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Goes over weeks 1-12 of prenatal development

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Fetal development: The 1st trimester

Fetal development begins soon after conception. Find out how your baby grows and develops during the first trimester.
By Mayo Clinic Staff
You're pregnant. Congratulations! You'll undoubtedly spend the months ahead wondering how your baby is growing and developing. What does your baby look like? How big is he or she? When will you feel the first kick?

Fetal development typically follows a predictable course. Find out what happens during the first trimester by checking out this weekly calendar of events. Keep in mind that measurements are approximate.

Weeks 1 and 2: Getting ready

It might seem strange, but you're not actually pregnant the first week or two of the time allotted to your pregnancy. Yes, you read that correctly!

Conception typically occurs about two weeks after your last period begins. To calculate your estimated due date, your health care provider will count ahead 40 weeks from the start of your last period. This means your period is counted as part of your pregnancy - even though you weren't pregnant at the time.

Week 3: Fertilization

The sperm and egg unite in one of your fallopian tubes to form a one-celled entity called a zygote. If more than one egg is released and fertilized or if the fertilized egg splits into two, you might have multiple zygotes.

The zygote typically has 46 chromosomes - 23 from the biological mother and 23 from the biological father. These chromosomes help determine your baby's sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it will begin dividing to form a cluster of cells resembling a tiny raspberry - a morula.

Week 4: Implantation

The rapidly dividing ball of cells - now known as a blastocyst - has begun to burrow into the uterine lining (endometrium). This process is called implantation.

Within the blastocyst, the inner group of cells will become the embryo. The outer layer will give rise to part of the placenta, which will nourish your baby throughout the pregnancy.

Week 5: Hormone levels increase

The fifth week of pregnancy, or the third week after conception, the levels of HCG hormone produced by the blastocyst quickly increase. This signals your ovaries to stop releasing eggs and produce more estrogen and progesterone. Increased levels of these hormones stop your menstrual period, often the first sign of pregnancy, and fuel the growth of the placenta.

The embryo is now made of three layers. The top layer - the ectoderm - will give rise to your baby's outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears.

Your baby's heart and a primitive circulatory system will form in the middle layer of cells - the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, ligaments, kidneys and much of the reproductive system.

The inner layer of cells - the endoderm - is where your baby's lungs and intestines will develop.

Week 6: The neural tube closes

Growth is rapid this week. Just four weeks after conception, the neural tube along your baby's back is closing. The baby's brain and spinal cord will develop from the neural tube. The heart and other organs also are starting to form.

Structures necessary to the formation of the eyes and ears develop. Small buds appear that will soon become arms. Your baby's body begins to take on a C-shaped curvature.

Week 7: Baby's head develops

Seven weeks into your pregnancy, or five weeks after conception, your baby's brain and face are growing. Depressions that will give rise to nostrils become visible, and the beginnings of the retinas form.

Lower limb buds that will become legs appear and the arm buds that sprouted last week now take on the shape of paddles.

Week 8: Baby's nose forms

Eight weeks into your pregnancy, or six weeks after conception, your baby's lower limb buds take on the shape of paddles. Fingers have begun to form. Small swellings outlining the future shell-shaped parts of your baby's ears develop and the eyes become obvious. The upper lip and nose have formed. The trunk and neck begin to straighten.

By the end of this week, your baby might be about 1/2 inch (11 to 14 millimeters) long from crown to rump - about half the diameter of a U.S. quarter.

Week 9: Baby's toes appear

In the ninth week of pregnancy, or seven weeks after conception, your baby's arms grow and elbows appear. Toes are visible and eyelids form. Your baby's head is large but still has a poorly formed chin.

By the end of this week, your baby might be a little less than 3/4 inch (16 to 18 millimeters) long from crown to rump - the diameter of a U.S. penny.

Week 10: Baby's elbows bend

By the 10th week of pregnancy, or eight weeks after conception, your baby's head has become more round.

