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"Fetus of about eight weeks, enclosed in the amnion," from Gray's Anatomy. Part of the placenta is shown as the lowest feature in this image, and the amnion extends above the fetus.[1]

A fetus (or foetus or fœtus) is a developing mammal or other viviparousvertebrate, after the embryonic stage and before birth. The plural is fetuses, or sometimes feti.

In humans, the fetal stage of prenatal development begins about eight weeks after fertilization, when the major structures and organ systems have formed,[2] and lasts until birth.[3]

he fetal stage begins about eight weeks after fertilization. The fetus is not as sensitive to damage from environmental exposures as the embryo was, though toxic exposures can often cause physiological abnormalities or minor congenital malformation.[7] Fetal growth can be terminated by various factors, including miscarriage, feticide committed by a third party, or induced abortion. Throughout the fetal stage, the fetus continues to be connected to the placenta by the umbilical cord, which supplies the fetus with oxygenated, nutrient-rich blood from the placenta, and conversely returns the deoxygenated, nutrient-depleted blood to the placenta. The fetus is surrounded by amniotic fluid, which (among other things) cushions against blows to the mother's abdomen, allows for easier fetal movement, promotes muscular/skeletal development, and helps protect the fetus from heat loss.

The following timeline describes some of the specific changes in fetal anatomy and physiology by fertilization age (i.e. the time elapsed since fertilization). Obstetricians often find it convenient to instead measure gestational age from two weeks earlier than fertilization, but the term “gestational age” is often ambiguous, and there is no uniform method for counting it.[8]Embryologists typically measure age from fertilization.[9] For purposes of this article, age is measured from fertilization, except as noted.

8 weeks: condition at start of fetal stage

The risk of miscarriage decreases sharply at the beginning of the fetal stage, and loss is "virtually complete by the end of the embryonic period."[11][12] At this point, all major structures, including hands, feet, head, brain, and other organs are present, but they will continue to grow, develop, and become more functional.[13] When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and weighs about 8 grams.[14] The heart is beating.[15] The fingers and toes have separated from each other,[16] and some fingerprint formation can be seen from the beginning of the fetal stage.[17][18] By the fetal stage, the embryonic tail is gone, and an atrophied embryonic tail bud remains.[19][20]

The first movements occur around this time, and those movements include sideward bendings of the head.[21] Generally speaking, fetal movement can be classified as either elicited (e.g. reflexive) or spontaneous, and spontaneous movements may be triggered by either the spine or the brain. Whether a spontaneous movement is supraspinally determined (i.e. caused by signals from the brain) can be inferred by comparison to movements of an anencephalic fetus, which has no brain.[22]

8 to 15 weeks after fertilization

The fetus makes general movements and startles from about nine weeks.[21] Movements include complex motor patterns, and localized movement of the arms and legs, together with hiccups, stretches and yawns.[21][23] The breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.[24]

At nine weeks the fetus is able to bend fingers around an object; in response to a touch on the foot, the fetus bends the legs or curls the toes to move away from an object.[25] During weeks 9-12, the face is “well-formed” and the fetus is three to six inches (152 mm) in length.[26] From weeks 9 to 12, the head makes up nearly half of the fetus' size, the fetal eyelids close and remain closed for several months, and the appearance of the genitals in males and females becomes more apparent.[26] The fetus begins drinking amniotic fluid about twelve weeks after fertilization,[27] for hydration and nutrition.[28]Tooth buds appear, the limbs are long and thin, and red blood cells are produced in the liver, however the majority of red blood cells will be made later in gestation (at 21 weeks) by bone marrow.[26] A fine hair called lanugo develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming meconium (stool). [26] Fetal skin is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of EEG movement occur in the 12th week.[29] By the end of this stage, the fetus has reached about 15 cm (6 inches).

16 to 25 weeks after fertilization

The lanugo covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development. Alveoli (air sacs) are forming in lungs. The nervous system develops enough to control some body functions. The cochlea are now developed, though the myelin sheaths in the neural portion of the auditory system will continue to develop until 18 months after birth. The respiratory system has developed to the point where gas exchange is possible. The quickening, which refers to the first maternally discernible fetal movements, are often felt during this period, although the fetus has been moving throughout the fetal stage. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 18–19 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements around 16 weeks.[30] By the end of the fifth month, the fetus is about 20 cm (8 inches).

26 to 38 weeks after fertilization

The amount of body fat rapidly increases. Lungs are not fully mature. Thalamic brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Fingernails reach the end of the fingertips. The lanugo begins to disappear, until it is gone except on the upper arms and shoulders. Small breast buds are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week. The fetus is considered full-term between weeks 35 and 40,[31] which means that the fetus is considered sufficiently developed for life outside the uterus.[32] It may be 48 to 53 cm (19 to 21 inches) in length, when born. Control of movement is limited at birth, and purposeful voluntary movements develop all the way until puberty.[33][34]

Variation in growth

There is much variation in the growth of the fetus. When fetal size is less than expected, that condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be maternal, placental, or fetal.[35]

Maternal factors include maternal weight, body mass index, nutritional state, emotional stress, toxin exposure (including tobacco, alcohol, heroin, and other drugs which can also harm the fetus in other ways), and uterineblood flow.

Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization and nutrient production.

Fetal factors include the fetus genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.[35]

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).[36] SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases risk for perinatal mortality (death shortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, immune dysfunction, neurologic abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.

