations that identify the region of the placenta. At this stage, the embryo grows rapidly and all external characteristics of the future baby start to take shape.

It’s difficult to talk about size at this stage and you’ll only get the first length parameters on week seven or eight.


pregnancy week by week

In the sixth week of pregnancy, the embryo assumes a C shape, folding on itself. An ultrasound can distinguish a head and a tail, and the baby has more or less the size of a coffee bean while the appearance is more reminiscent of a small tadpole.

The heart, which initially has only two cavities, begins to beat at about 22 days after conception and only two days later it connects to the vascular network. Gradually, the heart acquires a regular rhythm while the neural tube that connects the brain to the spinal cord also develops.

Besides brain and heart, the digestive system also begins to develop during this week, as well as the kidneys, pancreas, and liver.

The embryo also starts to develop a skeleton, beginning with the vertebral column. His face also begins to have defined traits while the first buds of the hands and feet take shape. By the end of the week, the embryo will start moving, although it’s impossible for you to feel him.


The seven-week threshold has arrived and now you’ll probably receive the first official measurements of your baby. On average, the baby measures about 0.63 inches and weighs less than 0.04 ounces. The shape pretty much resembles the shape of a bean.

During this week, the embryo starts to develop nostrils and lips, but also the tongue and the teeth. By now, the heart got its four cavities while the limbs are more evident, with sketches of little hands and feet.

The sexual organs also started to develop, although it’s impossible to tell whether it is a boy or a girl. At the same time, the embryo got more active and the umbilical cord starts to develop from the yolk sac.


An ultrasound in the eighth week shows the embryo as an astronaut floating in the space. At this stage, the baby has the size of a blueberry and the tail that characterized the first stages of intrauterine development has disappeared.

You’re still unable to fill his movements – you won’t feel any real kicks until week 20. The sexual organs start to develop into testicles or ovaries, and you’ll soon know where it’s a boy or a girl. The heart is pumping blood efficiently and you can even see thin blood vessels under the skins.

Lungs are also formed while the head is huge compared to the rest of the body. At this stage, the eyes are covered by a thin layer of skin that will turn into eyelids while the ears are not yet fully formed.

This development period is very delicate and is prone to the action of various pathogens, such as the toxoplasmosis or cytomegalovirus.


The ninth week of pregnancy marks the end of the embryonic period and from the next week, you’ll be able to refer to the baby as a fetus. Now, your child resembles a little human being and the reproductive organs are fully formed though not recognizable just yet.

The eyelids are also formed and the hair follicles and nipples start growing. Blood begins to circulate through the umbilical cord and the placenta begins to perform its function.


By now, the brain has grown so much that the head of the fetus is still much larger than the body. The baby measures a little more than an inch and weighs about 0.22 ounces. The limbs are now longer and the hips and wrists are formed.

At this gestational stage, the baby would be able to grasp objects and the more he grows the more sensitive he becomes. The heart beats about 180 times per minute, practically at double the speed that an adult heart.

The fetus is also very active during this stage and in broad terms, the baby is about ten thousand times larger than the oocyte which has been fertilized.


By the eleventh week, your baby has all vital organs and they begin to perform their function. The most critical part of fetal development has ended and the rest of the pregnancy serves only for the growth and maturation of the organs.

The baby now measures 1.61 inches and weighs about 0.25 ounces, almost the size of an adult fingernail. Almost half of the body length is occupied by the head and, although the eyelids are completely fused, the irises have started to develop.

The hearing is not yet functional but the ears are almost completely developed. The blood vessels in the placenta have also increased and they provide the baby with nutrients and oxygen. Your baby’s diaphragm has also developed and the fetus may start having the first hiccups.

The baby also can open and close his mouth and yawn. He grows so fast that by the end of the week it has almost doubled his length.


Week 12 marks an important milestone. By now, the baby is fully formed from head to toe while his organs continue to develop. The brain, despite being small, acquires the same structure it will have at birth and the continuous development of the nerves allow the fetus to feel pain.

Hair and nails also grow, while tiny teeth develop in the mouth. The baby’s heart pumps a lot of blood through the placenta and the genitals are clearly visible by now.