Your baby can now bend his or her elbows. Toes and fingers lose their webbing and become longer. The eyelids and external ears continue to develop. The umbilical cord is clearly visible.

Week 11: Baby's genitals develop

At the beginning of the 11th week of pregnancy, or the ninth week after conception, your baby's head still makes up about half of its length. However, your baby's body is about to catch up.

Your baby is now officially described as a fetus. This week your baby's face is broad, the eyes widely separated, the eyelids fused and the ears low set. Buds for future teeth appear. Red blood cells are beginning to form in your baby's liver. By the end of this week, your baby's external genitalia will start developing into a penis or a clitoris and labia majora.

By now your baby might measure about 2 inches (50 millimeters) long from crown to rump - the length of the short side of a credit card - and weigh almost 1/3 ounce (8 grams).

Week 12: Baby's fingernails form

Twelve weeks into your pregnancy, or 10 weeks after conception, your baby is sprouting fingernails. Your baby's face now has taken on a more developed profile. His or her intestines are in the abdomen.

By now your baby might be about 2 1/2 inches (61 millimeters) long from crown to rump - the length of the short side of a U.S. bill - and weigh about 1/2 ounce (14 grams).

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Show references

1. Pregnancy: Stages of pregnancy. Office on Women's Health. https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/stages-pregnancy. Accessed Feb. 25, 2020.

2. Frequently asked questions: Pregnancy FAQ156: Prenatal development: How your fetus grows during pregnancy. American College of Obstetricians and Gynecologists. https://www.acog.org/patient-resources/faqs/pregnancy/how-your-fetus-grows-during-pregnancy. Accessed Feb. 25, 2020.

3. American College of Obstetricians and Gynecologists. Your Pregnancy and Childbirth Month to Month. 6th ed. American College of Obstetricians and Gynecologists; 2015.

4. Moore KL, et al. The Developing Human: Clinically Oriented Embryology. 11th ed. Elsevier; 2020. https://www.clinicalkey.com. Accessed Feb. 25, 2020.

See more In-depth

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Fetal development: What happens during the 1st trimester?

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Fetal development begins soon after conception. Find out how your baby grows and develops during the first trimester. By Mayo Clinic Staff You're pregnant. Congratulations! You'll undoubtedly spend the months ahead wondering how your baby is growing and developing. What does your baby look like? How big is he or she?

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How did you come to be who you are? From beginning as a one-cell structure to your birth, your prenatal development occurred in an orderly and delicate sequence. There are three stages of prenatal development: germinal, embryonic, and fetal. Keep in mind that this is different than the three trimesters of pregnancy. Let's take a look at what happens to the developing baby in each of these stages.

"The body of the unborn baby is more complex than ours. The preborn baby has several extra parts to his body which he needs only so long as he lives inside his mother. He has his own space capsule, the amniotic sac. He has his own lifeline, the umbilical cord, and he has his own root system, the placenta. These all belong to the baby himself, not to his mother. They are all developed from his original cell."

Day & Liley, The Secret World of a Baby, Random House, 1968, p. 13

Let's take a look at some of the changes that take place during each of the three periods of prenatal development: the germinal period, the embryonic period, and the fetal period.

The Germinal Period (Weeks 1-2)

Conception occurs when a sperm fertilizes an egg and forms a zygote, which begins as a one-cell structure. The mother and father's DNA is passed on to the child at the moment of conception. The genetic makeup and sex of the baby are set at this point. The germinal period (about 14 days in length) lasts from conception to implantation of the zygote (fertilized egg) in the lining of the uterus.

During the first week after conception, the zygote divides and multiplies, going from a one-cell structure to two cells, then four cells, then eight cells, and so on. The process of cell division is called mitosis. After the fourth division, differentiation of the cells begins to occur as well. Differentiated cells become more specialized, forming different organs and body parts. After 5 days of mitosis, there are 100 cells, and after 9 months there are billions of cells. Mitosis is a fragile process, and fewer than one-half of all zygotes survive beyond the first two weeks (Hall, 2004).

