The Stages of Fetal Development

Each month, a woman is fertile for about 5 days. Though an egg is only able to be fertilized for 12-24 hours, sperm can live inside the uterus and fertilize any egg that is released for about 5 days after intercourse.

There are two ways to measure what stage of development a baby is in: fertilization age and gestational age.

These terms can be somewhat confusing since they are based upon different starting points. Fertilization age is calculated from the date of conception onwards. The gestational (or menstrual) age timeline begins at the start of the mother’s last menstrual period and is figured from her point of view. Gestational age is usually estimated to be about two weeks earlier than fertilization age, though the actual date of conception can vary. The framework most often used in discussions about the development of the unborn child is gestational age.

Thus, fertilization takes place at roughly two weeks of gestational age and implantation about one week later. At fertilization, genetic instructions from the mother and the father combine to form a baby, who is often called a zygote at this stage, barely visible to the human eye.¹ This single cell contains more information than fifty sets of the physical 33-volume set of the Encyclopedia Britannica.

Stages of Fetal Development

When a woman ovulates, an ovary releases an egg (or several), which travels into the Fallopian tube to be fertilized. After an egg is fertilized, usually within the Fallopian tubes, the germinal stage begins. This stage is the shortest of the fetal development stages. Multiple characteristics are already determined at the moment of conception, including the sex of the baby! Let’s call this newly formed baby Andrew.

The fertilized egg, or newly formed human being, is often called a zygote at this stage. On the very first day, the first four cell divisions take place as baby Andrew travels down the mother’s Fallopian tubes towards the uterus, all the while being nourished and protected by the mother’s body.

About a week after conception (5-9 days), the newly formed baby (often called a blastocyst, a baby that is rapidly growing), implants in the uterine lining. From this point onwards to about eight weeks, the newly formed baby is called an embryo. Pro-abortionists often make the unscientific claim that pregnancy begins at implantation instead of fertilization. The change of terminology regarding the beginning of pregnancy was made in the 1960s based not on medical evidence, but simply to clear the way for abortifacient means of birth control that were under development at the time.² Implantation is the end of the germinal stage and the beginning of the embryonic stage.

Related: What Are Abortifacients?

The first three months of pregnancy are known as the first trimester. Once the second trimester starts, critical development has already taken place.

Around 4 weeks gestational age (two weeks fertilization age), the first missed menstrual period happens. The mother’s menstruation is suppressed by chemical signals emitted by the unborn child. The child’s first completed brain cells appear.

At week 5, Andrew’s heart is in an advanced stage of formation. His eyes begin to form, and his brain, spinal column, and nervous system are virtually complete. Around this time, the mother will be able to get a positive home pregnancy test.

At just over 5 weeks, Andrew’s heart begins to beat and will beat 110 times per minute.

At week 6, the child’s muscles are developing. His arm and leg buds are visible, and his first neocortical cells appear. The neocortex is the seat of complex thinking and reasoning, a feature present in no other mammal. He has grown in size by a factor of 10,000 since fertilization and is now 6 to 7 millimeters long (about 1/4 inch). Blood flows in the baby’s veins, separate from his mother’s blood.

At the 7th week of pregnancy, the baby’s pituitary gland is forming. The pituitary is often called the “master gland” since it controls the functions of most of the other endocrine glands. Additionally, the baby’s mouth, ears, and nose are taking shape. His cartilage is beginning to be replaced with bone, and the genitals are forming.

At 8 weeks of pregnancy, Andrew’s heart energy output is an incredible 20% that of an adult. His cartilage skeleton is completely formed, and ossification (bone formation) begins. The umbilical cord has developed. His brain coordinates voluntary movement of muscles and the involuntary movement of organs. Reflex responses are present. At this stage, the baby will have reflexes in reaction to invasive procedures.

At around 43 days after conception, the baby’s brain waves can be recorded.

At 45 days after conception, Andrew begins spontaneous and voluntary body movements, and his milk teeth buds are present.

At around 9 weeks, the embryonic stage ends and the third and final stage of fetal development begins – the fetal stage.

At week 9, Andrew’s lips are sensitive to touch, and his ears resemble his family’s pattern. His teeth and taste buds are beginning to form. The first fully developed neurons (nerve cells) appear on the top of his spinal cord, beginning the construction of the brain stem, which regulates vital functions such as breathing, the heartbeat, and blood pressure.

