What are the Fetal Development Stages of Pregnancy?

An average pregnancy lasts 40 weeks, or about nine months. Pregnancies are divided into three trimesters lasting about three months each. For a visual journey through the stages of development, check out fetal development by week.

Each day a fetus spends in the womb is vital for a baby’s healthy development up until they reach full term, or at least 37 weeks of pregnancy. Though modern medicine has made great strides in caring for preterm infants, the womb is undoubtedly the best place for a baby to grow. While not all premature babies have health issues, those born closer to their due date tend to have improved outcomes, with less time spent in the hospital. For more information on prematurity, check out Premature Birth 101.  

To learn more about the latest advances in preventing preterm birth, check out our blog post.

Conception: The Start of Your Pregnancy

Each month during a woman’s childbearing years, an egg matures within the ovary inside a cyst called a follicle. When the follicle releases the egg into the fallopian tube, this is called ovulation. Ovulation usually occurs two weeks into a woman’s monthly cycle.  Most months, the egg is not fertilized by a sperm, so the lining of the uterus is shed through the vagina in a normal menstrual period.

If the egg is fertilized, this is known as conception. The fertilized egg, now called a zygote, divides rapidly into many cells and becomes a blastocyst (also called a blastocyte). It stays in the fallopian tube for about three days, until it begins its journey to the uterus.

How Soon Will I Know I’m Pregnant?

From the time of conception, the hormone human chorionic gonadotropin (hCG) will be present in your blood. Although conception usually occurs about two weeks after your last period, this hormone won’t reach detectable levels for a pregnancy test for another week or two, meaning three to four weeks after your last period.

How Do I Calculate My Baby’s Due Date?

Your baby’s due date is based on the menstrual or gestational age of the embryo or fetus, which is the date of the start of your last period. Gestational age is most accurate when also confirmed by an ultrasound.  Your healthcare provider will calculate your due date as 40 weeks from this date. Because not all months are the same number of weeks, a 40 week pregnancy may last between nine and ten months. If you’d like to see an estimate of your due date, check out the March of Dimes Due Date Calculator.

Generally, fetal development stages are discussed in terms of weeks of pregnancy. So, a four-week pregnancy is four weeks from the start of your last period, or about two weeks after actual conception. As your pregnancy progresses, your healthcare provider will refer to the gestational age in weeks and days. For example, if you are on the fifth day of your 21^st week of pregnancy, the gestational age will be noted as “21 5/7.”

When should I schedule my first pregnancy appointment for optimal prenatal health?

When you receive a positive home pregnancy test, you should call your healthcare provider to schedule your first prenatal medical visit. Even before this appointment, you should make sure to get a healthy start on your pregnancy by avoiding tobacco, drugs, and alcohol; and by taking a prenatal vitamin supplement every day. Check out the CDC Pregnancy page to learn more about the steps you can take to maintain a healthy pregnancy.

Fetal Development in the First Trimester

During the first three months of pregnancy, your baby develops from an egg to a fertilized egg or zygote, to a blastocyst, an embryo, and then a fetus.

Tests available during the first trimester include:

• Ultrasound. In the first trimester, a nuchal translucency screening by ultrasound can help your doctor assess your baby’s risk of certain abnormalities. An ultrasound also helps confirm the due date, determine the number of fetuses present, and check the health of your uterus and cervix.
• Blood tests. These measure plasma protein A (a possible marker of chromosomal abnormality), and human chorionic gonadotropin (hCG).

Month 1 (Weeks 1-4) – The Embryo Stage

Baby’s Development

The first two weeks after conception are called the germinal stage of development. In this stage, the single-celled zygote begins to travel from the fallopian tube to the uterus. Within the first two days after conception, cell division begins. When the zygote reaches the eight-cell stage at days five to six, the cells begin to differentiate into two different layers: the inner layer will form an embryo, and the outer layer will develop into the placenta. When this differentiation occurs, the zygote is now called a blastocyst. When the blastocyst arrives at the uterine wall, it attaches in a process called implantation.

Within three weeks of implantation, the blastocyst forms into the little ball called an embryo, which begins the embryonic stage of development. During this period, the embryo becomes distinctly human in its cell types, and the neural tube forms. The neural tube is the precursor of the central nervous system, which will eventually include the brain and spinal cord.

At four weeks, the embryo begins to develop early structures that will form the face, neck, heart, blood vessels, lungs, stomach, and liver.

