What Happens Each Week During Pregnancy?

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You and your baby go through rapid changes each week during your pregnancy. For 40 weeks, your body works to create a new life and form the intricate body systems in your baby.

Pregnancy is divided into three trimesters that signal different periods of development. The first trimester encompasses weeks one to 13. This is when fertilization occurs and organs are developed. The second trimester refers to weeks 14 to 27. Rapid growth and development occur in this stage. The third trimester is weeks 28 to 40, during which the baby gains weight and their organs mature in preparation for birth.

Pregnant woman sitting on bed

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First Trimester

The first trimester begins before you are actually pregnant. It starts on the first day of your last period. It is the most delicate time in pregnancy when all the major organs and nerves are forming in your baby. It is also during the first trimester that your baby is most susceptible to damage from substances like alcohol, drugs and certain medicines, and illnesses. A healthy first trimester is therefore crucial to the normal development of your baby.

Week 1

During the first week of the first trimester, you are actually not pregnant yet. This is the first day of your last menstrual period and is used to determine the start date of your pregnancy.

Week 2

This is the week that an egg is released from your ovary. It is during this week that, if you have unprotected intercourse, you are most likely to become pregnant.

Week 3

After intercourse, sperm will enter the uterus through the cervix. When the strongest sperm reaches the fallopian tubes and meets an egg, conception takes place. The sperm fertilizes the egg, and it becomes a zygote, a group of cells made up of half of each parent's DNA.

Throughout the rest of this week, these cells multiply and divide, creating a blastocyst. The blastocyst is the name given to the grouping of cells with an inner layer that will become the embryo and an outer shell that will form the membranes that house the baby in the uterus.

Most people discover they are pregnant through a home pregnancy test after they have missed their period. While some people may have a positive pregnancy test in the third week, most home pregnancy tests won't detect pregnancy until the fourth week.

Week 4

By this week, the blastocyst has moved down the fallopian tube into the uterus. The lining of the uterus is rich with blood and nutrients—a lining that is normally released during menstruation. The blastocyst implants into this lining to begin its transformation into an embryo.

You may see your obstetrician at this point to confirm your pregnancy with a lab test and check your hormone levels. They may be able to predict your due date at this time.

Week 5

This is the start of the embryonic period. Cells continue to multiply and divide and differentiate into groups that will become the baby's major body systems. Blood cells, nerve cells, the brain, heart, spinal cord, and other organs begin to develop. This is a risky time in your pregnancy because it is the point at which problems may occur with the baby's basic body systems.

Weeks 6 to 7

Your baby's brain is developing faster now, dividing into five different sections and forming cranial nerves. The heart begins to beat regularly and can be detected on ultrasound. Eyes, ears, arms, and legs begin to form, and tissues gather to create what will become the baby's spine and bones.

Week 8

Your baby's arms and legs grow longer, and their hands and feet begin to form. The brain continues to develop, and lung tissues are formed.

This may be the week people usually see a doctor for the first time. A vaginal ultrasound may be performed to check your baby's size and position in the uterus. Your doctor will review your medical history, ask about previous pregnancies, and assess you general health.

You may have blood drawn at this visit to check your blood type and Rh factor, immunity to certain diseases, current infections, and more. From this point on, your doctor should see you at least once a month. You will also be given instructions on prenatal care.

By the end of the eighth week, your embryo is about a half-inch long.

Week 9

Your baby's nipples, hair follicles, and toes are forming. Elbows develop on arm buds, and all essential organs have been formed and begin to grow larger.

At this point in your pregnancy, your body is also preparing in ways you may not realize. Your blood volume increases, and your heart beats faster to bring extra blood to your embryo.

Week 10

Eyelids, outer ears, and facial features develop this week. Intestines mature, and the baby transitions from being an embryo to a fetus.

Weeks 11 to 14

The end of the first trimester leading into the second trimester is a busy time. Your embryo is now a fetus, and its face and limbs are fully formed. Nails form on your baby's hands, and it can now make a fist. Eyes and eyelids are formed but will remain closed until about the 28th week of pregnancy.

Body systems begin basic functions—the pancreas makes insulin, the kidneys make urine, and your baby makes breathing-like movements to swallow amniotic fluid. Lung tissues will continue to develop and mature throughout your pregnancy.

During the first trimester, you may experience fatigue, morning sickness, and mood swings as your body is put through physical and hormonal strain. It's a good time to establish a strong support system. At this point, your doctor may perform a first trimester screening exam with additional ultrasounds, pelvic exams, and prenatal tests like:

  • Chorionic villus sampling to detect Down syndrome and other genetic disorders
  • Nuchal translucency screening to check the baby's heart, size, and any chromosomal abnormalities
  • Maternal serum screen to detect birth defects or disorders

By the end of week 12, the fetus is about 2 inches long and weighs just half an ounce.

Second Trimester

You may feel a burst of energy in the second trimester. Your baby's organs are all formed, and your pregnancy will transition to a period of growth and maturation. You may now have a bulging pelvic area that makes your pregnancy visible to others.

