Week-by-Week Pregnancy Stages

Dr. Panagiotis Polyzos MD PhD MSc

Obstetrician Gynaecologist
Doctor of Medicine, University of Athens Medical School

Panagiotis Polyzos, Gynaecologist Obstetrician, is active at the Institute of Life - IVF Unit of Iaso Maternity Hospital.

Contents

Στάδια εγκυμοσύνης ανά εβδομάδα

First Trimester of Pregnancy

Your pregnancy: 2 weeks

In the previous week, there is an increase in the amount of estrogen and progesterone circulating in your blood, acting in a way that allows your uterus to form a fertile environment to receive and support a potentially fertilized egg. At the same time, in your ovaries, the eggs are maturing within their follicles. At the beginning of this week (around day 14 of a 28-day cycle), during ovulation, one of your eggs is released from its follicle and is swept into a fallopian tube. Over the next 12 to 24 hours, the egg may be fertilized if one of the millions of sperm manages to swim the entire distance from your vagina, through your cervix and uterus, into the fallopian tube, and penetrate the egg. Only about 400 sperm survive the demanding, roughly ten-hour journey to the egg, and only one will succeed in passing through its outer membrane. During the following 10 to 30 hours, the sperm’s nucleus will fuse with the egg’s nucleus and combine their genetic material. If the sperm carries a Y chromosome, your baby will be a boy. If it carries an X chromosome, you will welcome a girl. During a 3–4 day journey from the fallopian tube to the uterus, the fertilized egg, now called a zygote, divides into 16 identical cells. Once it enters the uterus, the zygote is called a morula. One or two days later, it will implant into the welcoming uterine lining, continuing its rapid development and transformation. At this stage, the developing embryo is a tiny ball of cells formally referred to by scientists as a blastocyst: It has an inner cell mass that will become the embryo itself, a fluid-filled cavity that will become the amniotic sac and an outer cell mass that will become the placenta — the flattened organ that provides oxygen and nutrients to your baby and removes waste products.

Your Pregnancy: 3 weeks

The developing embryo is a small ball made up of several hundred cells that are multiplying at a very rapid rate. Once this ball of cells (called a blastocyst) settles into your uterus, the part that will become the placenta begins producing the pregnancy hormone hCG (human chorionic gonadotropin), which signals your ovaries to stop releasing eggs, causing an increase in the production of estrogen and progesterone. Meanwhile, amniotic fluid begins to collect around the ball of cells in the cavity that will become the amniotic sac. This fluid will surround your baby until the end of pregnancy. At this time, the blastocyst receives oxygen and nutrients — and eliminates what it does not need — through a primitive circulation system made up of tiny channels that connect it to blood vessels in the lining of your uterus. The placenta will not yet be sufficiently developed to take over this work, which will occur by the end of next week.

Your Pregnancy: 4 weeks

This week marks the beginning of the embryonic period. From now until week 10, all of your baby’s organs will begin to develop, and some will even start to function. As a result, this is the time when your baby is most vulnerable to anything that could affect its development. At this stage, your baby is about the size of a poppy seed. The placenta consists of two layers at this point. The cells inside your uterus are creating spaces for blood flow so that the placenta can develop and soon be able to provide nutrients and oxygen for your baby’s growth when it becomes functional at the end of this week.

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Your Pregnancy: 5 weeks

Inside your uterus, the embryo is growing at a rapid pace. At this point, it is about the size of a sesame seed. It is made up of three layers — the ectoderm, the mesoderm, and the endoderm — which will later form all organs and tissues of the body. The neural tube — which will become your baby’s brain, spinal cord, nerves, and spine — begins to form in the upper layer called the ectoderm. This layer will also develop into your baby’s skin, hair, nails, mammary and sweat glands, and the enamel of the teeth. The heart and circulatory system begin to form in the middle layer, the mesoderm. This week, your baby’s tiny heart begins to divide into chambers.
 
The mesoderm will also develop into your baby’s muscles, cartilage, bones, and subcutaneous (under the skin) tissues. The third layer, the endoderm, will create the lungs, intestines, and urinary system, as well as the thyroid, liver, and pancreas. It also develops into the umbilical cord and placenta, which provide nourishment and oxygen to your baby.
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Your Pregnancy: 6 weeks

Important developments this week: The nose, mouth, and ears are beginning to take shape. If you could see inside your uterus, you would find a disproportionately large head with dark spots where your baby’s eyes and nostrils are starting to form. The ears look like small indentations on the sides of the head, and the arms and legs appear as tiny protruding buds. The heart beats about 100 to 160 times per minute — nearly twice as fast as your own — and blood is beginning to circulate throughout the body.
 