After the zygote divides for about 7-10 days and has 150 cells, it travels down the fallopian tubes and implants itself in the lining of the uterus. It's estimated that about 60 percent of natural conceptions fail to implant in the uterus. The rate is higher for in vitro conceptions. Once the zygote attaches to the uterus, the next stage begins.

The Embryonic Period (Weeks 3-8)

The embryonic period begins once the zygote is implanted in the uterine wall. It lasts from the third through the eighth week after conception. Upon implantation, this multi-cellular organism is called an embryo. Now blood vessels grow, forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord.

During this period, cells continue to differentiate. Basic structures of the embryo start to develop into areas that will become the head, chest, and abdomen. During the embryonic stage, the heart begins to beat and organs form and begin to function. At 22 days after conception, the neural tube forms along the back of the embryo, developing into the spinal cord and brain.

Growth during prenatal development occurs in two major directions: from head to tail (cephalocaudal development) and from the midline outward (proximodistal development). This means that those structures nearest the head develop before those nearest the feet and those structures nearest the torso develop before those away from the center of the body (such as hands and fingers).

The head develops in the fourth week and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent. But by the end of this stage, they disappear and the organism takes on a more human appearance. The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time.

About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant. It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. Potential mothers are not often aware of the risks they introduce to the developing child during this time.

The Fetal Period (Weeks 9-40)

When the organism is about nine weeks old, the embryo is called a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being as the "tail" begins to disappear.

From 9-12 weeks, the sex organs begin to differentiate. By the 12th week, the fetus has all its body parts including external genitalia. In the following weeks, the fetus will develop hair, nails, teeth and the excretory and digestive systems will continue to develop. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams.

At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. During the 4-6th months, the eyes become more sensitive to light and hearing develops. The respiratory system continues to develop. Reflexes such as sucking, swallowing and hiccuping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well. Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nurse cells that support neurons continue to grow. At 24 weeks the fetus can feel pain (Royal College of Obstetricians and Gynecologists, 1997).

The first chance of survival outside the womb, known as the age of viability is reached at about 22 to 26 weeks (Moore & Persaud, 1998). By the time the fetus reaches the sixth month of development (24 weeks), it weighs up to 1.4 pounds. The hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the mother's womb.

Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.

Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus's organ systems are developed enough that it could survive outside the mother's uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent.

Watch It

Video 1. Prenatal Development explains many of the developmental milestones and changes that happen during each month of development for the embryo and fetus.

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How did you come to be who you are? From beginning as a one-cell structure to your birth, your prenatal development occurred in an orderly and delicate sequence. There are three stages of prenatal development: germinal, embryonic, and fetal. Keep in mind that this is different than the three trimesters of pregnancy.

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American College of Obstetricians and Gynecologists (ACOG) 

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American College of Obstetricians and Gynecologists (ACOG) 

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ACOG contains articles breaking down fetal development every 3 weeks until birth, answers to commonly asked questions, and other helpful information

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How does pregnancy begin?

Fertilization, the union of an egg and a sperm into a single cell, is the first step in a complex series of events that leads to pregnancy. Fertilization takes place in the fallopian tube. Over the next few days, the single cell divides into multiple cells.

At the same time, the small cluster of dividing cells moves through the fallopian tube to the lining of the uterus. There it implants and starts to grow. For 8 weeks after fertilization, it is called an embryo. From 9 weeks after fertilization until birth, it is called a fetus.

What is the placenta?

The placenta is formed from some of these rapidly dividing cells. The placenta functions as a life-support system during pregnancy. Oxygen, nutrients, and hormones are transferred across the placenta to the fetus. Waste products from the fetus are transferred back across the placenta for removal.

How will my uterus change during pregnancy?