Baby’s Size

At the end of the first month, the embryo is about the size of a grain of rice: only about a quarter of an inch long.

Changes in Your Body

By the end of the first month, you have only been pregnant for two weeks or so. Most mothers do not know they are pregnant by now, although hCG may be detectable in the urine.

Month 2 (Weeks 5-8)

Baby’s Development

During the second month of pregnancy, the embryo begins to develop facial features and buds that will become arms and legs. The central nervous system continues to develop, along with digestive organs. By the sixth week, healthcare workers can begin to detect baby’s heartbeat.

Baby’s Size

At eight weeks, your baby is just over a half inch long, about the size of a kidney bean.

Changes in Your Body

With the new influx of hormones that support your baby’s development, you may notice an increased need for urination, high levels of fatigue, and the start of nausea and vomiting known as morning sickness.

Month 3 (Weeks 9-12) – From Embryo to Fetus

Baby’s Development

After the eighth week of pregnancy, your growing baby is now called a fetus instead of an embryo. This begins the final stage of development, called the fetal stage. By the end of this month, the fetus is fully formed, with all organs and extremities in place that will continue to grow and develop throughout the pregnancy. At this stage, baby’s circulatory and urinary systems begin to function, and bones begin to form in a process called ossification. Your baby will even begin to make small movements, like opening the mouth and making fists. This month, the umbilical cord is fully functional, supplying your baby with nutrients.

Baby’s Size

At week 12, the fetus is about two inches long and weighs about two ounces, about the size of a small lime.

Changes in Your Body

By now, you may begin showing as your uterus grows. You may have gained a little weight, but not if you’ve been experiencing nausea and vomiting. Your skin may take on a new glow, or you could develop acne or dark spots.

Fetal Development in the Second Trimester

The start of the second trimester is a big milestone in your pregnancy. Morning sickness and other symptoms of the first trimester may be fading by now, leading you to feel more like your old self.

Tests available from your healthcare provider during the second trimester include:

• Blood tests. Blood tests during this trimester look for abnormal levels of alpha fetoprotein (AFP), estriol, inhibin, and hCG. During the second trimester – at weeks 18 through 20 – most pregnant women can receive the PreTRM Test to determine their risk of delivering too early. This test provides your doctor with your individualized risk of going into early labor before 37 weeks of pregnancy, allowing both you and your doctor to make changes to your pregnancy care plan as needed.
• Ultrasound. An ultrasound can be performed in your second trimester to confirm that your pregnancy is progressing properly.
• Amniocentesis. If you and your doctor decide that an amniocentesis is warranted based on your risk factors or the results of other screening tests, this is usually performed between the 15^th and 20^th week of pregnancy.

Month 4 (Weeks 13-16)

Baby’s Development

During the fourth month, the nervous system begins to function, and the reproductive organs are developed enough for your doctor to possibly determine the sex of your baby through an ultrasound.  The fetus’s teeth and bones become denser, and baby will be able to make complex movements like yawning and sucking the thumb. This month, your baby’s urinary system is working well, as amniotic fluid is swallowed, filtered, and passed as urine every half hour.

Baby’s Size

At sixteen weeks, your baby is about the size of a large avocado: around seven inches long.

Changes in Your Body

Now that the placenta has taken over the production of hormones for the pregnancy, your moods may become more regulated, and fatigue and morning sickness symptoms can start to fade.

Month 5 (Weeks 17-20)

Baby’s Development

The fifth month brings noticeable movement from the fetus, or “quickening.” Your baby will continue to grow and exercise muscles throughout the pregnancy. This month, the fetus starts to grow hair on the head, along with a fine layer of protective hair elsewhere on the body called lanugo.  Baby will also develop vernix caseosa, a waxy protective coating on the skin that is shed before labor begins. Eyebrows, eyelashes, fingerprints, and tooth buds have begun to form. By now the brain can pick up electrical signals from the rest of the body, allowing baby to sense temperature and pressure.

Baby’s Size

At 20 weeks, the fetus is about 10 inches long and 10 ounces, roughly the length of a banana.

Changes in Your Body

By now, your pregnancy is more noticeable to strangers, as your uterus expands to the level of your belly button. Because of the increased blood supply to your baby, your heart may be beating faster, and your kidneys are working harder than before, which may cause more frequent urination.