Fetal pain

The subject of fetal pain and suffering is controversial. There may be an "emerging consensus among developmental neurobiologists that the establishment of thalamocortical connections" (at about 26 weeks) is a critical event with regard to fetal perception of pain.[44] Nevertheless, because pain can involve sensory, emotional and cognitive factors, it is "impossible to know" when painful experiences may become possible, even if it is known when thalamocortical connections are established.[44] Some experts have asserted that a fetus can sense pain as early as 13 ½ weeks.[45] In summary, there is good evidence that from about 26 weeks gestation the fetal brain can be considered a functional unit capable of processing noxious sensory input,[44] and pain before that point may also be possible.[46] The science of fetal pain is not completely certain, and the nature of pain remains elusive.[47]

Whether a fetus has the ability to feel pain and to suffer is part of the abortion debate.[48][49] For example, in the United States, legislation has been proposed by pro-life advocates requiring abortion providers to tell a woman that the fetus may feel pain during the abortion procedure, and that require her to accept or decline anesthesia for the fetus.[50]

Abortion Case Study

Issue: Abortion

Setting: It’s a rainy Friday afternoon in Biology on the second floor of Header High School. The classroom is full of high school seniors’ ready to graduate and move on to college. Mr. Baker the Biology teacher is beginning the lecture for the day when he is rudely interrupted.

Characters:

Mr. Baker (the teacher)

Kate (the pregnant teen)

John (the boyfriend)

Rick (student in the classroom)

Jamie (Kate’s best friend)

Class (all the other students in the classroom)

The Principal (leader of the school)

Mr. Baker:So, today we’re going to learn about mitosis so turn to page 275 in your books.

John:No! I can’t take this any longer! You can’t do that it’s an innocent child. That is not an option! For crying out loud it’s our child!

Jamie:It’s not your decision John! I told you Kate he’s not someone you want to have a child with. He’s flipping out in the middle of class screaming and shouting.

John:Listen up! I’m sick and tired of your input on our relationship. This is not a threesome Jamie.

Class: (Breaks out in laughter)

Mr. Baker: What’s going on here? I’m trying to teach a class! Leave your problems at the door.

John: I can’t. Really, I don’t want to. We must talk about this now! It’s a life and death situation!

Mr. Baker: What!?

Rick: Awesome.

Kate: Why do you want to keep it so badly? We’re just kids. We have our whole lives ahead of us. I am not prepared to be a mother at 18. I have dreams too.

Mr. Baker: Ok! I see what’s going on here. We must have order in the class! Hey! Pay attention to me! We must have……..

A Tornado Drill Alarm goes off blaring in the class. The Principal comes on the intercom.

The Principal:This is a tornado drill warning. All students must follow the instructions of your instructor. We have practiced this before as a school, don’t panic but stay alert. I will give you further instructions on when it is safe to leave your classroom with another intercom report.

Mr. Baker: Well you heard the man everyone face the walls of the classroom sit on your knees bent over with your hands clasp behind your neck and back. Go! Don’t run!

Class: (The class follows the instructions.)

Rick: Now we have all the time in the world to discuss the matters of life and death. Right, John. HAHA!

John: It’s not a joke Rick. Kate. All I want is for us to have this child together. Yes we made a mistake but we have to live with the choices we make. The bible says in Psalms chapter 139 versus 13 through 16 “For thou didst form my inward parts: Thou didst cover me in my mother's womb. I will give thanks unto thee; For I am fearfully and wonderfully made: Wonderful are thy works; And that my soul knoweth right well. My frame was not hidden from thee, When I was made in secret, And curiously wrought in the lowest parts of the earth. Thine eyes did see mine unformed substance; And in thy book they were all written, Even the days that were ordained for me, When as yet there was none of them.” So you see we can’t just abort our baby it would be an abomination. God knew us before we were fully formed in the womb. There are many scriptures that speak against it. We can do this together Kate.

Jamie: Here he goes with his Jesus stuff again. He makes himself sound so noble and honest like he really cares. He’s supposed to be a Christian. How about you tell everyone of the garbage you spoke to Kate when you said to her that you would wait to have intimate relations with her until you two we’re married. Who says that anyway? Oh! But wait. As soon as you heard Kate got a full ride scholarship to NYU things changed because you didn’t want your boo to leave you and make something of herself. You tricked her into doing it!

Rick: Oh snap you dirty dog! Nice. HAHAH!

John: That is not true.

Kate: I don’t believe in God like you do John! I also did some research on my own and one month in a pregnancy the so called baby that God knew is just a ball of cells no bigger than a kidney bean. How can something small like that feel any pain or even be alive? There are also laws that have been passed that defend my rights! I love you too John but we can wait to have children when we’re old enough. I don’t want to ruin my chances of going to NYU. I have wanted this for a very long time. If you truly loved me you would understand.

Mr. Baker: Did either of you inform your parent of this? There are laws of consent.

Jamie: Not here in Vermont. We live in one of the few states that require consent. That’s the way the cookie crumbles John. Your selfish act has been foiled! Just accept it! Kate is not going to have the baby and she’ll leave you for some New York City hunk that is not as conniving and selfish as you.

Kate: Stop it Jamie! I really haven’t made a definite decision yet!

Rick: Dude! Jamie really doesn’t like you man.

The Class: (the class giggles)

Kate: Mr. Baker what do you think?

John: I didn’t bring this up for everyone else to give there two cents. It is between me and you Kate.

Mr. Baker: I don’t know why I let this go on for so long? We shouldn’t be talking of this in class.

Kate: I really would like to know your opinion Mr. Baker.

Mr. Baker: My opinion is that…….

The intercom signals an announcement.

The Principal: I have been notified that our area is now safe and clear from any threat of a tornado. It is clear for you all to leave the classroom. Class is dismissed I hope you all have a safe afternoon.

The entire class looks at Mr. Baker in his response to the situation anxiously.