At 10 weeks, the preborn baby is well-proportioned, about 1 1/2 inches long and 1/30 of an ounce in weight. His limbs are fully formed. All organs are present, complete, and functioning (except the lungs). His heart beats sturdily. His stomach produces digestive juices, his liver makes blood cells, and his kidneys are functioning. The baby’s unique fingerprints are being engraved. His eyelids and the palms of his hands are sensitive to touch. Of the 45 total generations of cell replication that will take place by mature adulthood, fully two-thirds (30) have already taken place. He now consists of about one billion cells, containing more genetic information than every word communicated by every human being who has ever lived since the beginning of the human race.³

At 11 weeks, Andrew will bend his fingers around an object placed in his palm, and he can open and close his mouth. His fingernails are forming, and he sucks his thumb.

At 12 weeks, all sections of the preborn baby’s body are sensitive to touch. He swallows, squints, frowns, and puckers up his brow. Several systems are now functioning – the circulatory, digestive, and urinary. Baby’s liver produces bile, as well. At this point, all of the organs, limbs, muscles, and bones are present. Research shows that babies around this time may feel pain.

The first trimester of pregnancy ends at the end of week 12 (gestational age). By the end of the first trimester (the point at which most surgical abortions are done), vigorous activity shows the baby’s distinct personality. Sleep patterns differ; some babies hiccup constantly, others may cry. Andrew can kick, turn over, curl and fan his toes, make a fist, open his mouth and press his lips tightly together, and practice breathing.

The second trimester will run from week 13 until the end of week 27.

Week 13 is the start of the second trimester. During this week, baby Andrew makes all facial expressions, including smiles. He is now breathing amniotic fluid steadily and will continue to do so until birth. His fingernails and toenails are now present. His taste buds are working; he will drink more amniotic fluid if it is artificially sweetened, and less if it is given a bitter taste.

At 13 weeks, the preborn baby’s facial expressions resemble those of his parents. His movements are vigorous and graceful. His vocal cords and external sex organs are present, and the sex of the baby can be determined. The baby is also beginning to create urine, which is released into the amniotic fluid.

At 14 weeks, the baby’s skin is thickening. His fingerprints are forming, and red blood cells are forming in the spleen. Andrew can grasp with his hands, swim, and turn somersaults. His mother may first feel his movements at this time. His eyelashes are now present and rapid eye movements (REM), indicating dreaming, can be recorded. A very bright light shined on the mother’s abdomen will cause the baby to slowly move his arms to cover his eyes. Very loud music will cause him to cover his ears.

In week 15, the baby’s lungs are forming. He is now doing more significant movements, like sucking his thumb or smiling. His hair pattern on his scalp is also now forming.

In week 16, Andrew’s ears are well-formed enough to hear and he can react to light. His eyes can also move.

At 17 weeks, the baby’s heart is already pumping around 100 pints of blood per day! He is starting to roll around in the womb, his toenails are beginning to form, and he is beginning to put on fat. He’s also now covered in a white substance called vernix, which is thought to protect him.

In week 18, the baby may already have a sleeping schedule and may be awoken by loud noises. He’s covered in hair similar to peach fuzz called lanugo, which helps keep him warm and protects him from the amniotic fluid. His digestive system has begun functioning.

At week 19, all five senses are developing, and baby can now swallow! His adult teeth are beginning to form, as well, and around this time, the mother may begin feeling kicks. The bronchioles of his lungs are starting to develop.

At 20 weeks, baby’s taste buds are now formed and working. He is also drinking several ounces of amniotic fluid per day, and his sleep/wake cycles are beginning to be established, so the mother may feel more movements at certain times of the day.

In week 21, Andrew now has fully formed bone marrow, which is pumping out red blood cells. He can also now hear sounds going on inside the mother’s body, like her heartbeat, and the baby has a preference for his mother’s voice. Once he is born, he is likely to recognize his parents’ voices and the songs they sang frequently during pregnancy or the stories they read to the baby. The most premature baby on record to survive outside the womb was born at just 21 weeks and 1 day.

At 22 weeks, the baby has eyelashes, eyebrows, and nails. He can also respond to light and sounds outside of the womb. He has also formed his own unique sleeping habits, and a loud sound such as a slammed door may startle him. His hearing has a wider range of frequency than an adult’s in both the higher and lower ranges. He may be soothed to sleep by gentle music.