The intestines are developing, and the tissue that will become the lungs has appeared. The pituitary gland, muscles, and bones are also forming. At this stage, your baby is about the size of a lentil.
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Your Pregnancy: 7 weeks

Practically, your baby is still considered an embryo and has a small tail, which is an extension of the coccyx. This tail will disappear within a few weeks, but it will be the only thing that gets smaller. Your baby has doubled in size since last week and is now about the size of a blueberry.
 
If you could see inside your uterus, you would notice the folds of the eyelids, the tip of the nose, and tiny veins just beneath the delicate skin. Both hemispheres of your baby’s brain are growing. The pancreas is also beginning to develop, which will eventually produce the hormone insulin to aid digestion. During this week, the umbilical cord begins to form, which from now on will transport blood and nutrients to and from the embryo.

Your Pregnancy: 8 weeks

The news for this week is that the fingers and toes are beginning to emerge faintly, the eyelids almost cover the eyes, the respiratory tubes are extending from the throat into the developing lungs, and the “tail” has nearly disappeared. In the brain, nerve cells are branching outward to connect with others, forming primitive neural pathways. The external genital organs have not yet developed enough to reveal whether the baby is a boy or a girl. Your baby — about the size of a bean — is constantly moving and changing, although you still cannot feel it yet.

Your Pregnancy: 9 weeks

Your baby is almost 2.5 centimeters — about the size of a grape — and weighs just a few grams. The baby’s heart has nearly completed its division into four chambers, and the valves are beginning to form. The embryonic “tail” is now gone. The external genital organs will not differentiate as male or female for a few more weeks. The eyes are fully formed, but the eyelids are tightly fused and will not open until around week 27. The earlobes, mouth, nose, and clearly visible nostrils have now formed. The placenta has grown enough to take over most of the important work of hormone production. The basic physiology of your baby is now established and ready for rapid weight gain.
 

Your Pregnancy: 10 weeks

Although your baby is just barely over 2.5 centimeters in length and weighs less than 8 grams, it has now completed the most critical part of its development. This is the beginning of the so-called fetal period, a stage during which the tissues and organs in the body mature very rapidly. The vital organs — including the kidneys, intestines, brain, and liver — are in place and beginning to function, although they will continue to develop throughout your pregnancy. Your baby’s limbs can now bend. The outline of the spine is clearly visible through the transparent skin. The forehead appears prominent, temporarily sitting high on the head due to the rapid development of the brain, which currently accounts for half the length of the body. Over the next few weeks, your baby will double in size.

Your Pregnancy: 11 weeks

Your baby is now about 4 centimeters long, approximately the size of a fig, and is almost fully formed. Soon, its palms will be able to open and close, and some of the bones are beginning to harden. Your baby is already busy with tiny kicks, stretches, and small movements. These movements will become more frequent as the body continues to grow and become more functional. You will not feel your baby’s movements for another two months.

Your Pregnancy: 12 weeks

This week, your baby is developing reflexes. The fingers and toes will soon begin to open and close, hair is starting to grow, the eye muscles tighten, and the mouth opens and closes making sucking motions. The intestines will begin moving into the abdominal cavity, and the kidneys will start functioning by producing urine in the bladder. Meanwhile, nerve cells are rapidly multiplying in your baby’s brain. The face looks unmistakably human. The eyes have moved from the sides to the front of the head, and the ears are exactly where they should be. Your baby is now just over 5 centimeters long.

Your Pregnancy: 13 weeks

Fingerprints have formed on your baby’s tiny hands, the veins and organs are clearly visible beneath the thin skin, and the body is growing and beginning to catch up with the head, which until now has been much larger. If you are having a girl, she already has more than 2 million eggs in her ovaries. Your baby is about 7.5 centimeters long (about the size of a shrimp).

Second Trimester of Pregnancy

Your Pregnancy: 14 weeks

Your baby can now make facial expressions, urinate regularly, and may even suck its thumb! The kidneys are producing urine, which circulates into the amniotic fluid around the baby — a process that will continue until birth. Your baby is growing and beginning to stretch out.
 
It is about 9 centimeters long. The body is growing faster than the head, which is now positioned on a tiny neck. By the end of this week, the arms will have grown to a length more proportional to the rest of the body. A very fine layer of lanugo (soft hair) has begun to develop all over the body. The baby’s liver starts producing bile this week, and the spleen begins to help with red blood cell production. Your baby is now more flexible and active.