During pregnancy, the lining of your uterus thickens and its blood vessels enlarge to provide nourishment to the fetus. As pregnancy progresses, your uterus expands to make room for the fetus. By the time your baby is born, your uterus will have expanded to many times its normal size.

How long does pregnancy last?

A normal pregnancy lasts about 40 weeks from the first day of your last menstrual period (LMP). Pregnancy is assumed to start 2 weeks after the first day of the LMP. So an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant. The 40 weeks of pregnancy includes those extra 2 weeks.

How is the length of my pregnancy measured?

Pregnancy can be divided into weeks and sometimes days. A pregnancy that is "36 and 3/7 weeks" means "36 weeks and 3 days of pregnancy." The 40 weeks of pregnancy often are grouped into three trimesters. Here's how the trimesters are defined:

First trimester (first day of LMP to 13 weeks and 6 days): The time when fertilization and major organ development occurs.

Second trimester (14 weeks and 0 days to 27 weeks and 6 days): The time of rapid growth and development.

Third trimester (28 weeks and 0 days to 40 weeks and 6 days): The time when the fetus's weight increases and the organs mature so they will be ready to function after birth.

What is the estimated due date?

The day your baby is due is called the estimated due date (EDD). Only about 1 in 20 women give birth on their due dates. Still, the EDD is useful for a number of reasons. It determines your fetus's gestational age throughout pregnancy so that the fetus's growth can be tracked. It also provides a timeline for certain tests that you will have throughout your pregnancy.

How is my estimated due date calculated?

Your EDD is calculated from the first day of your LMP. But when the date of the LMP is uncertain, an ultrasound exam may be done during the first trimester to estimate the due date.

During this ultrasound exam, the "crown-rump length" is measured. This is the length of the embryo or fetus from the top of the head ("crown") to the bottom of the area that will become the buttocks ("rump"). If you are less than 7 weeks pregnant, it is not possible to see the embryo's crown or rump, so the greatest length of the embryo is measured. A formula is then used to estimate the gestational age based on this measurement.

If you have had in vitro fertilization (IVF), the EDD is set by the age of the embryo and the date that the embryo is transferred to the uterus.

What happens during weeks 9 to 12 of pregnancy?

Cartilage for the limbs, hands, and feet start to form but won't harden into bones for a few weeks.

Eyelids form but remain closed.

Liver begins development.

Kidneys begin making urine.

Pancreas starts making insulin.

Fingernails form.

What happens during weeks 17 to 20 of pregnancy?

The part of the brain that controls motor movements is fully formed.

Digestive system is working.

Ears, nose, and lips are recognizable on an ultrasound exam.

In girls, the uterus and vagina are starting to form.

Soft, downy hair called lanugo starts to form and cover the body.

What happens during weeks 21 to 24 of pregnancy?

Kicks and turns are stronger.

Sucking reflex is developing.

Fat is forming under the skin.

Ridges form in the hands and feet that later will be fingerprints and footprints.

Skin is wrinkled and reddish from visible blood vessels.

What happens during weeks 33 to 36 of pregnancy?

Bones harden but the skull remains soft and flexible.

Fingernails have grown to the end of the fingers.

Limbs begin to look chubby.

The fetus may turn into a head-down position for birth.

What happens during weeks 37 to 40 of pregnancy?

Circulatory system is done developing.

Musculoskeletal system also is done developing.

Lungs, brain, and nervous system are finishing their development.

Fat continues to be added all over to keep the baby warm after birth.

Glossary

Cell: The smallest unit of a structure in the body. Cells are the building blocks for all parts of the body.

Egg: The female reproductive cell made in and released from the ovaries. Also called the ovum.

Embryo: The stage of development that starts at fertilization (joining of an egg and sperm) and lasts up to 8 weeks.

Fallopian Tube: One of a pair of tubes through which an egg travels from the ovary to the uterus.

Fertilization: A multistep process that joins the egg and the sperm.