This week is characterized by spontaneous movements and the baby can smile, open and close his fists, rotate the ankles and wrists, frown, and tighten the lips.


The fetus appears fully formed in this week but the organs are still developing and perfecting; therefore, it’s impossible for the little one to survive outside of the womb.

This week marks many changes for your child. The intestinal villi are formed, which will later promote the peristaltic movements and the digestion. The liver begins to secrete the bile and the pancreas produces insulin.

At this stage, the child also begins to swallow amniotic fluid, smiles and the vocal cords are developing. Although the ears are not fully formed, the baby also starts hearing the first sounds.

In the thirteenth week, the fetus also gets fingerprints.  The fetal blood cells, produced by the yolk sac until now, start being produced by the liver until the bone marrow and spleen will take up this function. The neck can support the movements of the head and the baby measures a little less than three inches.

At this stage, the placenta doesn’t filter the substances coming from the mom, so you should limit alcohol and caffeine intake, as well as the use of medicines and drugs.



The first trimester has passed and the baby’s organs and systems are working alone. The digestive system can now process food while the urinary system produces and eliminates urine. Much of that urine form the amniotic fluid, which the baby will swallow again.

At this stage, the baby’s skin is also permeable to water, electrolytes, urea, and creatinine, so the composition of the amniotic fluid is similar to that of the maternal and fetal blood.

Thin and soft nails cover both the fingertips and the toes of your little one while the body begins to cover with a sort of thin hair called lanugo.

The eyes and ears start to move towards their final position and the spleen starts producing red blood cells. By now, the arms and legs are also more proportionate and the muscles and joints allow the baby to move. In fact, the baby already knows how to roll from one side to another and can even suck his thumb.


You bundle of joy continues to grow fast and the taste buds have almost reached their final developmental stage. Yet, the baby can’t taste anything yet.

The bones continue to develop and require a lot of calcium. Eyebrows start growing above the eyelids and the hearing mechanism is perfected. Although the eyelids are still closed, the baby can already perceive light.

By now, the genitals are clearly visible and you’ll be able to know whether it’s a pink or a blue bow.

At the moment, the fetus takes much of his energy from glucose derived from the mom’s diet but also uses fatty acids and other nutrients. In terms of size, the fetus measures more or less four inches and weighs almost 2.5 ounces.


The baby’s nails are fully formed by now and the child continues to inhale amniotic fluid. The movements of the limbs get more and more coordinated and vigorous, the head is erect and the legs well developed.

Your baby moves a lot and changes position about 20 times per hour. She plays with the umbilical cord and touches her face.

You could start feeling the first movements during this week, although at first, they will much resemble the movements of a butterfly. Yet, these light touches will get stronger and stronger in the next weeks.


The respiratory system of the child is in continuous development; the trachea is correctly positioned and the bronchi and bronchioles start growing. During this stage, the child begins to form and deposit fat under the skin, a change that boosts metabolism and regulates body temperature.

The movements are stronger and frequenter. Your bundle of joy is drinking lots of amniotic fluid while the skeleton continues to ossify. However, the bones are still flexible to facilitate the passage through the birth canal.

In terms of sizes, the baby measures more than five inches and weighs a little less than 7 ounces.


Your child is growing rapidly and the bones are continuing to harden. Fingertips and toe tips are already formed while the unique set of fingerprints is fully developed.

If you’re waiting for a baby girl, her uterus, fallopian tubes, and the vagina are fully formed and clearly visible during the ultrasound.

Baby’s heart pumps between 25 and 30 liters of blood a day and the kidneys may start functioning during this week. The taste buds already distinguish bitter from sweet and hearing is very sensitive. Your baby may start to respond to outside stimuli with kicks.


The nineteenth week is extraordinary from the point of view of fetal development. Your baby will cover in a protective layer of cells that will keep the skin intact in the amniotic fluid until birth. The buds of the teeth are developed and start storing calcium.

But the true milestone is represented by dreaming. Your baby will experience the first REM sleep by the end of this week.


