Pregnancy Week by Week: Stages and Fetal Development

First Trimester of Pregnancy

Your Pregnancy: 2 weeks

In the previous week, there is a rise in the amount of estrogen and progesterone circulating in your blood, which acts so that your uterus forms fertile ground to receive and hold a possibly fertilized egg. At the same time, in your ovaries, eggs are maturing in the follicles. Early this week (around day 14 of a 28-day cycle), at ovulation, one of your eggs is released from its follicle and drifts into a fallopian tube. Over the next 12 to 24 hours, the egg will be fertilized if one of the millions of sperm manages to swim all the way from your vagina through your cervix and into the fallopian tube, and penetrate the egg. Only about 400 sperm will survive the grueling journey of about ten hours to the egg, and only one will manage to break through its outer membrane. Over the next 10 to 30 hours, the nucleus of the sperm will fuse with the egg and they will 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 to 4 day journey from the fallopian tube to the uterus, the fertilized egg, now called a zygote, will divide into 16 identical cells. Once it enters the uterus, the zygote is called a morula. One or two days later, it will settle into the welcoming environment of the uterus, continuing its rapid growth and transformation. The developing embryo is now a tiny ball of cells that scientists officially call a blastocyst: it has an inner cell mass, which will become the embryo itself, a fluid-filled cavity that will become the amniotic sac, and an outer mass of cells that will become the placenta, the flattened organ that supplies your baby with oxygen and nutrients and carries away 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.

Your Pregnancy: 5 weeks

Your Pregnancy: 6 weeks

Your Pregnancy: 7 weeks

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 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 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.

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 Pregnancy: 26 weeks

Your Pregnancy: 27 weeks

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 Pregnancy: 30 weeks

Your Pregnancy: 31 weeks

Your Pregnancy: 32 weeks

Your Pregnancy: 33 weeks

Your Pregnancy: 34 weeks

Your Pregnancy: 35 weeks

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 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.

The pregnancy belly month by month

Every belly grows at its own pace, depending on height, body type, abdominal muscles and whether it is a first pregnancy. As a rough guide, the pregnant belly month by month:

• Months 1 to 3: the uterus stays inside the pelvis. The bloating you feel is mainly due to hormones.
• Month 4: the uterus rises above the pubic bone and a small curve appears, earlier in a second pregnancy.
• Month 5: at 20 weeks, the top of the uterus reaches roughly the navel. The belly is now clearly visible.
• Months 6 to 7: the belly grows about one centimeter per week. Stretch marks and round ligament pain may appear.
• Month 8: the uterus reaches close to the ribs and presses on the stomach and diaphragm. That’s why heartburn and shortness of breath are common.
• Month 9: a few weeks before delivery, the belly often “drops” as the baby’s head engages in the pelvis.

The doctor monitors growth by measuring fundal height and with ultrasound. Worrying pains are described in the article on lower abdominal pain in pregnancy, while the first signs are covered in the article on pregnancy symptoms.

Frequently Asked Questions