Your Pregnancy: 15 weeks

This week, the embryo is about 10 centimeters long. It is beginning to move around in the amniotic fluid, while the primitive air sacs in its lungs are starting to develop. The legs are now growing longer than the arms. Even though the eyelids are still closed, your baby can sense light. If you shine a flashlight on your abdomen, for example, it is likely to move away from the beam. Finally, the genital organs are now clearly distinguishable.

Your Pregnancy: 16 weeks

Over the next few weeks, your baby will double its weight and noticeably increase in length, which this week is about 11.5 centimeters. The legs are now much more developed, the head is upright, and the eyes have moved closer to the front of the head. The ears are near their final position. Hair growth on the scalp has begun, and the nails are starting to grow. The heart is now pumping about 25 liters of blood each day — a volume that will continue to increase as your baby grows.

Your Pregnancy: 17 weeks

Your baby’s skeleton is changing, gradually transforming from soft cartilage into stronger bone, while the umbilical cord — which connects the fetus to the placenta — is becoming thicker. Your baby now weighs about 150 grams. It can move its limbs and joints, and the sweat glands are beginning to develop.

Your Pregnancy: 18 weeks

This week, your baby is about 14 centimeters long. It is busy moving its arms and legs — movements that you will start to notice in the coming weeks. The blood vessels are visible through the thin skin, and the ears are now in their final position. A protective coating called myelin is beginning to form around the nerves — a process that will continue for a year after birth. If you are having a girl, the uterus and fallopian tubes are forming and are in place. If you are having a boy, the genital organs are now visible.

Your Pregnancy: 19 weeks

Your baby’s senses are developing rapidly, with the brain now designating specialized areas for smell, taste, hearing, vision, and touch. Some research suggests that your baby may already be able to hear your voice, so feel free to read or sing with a cheerful tone. Your baby is about 15 centimeters long. The arms and legs are now proportional to each other and to the rest of the body. The kidneys continue to produce urine, and hair on the scalp is growing. A waxy protective coating called vernix caseosa is forming on the skin to shield it from the amniotic fluid.

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Your Pregnancy: 20 weeks

Your baby now weighs about 300 grams. It is practicing swallowing movements — great preparation for the digestive system. Meconium is also being produced, a black, sticky byproduct of digestion. This sticky substance will accumulate in the intestines and you will see it in your baby’s first diaper.

Your Pregnancy: 21 weeks

Your baby now weighs about 500 grams and measures approximately 25 centimeters in length. You may already feel the early fluttering movements turning into full kicks and punches. Additionally, your baby’s eyebrows and eyelids are now present, and if you are having a girl, her vagina has begun to form as well.

Your Pregnancy: 22 weeks

At 27 centimeters in length and almost three-quarters of a kilogram, your baby is beginning to look like a tiny newborn. The lips, eyelids, and eyebrows are becoming more distinct. The eyes have formed, but the irises still lack pigment. If you could see inside your uterus, you would be able to spot a fine layer of lanugo covering the body, along with deep wrinkles in the skin. Inside the abdomen, the pancreas — essential for producing important hormones — is steadily developing.

Your Pregnancy: 23 weeks

Now that your baby is over 27 centimeters long and weighs just under one kilogram, you may be able to see its movements from the outside, under your clothes. The blood vessels in the lungs are developing in preparation for breathing, and sounds are becoming increasingly noticeable. Your baby is getting accustomed to loud noises — such as a dog barking or the sound of a vacuum cleaner — and will likely not be disturbed by them after birth.

Your Pregnancy: 24 weeks

Your baby is steadily growing, having gained about 4 ounces since last week. This puts the weight at a little more than one kilogram. Since the baby is now nearly a foot long, the body is filling out proportionately and will soon begin to look chubbier. The brain is also growing rapidly, and the lungs are developing their branches and the respiratory tree. The cells are producing surfactant, a substance that will help the air sacs inflate once your baby breathes outside the womb. The skin is still thin and transparent, but this will change soon.

Your Pregnancy: 25 weeks

Your baby now measures about 13.5 inches. Weighing around one and a half kilograms, the wrinkled skin will begin to smooth out, making the baby look more and more like a newborn.
 
Hair growth has also increased — and if you could see your baby, you would now be able to distinguish its color and texture.