Month 6 (Weeks 21-24)

Baby’s Development

As the fetus continues to grow and develop, they also become more active, even responding to sounds with movement. By six months, baby’s fingerprints are developed, and the eyes begin to open. At this stage, the fetus is working on hearing, with rapid development of the inner ear organs. The lungs are developing quickly.

Baby’s Size

At six months, the fetus is about a foot long and weighs 1.3 pounds, about the size of an ear of corn.

Changes in Your Body

Heartburn may be a symptom now, as your uterus grows and pushes on your digestive organs. You may be able to make out the shape of your baby more distinctly. Backaches may start to develop with the increased pressure from your growing baby.

The Third Trimester – Stages of Baby’s Growth

The third trimester is the home stretch of your pregnancy! The more time your baby has in the womb now, the better their odds for a safe and healthy delivery. Throughout this trimester, the baby is adding much-needed body fat that will help keep them healthy outside the womb.

Now is the time to firm up your labor and delivery plans. You should also be on the lookout for signs of early labor, and contact your healthcare provider immediately if you believe you may be experiencing any of these symptoms.

Tests during the third trimester include:

• Group B streptococcus screening. This bacterium is found in the lower genital tract of about one-fifth of all women. If left untreated, it can cause problems in mothers and babies. Screening usually occurs between weeks 35 and 37.
• Fetal monitoring. In the late stages of pregnancy, your doctor may want to monitor your baby’s heart rate to look for any abnormalities that may indicate problems in the womb or the need for a cesarean delivery.
• Ultrasound. In the third trimester, an ultrasound can help physicians monitor fetal growth, check the levels of amniotic fluid, and determine the position of the fetus.
• Glucose testing. A glucose challenge test is usually given between weeks 24 and 28 to examine your body’s reaction to sugars. Abnormal results will lead to glucose tolerance testing to explore a possible diagnosis of gestational diabetes.

Month 7 (Weeks 25-28)

Baby’s Development

In the seventh month, the fetus continues to develop and to grow fat reserves. By now, their sense of hearing is fully developed, and they can respond to light, sounds, and pain. This month, baby’s movements are getting stronger and stronger, and they may even have a regular sleep schedule.

Baby’s Size

By the end of the seventh month, your baby weighs about 2 2/3 pounds and measures 15 inches long.

Changes in Your Body

By month seven, a new level of fatigue may set in as your body continues to grow. Your body also has a blood level that is 50% higher than normal. Your uterus may also begin the practice contractions known as Braxton-Hicks contractions.

Month 8 (Weeks 29-32)

Baby’s Development

This month, the baby continues to add fat reserves to help them develop after birth. Many organ systems are functional by now, but this month provides crucial developments for the still- immature lungs. The brain continues to develop, and some researchers believe baby begins to dream during sleep. This month, the fetus will shed the lanugo. This provides an important source of protein when it is swallowed with the amniotic fluid.

Baby’s Size

By the end of the eighth month, your baby could be as long as 18 inches and weigh as much as five pounds.

Changes in Your Body

By the eighth month, you may have increasing fatigue and insomnia, so it’s important to rest when you can. You may feel that you have difficulty taking a full breath because of the size of your belly. In the eighth month, your breasts may begin to discharge fluid, which is a normal sign that your body is getting ready for baby’s arrival.

Month 9-10 (Weeks 37-42)

Baby’s Development

In the final weeks of your pregnancy, your baby will continue to get ready for life after birth, with the lungs finally mature, and plenty of fat reserves stored up. Baby’s reflexes are now coordinated, and they can make many of the movements that they’ll make when they’re born, like blinking, turning the head, and grasping.

Baby’s Size

In the last month of pregnancy, your baby will reach its mature size. Most full-term babies range in weight from about six to nine pounds, and in length from 19-21 inches.

Changes in Your Body

By now your body is getting ready for the imminent arrival of your baby. You may experience a “nesting” phase, where you have sudden bursts of energy that help you prepare your home for your new arrival. You may experience lightening, the sensation that the baby’s position has dropped. Your uterus may be undergoing Braxton-Hicks contractions more frequently, and you may notice some normal vaginal discharge.

Most pregnant women go into labor sometime between weeks 38 and 42. Signs of labor include regular contractions, which can range from mild to severe in intensity. Your cervix may release its mucus plug, and your water may break. Other signs of labor include leg cramps, back pain, and nausea. For details about the stages of labor, check out this guide from the National Institutes of Health.