Congratulations! You’ve made it halfway through pregnancy! In the last four weeks, the baby has quadrupled his weight and it now weighs more than the placenta. The fetus is now large enough to hurt you with the movements, a thing that makes you understand whether his sleeping or not.

Hair begins to grow and meconium accumulates in the intestine. From a developmental point of view, the baby can now taste, smell, hear, see, and touch. The nervous system also starts forming those complex junctions that serve memory and thinking.

All organs are almost fully formed and the baby is entering in a period of simple growth. By now, your baby measures 6.46 inches and weighs 10.58 ounces.


By now, the baby is the size of a carrot and continues to move regularly through the amniotic fluid. Yet, by the end of this quarter, she will start to stabilize her position.

Like in the previous weeks, the baby swallows large amounts of amniotic fluid, eliminating it through urine. The child has now eyelashes and eyebrows, while hair is also visible. If you’re expecting a boy, the testicles will fall from the abdomen.

By now, the child also hears you when you speak, so it’s a good idea to start to communicate with him.


The child continues to grow and starts practicing life outside of the womb. He is very sensitive to noise that can wake him up, causing nervous responses such as kicks. On the other hand, the sleep and waking rhythms become regular.

The skin is becoming less transparent and by now, the baby measures about 11 inches.


By week 23, the baby has the appearance of a newborn but is more skinny and wrinkled due to a poor accumulation of subcutaneous fat. She continues to drink amniotic fluid and the digestive system is mature enough to absorb nutrients.

Thanks to the development of the inner ear bones, the baby also starts gaining balance while the skin starts to accumulate melanin.

Your child has still plenty of room to move around, so she kicks very often. By now, the baby measures 11.38 inches.


In week 24, the baby is almost completely formed and continues to gain weight. Fat deposits are used to form muscles and to maintain the body temperature. The lungs start to produce surfactant and have reached a level of development that allows a child in four to survive if born in this week, even though 24-week preemies may have disabilities.

You may start to experience Braxton-Hicks contractions, which are painless and non-regular contractions of the uterus that have no effect on the child.

By the end of this week, the baby won’t only recognize your voice, but will also recognize the voice of the dad.


Woman holding an ultrasound scan of her unborn baby

By the twenty-fifth week, the baby has accumulated plenty of subcutaneous fat which makes him less skinny and wrinkled. Bone marrow produces blood cells and the sexual organs are completely developed. The vertebral column will also reach its final form during this week.

While the lungs continue to mature, the nostrils open and the senses get more and more sophisticated. The baby learns how to respond better to stress and the nerves around the mouth and lips increase their sensitivity.

Becoming more and more expert at using his hands, the baby also starts to explore the surrounding space.

If you’re having twins or multiples, in this week they will start exploring each other, giving life to that bond that will characterize them forever. Preemies born in this week have a 50% chance of survival.


Gradually the whole complex system of the inner ear has come to maturity, allowing the baby to hear a wide variety of sounds but also improving her control over the balance.

All in all, the uterus is a quiet place but the child will continue to be surrounded by noise for all nine months. Your heartbeat, digestion, and other body functions are noisy and your child will perceive all of them.

Because the uterus allows a little light to pass, the baby will also learn to distinguish darkness from light. In this week, the baby also opens his eyes for the first time.


By the twenty-seventh week, the baby is well proportioned and the head, body, and legs represent about a third of the total length each. If by now all babies have grown more or less in the same rhythm, genetic heritage and environmental factors will set the growth rate from now on.

The most important organ to develop during this week is the eye; by the end of the week, the baby will be able to identify the source of the light.



Week 28 marks the beginning of the third and last trimester. Baby’s eyes are now wide open and she opens and closes the eyelids. Eyelashes and eyebrows continue to grow as more and more subcutaneous fat are deposited.

Another important change is the child sleep rhythm, that becomes independent of yours. What does this mean? Goodbye peace and hello sleepless nights.

Thanks to medical advancement, a baby born in this week has 90% chances of survival and the main problems are linked to the incomplete development of the lungs.

Your baby is now big enough to occupy all available space and you can clearly feel him through your tummy. In detail, he’s measuring 14.41 inches and weighs 2.22 pounds.