Your Pregnancy: 26 weeks

The network of nerves in your baby’s ears is now better developed and more sensitive than before. Your baby may now be able to hear your voice and your partner’s voice when you speak to each other. Your baby is inhaling and exhaling small amounts of amniotic fluid, which is essential for lung development. These so-called breathing movements are also good practice for when your baby is born and takes its first breath of air. At the same time, your baby continues to deposit fat under the skin and now weighs about one and two-thirds kilograms and measures around 14 inches from head to heel. If you are having a boy, his testicles are beginning to descend into the scrotum — a journey that will take about two to three days.

Your Pregnancy: 27 weeks

This week, your baby weighs about 2 kilograms (similar to a cauliflower head) and measures about 14.5 inches in length. Your baby has regular sleep and wake cycles, opens and closes its eyes, and sometimes sucks on its fingers. With more growth in brain tissue, your baby’s brain is now very active. Although the lungs are still immature, they would be able to function — with significant medical support — if your baby were to be born now. You may feel tiny rhythmic movements caused by hiccups, which may become common from now on. Each episode usually lasts only a few minutes and does not bother the baby, so just relax and enjoy the tickling sensation.

Third Trimester of Pregnancy

Your Pregnancy: 28 weeks

This week, your baby weighs about 2.25 kilograms and measures 14.8 inches from head to heel. Your baby can open and close its eyes, which now have beautiful eyelashes. With improved vision, your baby may be able to see the light filtering through the uterus. Your baby has also developed billions of neurons in the brain and continues to deposit fat under the skin in preparation for life outside the womb.

Your Pregnancy: 29 weeks

Your baby now weighs about 2.5 pounds (about the size of a pumpkin) and, if it’s a boy, measures over 15 inches long from head to heel. The muscles and lungs continue to mature, and the head is growing to make room for the developing brain. To meet your baby’s increasing nutritional needs, you’ll need plenty of protein, vitamin C, folic acid, and iron. And because the bones require a lot of calcium, be sure to drink milk (or choose another good source of calcium, such as cheese, yogurt, or fortified orange juice). During this trimester, about 250 mg of calcium are used daily to strengthen your baby’s skeleton.

Your Pregnancy: 30 weeks

Your baby is now about 15.7 inches long and weighs approximately 3 kilograms. The volume of amniotic fluid surrounding the baby will begin to decrease as your baby continues to grow and take up more space in your uterus. Your baby’s vision continues to develop, although it is still not very sharp. Even after birth, your baby will keep its eyes closed for a large part of the day. When the eyes are open, they will respond to changes in light, but vision will be around 20/400 — meaning your baby can only see objects that are just a few centimeters from the face. (Normal adult vision is 20/20.)

Your Pregnancy: 31 weeks

This week, your baby measures over 16 inches long. Weighing about 3.3 pounds, your baby is heading into a growth spurt. They can turn their head from side to side, and the arms, legs, and body are beginning to plump up as fat continues to accumulate beneath the skin. Your baby is probably moving a lot — so much that you may be having trouble sleeping due to kicks and flips. All this activity is a sign that your baby is active and healthy.
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Your Pregnancy: 32 weeks

By now, your baby weighs about 3.75 pounds and is approximately 16.7 inches long, taking up a lot of space in your uterus. Your baby has gained about one pound this week. The nails and hair are now formed. The skin has become soft and smooth, and fat continues to accumulate as preparation for birth.

Your Pregnancy: 33 weeks

This week, your baby weighs just over 4 pounds and is past 17 inches in length. The bones in the skull are not yet fused and remain very soft, allowing them to shift — making it easier for the baby to fit through the birth canal. (The pressure on the head during delivery is so intense that many babies are born with a temporary “conehead” appearance.) These bones will not fully fuse until early adulthood, allowing the brain and surrounding tissues to continue expanding throughout childhood.

Your Pregnancy: 34 weeks

Your baby now weighs about 4¾ pounds (about the size of a cantaloupe on average) and is approximately 18 inches long. The layers of fat have increased, which will help regulate body temperature after birth. The skin is also smoother than ever. The central nervous system and the lungs continue to mature. If you are feeling nervous about the possibility of preterm labor, you’ll be relieved to know that babies born between 34 and 37 weeks generally do very well and are usually healthy. They may need a short stay in the incubator to avoid some temporary health issues, but in the long term their outcome is typically the same as full-term babies.

Your Pregnancy: 35 weeks

Your baby doesn’t have much room to maneuver now that it is over 18 inches long. Because it feels so snug in your belly, it’s likely not doing somersaults anymore. The kidneys are now fully developed, and the liver can process some waste products. Most of the major physical functions have developed and are now complete.