Fetus: The stage of human development beyond 8 completed weeks after fertilization.

Genitals: The sexual or reproductive organs.

Gestational Age: How far along a woman is in her pregnancy, usually reported in weeks and days.

Hormones: Substances made in the body that control the function of cells or organs.

In Vitro Fertilization (IVF): A procedure in which an egg is removed from a woman's ovary, fertilized in a laboratory with the man's sperm, and then transferred to the woman's uterus to achieve a pregnancy.

Lanugo: Soft, downy hair that covers the fetus's body.

Last Menstrual Period (LMP): The date of the first day of the last menstrual period before pregnancy. The LMP is used to estimate the date of delivery.

Oxygen: An element that we breathe in to sustain life.

Placenta: An organ that provides nutrients to and takes waste away from the fetus.

Scrotum: The external genital sac in the male that contains the testicles.

Sperm: A cell made in the male testicles that can fertilize a female egg.

Surfactant: A substance made by cells in the lungs. This substance helps keep the lungs elastic and keeps them from collapsing.

Testicles: Paired male organs that make sperm and the male sex hormone testosterone. Also called testes.

Trimesters: The 3-month periods of time in pregnancy. They are referred to as first, second, or third.

Ultrasound Exam: A test in which sound waves are used to examine inner parts of the body. During pregnancy, ultrasound can be used to check the fetus.

Uterus: A muscular organ in the female pelvis. During pregnancy, this organ holds and nourishes the fetus. Also called the womb.

Vagina: A tube-like structure surrounded by muscles. The vagina leads from the uterus to the outside of the body.

If you have further questions, contact your ob-gyn.

Don't have an ob-gyn? Learn how to find a doctor near you.

FAQ156
Last updated: January 2024
Last reviewed: January 2024

Copyright 2024 by the American College of Obstetricians and Gynecologists. All rights reserved. Read copyright and permissions information.

This information is designed as an educational aid for the public. It offers current information and opinions related to women's health. It is not intended as a statement of the standard of care. It does not explain all of the proper treatments or methods of care. It is not a substitute for the advice of a physician. Read ACOG's complete disclaimer.

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This video covers fetal development over the first trimester, alongside an explanation of pregnancy symptoms appearing at this time

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First Trimester | 3D Animated Pregnancy Guide

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From fertilization to blastocyst implantation

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Embrology - Day 0 7 Fertilization, Zygote, Blastocyst

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Reviews growth of weeks 14-22

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Stages of Fetal Development - Second Trimester | La Dept. of Health

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The Louisiana Department of Health protects and promotes health and ensures access to medical, preventive and rehabilitative services for all citizens of the State of Louisiana.

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This covers fetal development over the second trimester, the risk factors involved in premature birth, and explains the cause and effects of maternal changes.

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Second Trimester | 3D Animated Pregnancy Guide

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This covers the last steps of fetal development, alongside what the mother should expect to feel in the weeks leading up to labor.

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Third Trimester | 3D Animated Pregnancy Guide

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Goes over weeks 28-40

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Fetal development: The 3rd trimester

Fetal development continues during the third trimester. Your baby will open his or her eyes, gain more weight, and prepare for delivery.
By Mayo Clinic Staff
The end of your pregnancy is near! By now, you're eager to meet your baby face to face. Your uterus, however, is still a busy place. Here's a weekly calendar of events for fetal development during the third trimester. Keep in mind that measurements are approximate.

Week 28: Baby's eyes partially open

Twenty-eight weeks into your pregnancy, or 26 weeks after conception, your baby's eyelids can partially open and eyelashes have formed. The central nervous system can direct rhythmic breathing movements and control body temperature.

By now your baby might be nearly 10 inches (250 millimeters) long from crown to rump and weigh nearly 2 1/4 pounds (1,000 grams).

Week 29: Baby kicks and stretches

Twenty-nine weeks into your pregnancy, or 27 weeks after conception, your baby can kick, stretch and make grasping movements.