During week 29, the child develops taste preferences and you may start to experience cravings. His nutritional needs become maximum and you’ll have to supplement your diet with proteins, vitamins, folic acid, iron, and calcium.

Baby’s skeleton continues to grow stronger while the brain, lungs, and muscles also continue their development.

During this week, the baby may also assume the cephalic position, which marks the beginning of the end of your pregnancy.


In the thirtieth week, the baby gets chubby as a result of the accumulation of fat. This gives your bundle of joy some control over the body temperature although the fat deposits keep growing. The head and body are fully proportioned and your fetus starts looking like a newborn.

During this week, the bone marrow replaces liver completely in the production of blood cells while the child becomes aware of the surroundings.

Baby’s heartbeat can also be heard with a simple stethoscope; the child has a length of 15.71 inches and weighs a little less than three pounds.


From now until birth, the baby gains weight faster than increasing in length. The lungs and digestive system have almost reached their full development and the pupils contract and dilate in response to light. Thanks to the accumulation of fat, the skin gets a beautiful rosy color.

In this week, the thymus begins to function and some studies have shown that the heart rate may indicate music preference.


You’re in the thirty-second week and now the baby sleeps about 95% of the time. Her skin has become tougher and brighter, while movements reach their peak. Because the baby is sleeping a lot, it’s advisable to start counting the movements.

The wrinkles start to disappear and the child may have frequent episodes of hiccups that can be recognized due to the rhythmic tremor.


As weeks pass by, the child’s brain continues to grow although the bones of her skull are still flexible and not completely welded.

Getting ready for birth, the child’s head will probably lower into the pelvis, giving some release to your digestive system. Yet, this change causes the child to press on the bladder, so you’ll have to hit the bathroom frequently.

The baby is probably less active and by now has a length of 17.20 inches.


Your child now behaves like a baby. He keeps his eyes open when he is awake and closes them when he sleeps.

During this period, the fetus also continues to produce antibodies to fight birth infections while he measures 17.72 inches. The chances of survival in case of premature labor are now around 95%.


We’re almost there and the baby is getting ready for childbirth. He’s getting more chubby and takes up most of the available space. Therefore, his movements are reduced.

By now, the lungs are fully developed and produce surfactant, a substance that prevents lung collapse while breathing.

The movements are slower and less frequent, but they are perceived more clearly. Measuring a little over 18 inches, the baby is more and more ready to come to light.


During week 36, the baby keeps growing and except for the development of the immune system, there is nothing exceptional going on.

The baby sleeps most of the time but may kick hard when awake.


Baby continues to grow and gain weight and from this week on, expect to welcome your bundle of joy at any time.

If born now, the baby isn’t considered a preemie anymore. If you still have to wait for the baby, know that she’s training the respiratory movements, turns the head and listens to all sounds.


In this gestational phase, the baby weighs a little less than seven pounds and measures a little under 20 inches. The lungs continue to mature and the thin hair disappears. The amniotic fluid is comparable to a volume of about five cups and the circumference of the belly almost corresponds to the circumference of the baby’s head.

Meconium accumulates in baby’s intestine and your immune system pass antibodies to the little one.


By week 39, the thin hair is almost completely gone and what’s left will go away on its own after birth. The baby gains weight and all his body functions are fully developed. Only the lungs are still developing, but this organ will reach its full development after birth.

At this stage, the bones in the skull are very elastic to favor the passage during delivery while all muscles are strong enough to support baby’s weight.

Because the baby spends much of the time sleeping, make sure everything’s fine by counting the fetal movements. You should feel at least ten in a day.


You’ve made it! This is the last week of pregnancy and the baby can be born at any time. Keep in mind that some pregnancies can last for 42 weeks or longer, so just monitor fetal activity to make sure everything’s fine.

By now, the baby measures about 20 inches and weighs about 8 pounds. The skeleton is made up of 300 bones, some of which will merge after birth.

Birth will bring important and irreversible changes to the baby’s heart and arteries, and if your bundle of joy isn’t born yet, just wait for it to happen.

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