During this trimester, you will feel your baby start to move, and most parents begin preparations for birth with childbirth classes. Your doctor will also perform additional tests and ultrasounds to check your baby's progress throughout the second trimester.

Some common tests your doctor will perform during this time include:

  • Amniocentesis between the 14th and 20th week
  • Glucose tolerance test or glucose challenge screening between the 26th and 28th week

Weeks 15 to 18

Your baby's organs are working, and it begins to move and stretch in the womb. At this point, though, these movements are most likely too slight to feel. The baby also begins to make sucking motions. Muscles develop and bones harden, and your baby is covered in an almost transparent layer of skin and fine hairs called lanugo.

At week 16, your fetus is roughly 4 inches long and weighs about 3 ounces.

Weeks 19 to 21

Your baby can now swallow, and they are more active and float around the uterus. This is the time when you may feel your baby's first movements, called quickening. You can also start talking to your baby since it can hear you now.

Your baby is 6 inches long and weighs less than 11 ounces by week 20.

Week 22

You can certainly feel your baby moving now, and you should even be able to hear its heartbeat with a stethoscope. Your baby is growing lanugo over its entire body now as well as eyebrows and eyelashes. Nails grow, and the baby begins to form its first bowel movement called meconium.

Weeks 23 to 25

Your baby will start growing more quickly. Fat begins to be stored, and the baby is now making its own red blood cells with its bone marrow. The lower lungs are beginning to develop. Blood vessels become more visible through the baby's skin, and it is improving its muscle control and coordination.

The 24th week marks the point in your pregnancy when an infant is considered viable—or able to survive—if born preterm. Chances of survival at the 24th week are about 50%.

By week 24, your baby is about 12 inches long and weighs about 1 and a half pounds.

Week 26

Your baby's eyes and eyelashes are fully formed now. Fingerprints and footprints are starting to develop, as well as the air sacs in the lungs that will allow your baby to breathe air after delivery. Since your baby can hear you now, you may notice it is startled by loud noises.

Third Trimester

The third and final trimester is usually one of discomfort. Your baby is growing quickly. You may experience soreness and fatigue as your body prepares itself for labor.

With the beginning of the third trimester—around week 28—you will also start seeing your doctor more often. Your doctor will teach you how to start keeping track of your baby's movements and when to call if you baby's activity is low. Your doctor will also perform ultrasounds and pre-delivery tests to check for things like fetal movement (non-stress test), Group B streptococcus, and preeclampsia.

Expect visits with your doctor every two weeks between week 28 and week 36, then weekly from week 36 until birth. Pregnancies that are considered high-risk or pregnancies with other complications may require more frequent visits.

Weeks 27 to 30

The nervous system begins taking over some of the body's own functions, and your baby will begin to open and close their eyes. The lungs are still maturing, but a substance called surfactant is formed. This is the fluid that will help the air sacs in the baby's lungs fill with air after delivery.

At week 28, your baby measures about 15 inches long and weighs nearly 3 pounds.

Weeks 31 to 34

Your baby is now storing more and more fat, as well as vitamins and minerals. Their bones are soft but fully developed. Even though your baby is not breathing yet, it is developing a rhythmic breathing pattern.

While you still have some time until delivery, around week 33, your baby may begin to change positions in the uterus. Expect your baby to start moving into a head-down position to prepare for birth.

Your baby is almost 17 inches long and should weigh more than 4 pounds by week 32.

Weeks 35 to 37

At this stage in pregnancy, your baby is reaching its full size in terms of length but will continue to gain weight until delivery. Fat collects under the skin, and your baby is working on developing sleep patterns. The heart, blood vessels, muscles, and bones are now fully developed, and your baby is considered at full term by the end of the 37th week.

At week 36, your baby should be about 18 inches long and weigh around 6 pounds.

Weeks 38 to 40

Your baby is ready to be born at this stage. Most of the baby's lanugo has disappeared, and hair on the head is more developed. Labor may begin anytime.

You may want to be sure at this point to have a plan in place for when to go to the hospital, how you will get there, and who will be with you.

Your baby is fully grown at 40 weeks, and should be around 20 inches in length and weigh around 7 to 8 pounds.

Complications

While pregnancy is a very natural process, there are certain situations or conditions that could increase your risk of complications. These include:

If you have any of these risk factors, be sure to discuss them with your doctor early in your pregnancy.

Potential complications include:

If you develop any of these conditions, your doctor may develop a special treatment plan for you and increase the frequency of your prenatal appointments.

A Word From Verywell

Pregnancy is a busy time full of changes for both the parent and the baby. Adequate prenatal care is crucial, and you should discuss any conditions you have that may complicate your pregnancy early on with your doctor. Be sure to avoid using substances like alcohol and cigarettes, and take good care of your health during your pregnancy. Knowing what to expect throughout your pregnancy may be comforting, and your doctor will watch you closely for any potential complications.

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Article Sources
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