Week 30: Baby's hair grows

Thirty weeks into your pregnancy, or 28 weeks after conception, your baby's eyes can open wide. Your baby might have a good head of hair by this week. Red blood cells are forming in your baby's bone marrow.

By now your baby might be more than 10 1/2 inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds (1,300 grams).

Week 31: Baby's rapid weight gain begins

Thirty-one weeks into your pregnancy, or 29 weeks after conception, your baby has finished most of his or her major development. Now it's time to gain weight - quickly.

Week 32: Baby practices breathing

Thirty-two weeks into your pregnancy, or 30 weeks after conception, your baby's toenails are visible.

The layer of soft, downy hair that has covered your baby's skin for the past few months (lanugo) starts to fall off this week.

By now your baby might be 11 inches (280 millimeters) long from crown to rump and weigh 3 3/4 pounds (1,700 grams).

Week 33: Baby detects light

Thirty-three weeks into your pregnancy, or 31 weeks after conception, your baby's pupils can change size in response to a stimulus caused by light. His or her bones are hardening. However, the skull remains soft and flexible.

Week 34: Baby's fingernails grow

Thirty-four weeks into your pregnancy, or 32 weeks after conception, your baby's fingernails have reached his or her fingertips.

By now your baby might be nearly 12 inches (300 millimeters) long from crown to rump and weigh more than 4 1/2 pounds (2,100 grams).

Week 35: Baby's skin is smooth

Thirty-five weeks into your pregnancy, or 33 weeks after conception, your baby's skin is becoming smooth. His or her limbs have a chubby appearance.

Week 36: Baby takes up most of the amniotic sac

Thirty-six weeks into your pregnancy, or 34 weeks after conception, the crowded conditions inside your uterus might make it harder for your baby to give you a punch. However, you'll probably still feel lots of stretches, rolls and wiggles.

Week 37: Baby might turn head down

Thirty-seven weeks into your pregnancy, or 35 weeks after conception, your baby has a firm grasp.

To prepare for birth, your baby's head might start descending into your pelvis. If your baby isn't head down, your health care provider will talk to you about ways to deal with this issue.

Week 38: Baby's toenails grow

Thirty-eight weeks into your pregnancy, or 36 weeks after conception, the circumference of your baby's head and abdomen are about the same.

Your baby's toenails have reached the tips of his or her toes. Your baby has mostly shed all of his or her lanugo.

By now your baby might weigh about 6 1/2 pounds (2,900 grams).

Week 39: Baby's chest is prominent

Thirty-nine weeks into your pregnancy, or 37 weeks after conception, your baby's chest is becoming more prominent. For boys, the testes continue to descend into the scrotum. Fat is being added all over your baby's body to keep him or her warm after birth.

Week 40: Your due date arrives

Forty weeks into your pregnancy, or 38 weeks after conception, your baby might have a crown-to-rump length of around 14 inches (360 millimeters) and weigh 7 1/2 pounds (3,400 grams). Remember, however, that healthy babies come in different sizes.

Don't be alarmed if your due date comes and goes with no signs of labor starting. Your due date is simply a calculated estimate of when your pregnancy will be 40 weeks. It does not estimate when your baby will arrive. It's normal to give birth before or after your due date.

Show references

See more In-depth

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Fetal development: What happens during the 3rd trimester?

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Fetal development continues during the third trimester. Your baby will open his or her eyes, gain more weight, and prepare for delivery. By Mayo Clinic Staff The end of your pregnancy is near! By now, you're eager to meet your baby face to face. Your uterus, however, is still a busy place.

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Just for fun.

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How Big Is My Baby This Week? Here's Your Baby Fruit Size Chart

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We're not sure why it's so fun comparing your fetus to fruit, but it really is. From poppy seed to watermelon, get a sense of your baby's size by week with these growth averages and fruit-size analogies.