At 23 weeks, the baby’s face is now beginning to look much like how it will when he is born. There are ridges on his palms and on the soles of his feet, which will turn into fingerprints.

At 24 weeks, Andrew’s reflexes are developed, so he may jump at loud noises. His skin is still see-through, but it is gradually becoming opaquer and is taking on a pink tone. If someone puts an ear to the mother’s pregnant belly, they may even hear the baby’s heartbeat! At around this time, Andrew has reached viability, meaning that he has a chance of surviving outside of the womb if he is born early.

In week 25, the baby is learning which way is up and down. His hands are fully formed, and he can grasp things now (such as the umbilical cord).

In week 26, Andrew’s eyes are forming, which will open soon. He is beginning to react more to stimuli and may even already recognize the mother’s voice. His immune system is also forming as he is benefitting from her mother’s antibodies. His lungs are also producing surfactant, which is necessary to allow the air sacs to inflate.

Week 27 is the last week of the second trimester. He is now practicing breathing in and out, his nervous system is continuing to develop, and his brain shows activity.

Week 28 is the start of the third trimester, which will end at 40 weeks. In this week, the baby is adding on more fat, so his skin is becoming less wrinkled. If the baby is a boy, the testicles will descend from the abdomen around this time.

In week 29, the mother may begin noticing small, rhythmic taps – this is Andrew hiccupping! The white coating that covers him to protect his skin from the amniotic fluid – the vernix – is starting to disappear. Myelin begins to form around his nerves to protect them.

In week 30, his weight increases to over one kilogram (2.2 pounds). His lungs are capable of breathing air. His eyeteeth are present. His hands can support his entire weight at this time, and he recognizes his mother’s voice. Of the 45 total generations of cell replication that will take place by mature adulthood, 38 have already taken place. He now has about 300 billion cells. His brain is growing quickly and is beginning to form “wrinkles,” known as convolutions, which allow it to store more brain tissue. He may also be able to see some dim shapes!

Week 31 sees a rapid growth of brain connections and more sleep time. Andrew is also now receiving information from all five senses. His eyes can also begin to focus, and the pupils will dilate and constrict in response to light. He may even somersault!

At 32 weeks, the baby is preparing for birth! The peach fuzz-like hair, lanugo, is beginning to shed. He is also around 11 inches from head to rump and weighs a little under 4 pounds.

At 33 weeks, his brain and nervous system are both fully developed. His bones continue to harden, but his skull remains soft, making birth easier.

At 8 months (34 weeks), with only one month to go, his weight is about two kilograms (4.4 pounds). If born now, Andrew has more than a 90% chance of surviving and being entirely healthy. His fingernails also have grown to the end of his fingertips, and he responds to light, sound, and touch. His ears will also be fully formed soon!

In week 35, the baby’s kidneys are formed, and his first bowel movement is forming in his intestines. He also has set sleep cycles at this stage, and the mother may feel more movement when he is awake and less when he is asleep. There is less space in the uterus now, so baby will not be kicking but rather will be rolling and wiggling. Most babies at this stage have shifted to be head down in the uterus, preparing for birth.

At 36 weeks, the final month of pregnancy has officially begun. The baby’s lungs are fully formed. His digestive system is also fully formed, so if he is born now, he would be able to feed. His ears are now more sensitive to sound.

At 37 weeks, if the baby is born now, he will be considered early-term. 37 weeks was previously considered full-term, but research now shows that the brain, lungs, and liver develop significantly between weeks 37 and 39. The baby is increasing in dexterity and can now grab smaller objects, such as his toe. He also may have a full head of hair!

At 38 weeks, the baby is gaining about an ounce of weight per day. All of his senses are fully functioning, and his toenails and fingernails are complete. most of the lanugo (peach fuzz-like hair) has been shed, and his vocal cords are now developed. His eyes also have color, though it may not be their true color, which will be established by the time he turns 1.

At 39 weeks, Andrew is officially full-term. That means that he is fully developed and ready to be born. He now has coordinated reflexes, can turn his head, and once born, can see 6 to 10 inches.