Your Pregnancy: 36 weeks

Your baby now weighs about 6 pounds and is more than 18½ inches long. It begins to shed most of the fine hair covering its body, as well as the vernix caseosa, the waxy substance that coats and protects the skin during the nine months spent in the amniotic fluid. Your baby swallows both of these substances, along with other secretions, resulting in a dark, tar-like mixture called meconium, which will make up the contents of the first bowel movement. At the end of this week, your baby is considered full term. (Full term is from 37 to 42 weeks of pregnancy.) Most likely, your baby has now settled into position with the head facing downward.

Your Pregnancy: 37 weeks

Your baby is now considered full term, even though your due date is still three weeks away. If you were to give birth now, the lungs would most likely be mature enough to fully adapt to life outside the womb (though some babies may still need a little more time). Your baby weighs about 6⅓ pounds and measures just over 19 inches from head to heel. Many babies are born with a full head of hair, but don’t be surprised if your baby’s hair color is different from yours.

Your Pregnancy: 38 weeks

Your baby has really plumped up. It now weighs about 6.8 pounds and measures over 19½ inches long. Your baby has a strong heartbeat, which you will soon be able to feel when you hold that tiny hand for the first time! The organs have matured and are ready for life outside the womb. Wondering what color your baby’s eyes will be? You might not know right away. If your baby is born with brown eyes, they will likely stay brown. If born with gray or dark blue eyes, they may stay gray or blue—or turn green or brown by the time your baby is around 9 months old. This happens because the irises (the colored part of the eyes) can develop more pigment after birth, but rarely become lighter or more blue. (Green, hazel, and brown eyes contain more pigment than gray or blue eyes.)

Your Pregnancy: 39 weeks

Your baby is waiting to greet the world! It continues to build up a layer of fat to help control body temperature after birth, but it’s likely already measuring about 20 inches long and weighing just over 7 pounds. (Boys tend to be slightly heavier than girls.) The outer layers of your baby’s skin have shed as new layers form underneath.

Your Pregnancy: 40 weeks

It’s hard to say for sure how big your baby will be, but the average newborn weighs about 7 1/2 pounds and is around 20 inches long. The bones in your baby’s skull are still soft, allowing for easier movement through the birth canal during labor. This so-called “molding” is the reason your baby’s noggin may look a bit cone-shaped after birth. Rest assured — this is completely normal and temporary.

Your Pregnancy: 41 weeks

Just over 20 inches long, your baby has continued to grow and may now weigh around 8 pounds. Your baby can’t stay inside forever. For the safety of your baby, your doctor will talk with you about inducing labor if your little one hasn’t been born by next week. Most doctors won’t let you go more than two weeks past your due date because it puts both you and your baby at increased risk for complications. Babies born at 42 weeks and beyond may have dry skin and are often overweight. Prolonged waiting increases the chance of developing an infection in your uterus and could be dangerous for your baby or lead to an unfavorable outcome. Additionally, your labor is more likely to be prolonged or to stop progressing, which means both you and your baby have a higher risk of injury during a vaginal birth, and your chances of needing a C-section double.

Frequently Asked Questions

Can the first-trimester ultrasound detect Down syndrome?

During the first-trimester ultrasound, the thickness of the baby’s nuchal translucency is measured. Therefore, if the nasal bone is visible, as well as the blood flow in the fetus’s liver and heart, these findings are associated with the likelihood of Down syndrome.

Tips for the 2nd trimester of pregnancy:

  • Follow a balanced diet rich in vitamins and trace elements to provide your baby with all the healthy nutrients needed for proper development.
  • Do not smoke.
  • Do not consume alcohol.
  • Try to sleep as many hours as possible.
  • Include seafood in your diet.
  • Exercise. Physical activity during pregnancy helps improve sleep quality and relieve other pregnancy-related symptoms.

During the 2nd trimester of your pregnancy, you will be able to enjoy better sleep. Make sure to sleep at least 8 hours during the night. Prefer to lie on your side with your legs bent, and experiment with pillows that can provide you with more support (between your knees, under your belly, and behind your back).

The second-trimester anatomy scan is an extremely important ultrasound examination, which takes place between the 20th and 24th week of pregnancy, and it is necessary because it provides essential information related to the baby’s anatomy.

The second-trimester anatomy scan provides detailed information about your baby. We can focus on specific fetal organs and examine the heart, brain, stomach, and many other parts. During this ultrasound, your baby’s gender can also be clearly seen!