The baby may be born on the due date, or a little earlier or later. On his birthday, Andrew releases hormones that trigger labor. The lightest baby ever born to survive in good health weighed in at a mere 7.5 ounces, less than a can of soft drink. Of the 45 total generations of cell replication that will take place by mature adulthood, 41 have taken place by birth. The baby now has about two trillion (2,000,000,000,000) cells. The remaining four generations of cell replication will occupy all of the person’s childhood and young adulthood. This means that, in developmental terms, we spend more than 90% of our lives in utero.

Prenatal Care is Essential for a Healthy Pregnancy

How can a pregnant woman best prepare for a healthy birth and set up her baby for a healthy beginning of life? As always, diet and exercise are essential. In addition to avoiding certain foods and drinks, a pregnant woman must also ensure she is eating healthily and receiving the necessary nutrients. What exercises are safe for pregnancy can vary based on the woman and that particular pregnancy. Asking her OB/GYN which exercises they recommend for her and her baby and what kind of diet to follow will help ensure that the mother is staying active and healthy while also protecting her baby.

Prenatal care is also crucial for a healthy pregnancy. Prenatal can help identify issues early on and offer interventions, monitor fetal growth and maternal health, and establish a birth plan that is healthiest for both mother and child. As soon as a woman receives a positive pregnancy test, she should call her OB/GYN immediately to set an appointment to ensure the pregnancy is healthy. If she does not have an OB/GYN, she can look for ones in her area – and she can always switch doctors during pregnancy if it is not a good fit.

Between seeing her baby grow and move on the ultrasound, hearing his or her heartbeat, and already caring for him or her before birth as she nurtures the child and practices healthy habits, pregnancy is also a wonderful time for the mother to bond with her baby. It is the beginning of a wonderful adventure and of a lifelong love.

Fetal Development and the Pro-Life Cause

We should all be familiar with our own prenatal histories, not just because it is so fascinating, but because we might save an unborn child ourselves one day with this timeline of fetal development.

The greatest pro-life visual tools are not photos of aborted late-term preborn babies, although these are indeed very powerful images. The most powerful persuaders are the full-color, beautiful, clear ultrasounds of preborn babies peacefully floating in their mother’s wombs.

The pro-abortionists clearly know this. When pro0lifers put up posters or billboards depicting living unborn children in a public place, abortion advocates often loudly object and even vandalize the displays. Also, in those rare instances when abortion advocates debate pro-lifers, they try their best to censor such images of ultrasounds. For example, third-trimester abortionist Warren Hern has written, “We respond to all requests from schools for educational presentations concerning abortion. If the sponsors want both sides presented, however, the presentations must be made on different occasions. We insist that visual aid materials not be present by either side.”

Regarding ultrasonography, pro-abortionist Sarah Ackley said, “The anti-abortion movement can, with little effort, marshal such images as evidence of fetal personhood. With no comparably powerful set of imagery at its disposal thus far, the abortion-rights movements will likely remain on the defensive with respect to questions of fetal personhood.”⁴

Preborn children can be the pro-life movement’s most eloquent messengers – but only if we are their voice.

This article was originally published in April 2021 by Dr. Brian Clowes and was most recently updated in September 2024 by Marisa Cantu.

+ Endnotes

[1] All information on fetal development is from the most-used medical dictionary in the world: Donald Venes, M.D. (Editor). Taber’s Cyclopedic Medical Dictionary. 2021: 24^th Edition.  F.A. Davis Company.

[2] In 1963, the generally accepted definition of “abortion” was “all the measures which impair the viability of the zygote at any time between the instant of fertilization and the completion of labor” (Public Health Service leaflet No. 1066, United States Department of Health, Education and Welfare [HEW], 1963, page 27). Two years later, the definition of the beginning of pregnancy had magically changed to “the implantation of a fertilized ovum” (American College of Obstetrics and Gynecology [ACOG]. Terminology Bulletin, “Terms Used in Reference to the Fetus” [Chicago: ACOG], September 1965).

[3] The total amount of information communicated since Adam and Eve were created has been contained in about 12 billion hardback and softback volumes averaging 180 pages and 55,000 words apiece; about 500 billion separate copies of newspapers, magazines, periodicals and newsletters of every description, each averaging 45 pages and 25,000 words; and about 100 billion people having spoken an average of 7,500 words per day for an average 45‑year lifespan. This is a total of 12,331,910,000,000,000,000 words of communication transmitted in every form since human life began on Earth (give or take a few quadrillion).

[4] Warren Hern, M.D. Abortion Practice (New York City: B. Lippincott Company), 1984, page 323.