How Pregnancy Works

After roughly 40 weeks, a tiny mass of cells transforms into a newborn. See more pregnancy pictures.
Photo courtesy Anita Patterson/MorgueFile

During fertilization, the sperm undertake a long and perilous journey into the woman's body. Their quest is to cross the cervix before they die. Out of millions of sperm, only one will fertilize the egg. Once that one sperm crosses the threshold, an entirely new journey begins.

For the next 40 weeks or so, the mother's body will undergo a series of transformations as it nourishes and houses a new life. The egg will also transform -- from a tiny mass of cells into a living, breathing baby.


In this article, we'll look inside the womb to learn how the fetus develops during each trimester of the pregnancy, see how the baby finally makes its way into the world, and find out what could go wrong during pregnancy.

­­Starting a family is a big decision -- planning ahead is important. Health experts recommend that women who are trying to conceive see their doctor or Ob/Gyn (obstetrician/gynecologist) for a pre-pregnancy examination, to make sure that their body is up to the task.

During a typical exam, the doctor will:

  • Take medical history to determine whether the woman has a history of any disease (such as diabetes or high blood pressure) that might be dangerous during the pregnancy
  • Get family history and peform genetic testing to identify any hereditary disorders
  • Perform a health screening, including an examination for infections such as herpes and other STDs or urinary tract infections
  • Check to make sure the mother-to-be is up to date on all of her immunizations, especially rubella, which can cause miscarriage or birth defects

Women who are trying to conceive should watch their diet and take daily prenatal vitamins. Taking at least 400 micrograms of folic acid each day can significantly protect against birth defects. They should also stop smoking, because it can lead to low birth weight and other complications.

Next, let's review what happens during conception and the first trimester.


First Trimester: From Conception to Embryo

The fertilized egg makes the journey through the Fallopian tube to the uterus, where it will implant.

A man releases millions of sperm with each ejaculation. Once inside the woman's body, the sperm make a mad dash to find the egg. If one sperm reaches the egg and penetrates it, fertilization occurs.

Some women know immediately that they're pregnant -- call it a new mother's intuition. However, the only way to know for sure is to take a pregnancy test. All pregnancy tests look for the same thing: the presence of human chorionic gonadotropin (hCG), a hormone produced only during pregnancy.


Home pregnancy tests measure hCG in urine, while a test in the doctor's office will identify hCG in a blood sample The blood test can pick up very tiny amounts of the hormone and identify pregnancy earlier than a home pregnancy test. Still, most home tests are 97 to 99 percent accurate if taken correctly.

Then things start progressing rapidly inside the mother's womb. The sperm and egg merge to form a little single-celled organism called a zygote, which consists of the 23 chromosomes from the man's sperm and the 23 chromosomes from the female's egg. These chromosomes will determine the baby's hair color, eye color and whether the baby will be a boy or a girl.

Soon after fertilization, the zygote makes the trip through the Fallopian tubes to the uterus. During this journey, the zygote divides. Within 72 hours, it will have gone from one cell to eight cells.

Remember the zygote? This little ball of cells divides until it contains about 100 cells. Then it becomes known as a blastocyst. The inner group of cells will form the embryo. The outer group of cells forms the placenta, which will provide nourishment.

Three weeks into the pregnancy, the blastocyst implants itself into the mother's uterine wall and releases hCG. This occurs only a few days after conception. Her doctor will begin counting the 40 weeks of pregnancy from the start of her last period, although conception normally occurs about two weeks after that.

By the fifth week of the pregnancy, the brain, spinal cord, heart, and other organs begin to form. The embryo is now made up of three layers: the ectoderm, mesoderm, and endoderm. Every organ and tissue will develop out of these three layers. The ectoderm will form the nervous system and backbone; the mesoderm will form the heart and circulatory system; and the endoderm will form the lungs, gastrointestinal tract, thyroid, liver and pancreas. The placenta has already begun to form, as well as the umbilical cord, which will deliver nutrients to -- and remove wastes from -- the growing embryo.

Next, we'll look at the rest of the first trimester.


First Trimester: From Embryo to Fetus

The embryo at six weeks
Image courtesy A.D.A.M.

Even though the embryo is still the size of a sesame seed, the mother-to-be will probably start feeling the first twinges of pregnancy. Morning sickness, frequent urination, sleepiness, and food cravings or aversions are all common. Her breasts may swell and become tender. Some women start to put on weight, but others actually lose weight from morning sickness. At this point in the pregnancy, the woman will have her first prenatal Ob/Gyn visit. By the fifth week, an ultrasound may be able to pick up a heartbeat.

The mother needs to be especially careful during this first trimester, during the formation of the delicate organs. Pregnant women should avoid alcohol, certain medications, caffeine, and smoking. They should also continue to take prenatal vitamins containing folic acid, eat a healthy diet and exercise regularly.


Between 9 and 12 weeks, women over age 35 and those who have a family history of chromosomal abnormalities will probably have a chorionic villus sampling (CVS). This test detects Down syndrome and other chromosomal abnormalities. Using an ultrasound to guide him, the doctor will remove a small piece of placental tissue and test it for these conditions.

In the last few weeks of the first trimester, the embryo really starts to take shape. The facial structures begin to form and become recognizable. The neural tube, which will form the brain and spinal cord, develops. Little buds emerge and grow into arms and legs.

Around week 8, the embryo becomes a fetus. The kidneys, liver, brain, and lungs are all beginning to function. The fingers and toes are separate and the external genitalia are formed. At 12 weeks, the fetus is about three inches long and weighs about one ounce. At the end of the first trimester, many pregnant women find that their clothes are getting tight, although they may not yet be ready for maternity clothes.

In the next section, we'll discuss what happens during the second trimester.


Second Trimester: The Kicking Fetus

The fetus at 14 weeks
Image courtesy A.D.A.M.

For many women, the second trimester is a definite improvement. As the nausea recedes and exhaustion abates, many women feel better and have more energy -- and appetite -- during this period. However, some women develop other unpleasant symptoms, such as heartburn.

Other changes occur in the mother's body. Even though her fetus is just a few inches long, her belly is growing. Mammary ducts inside her breasts prepare to produce milk. During this trimester, the mother's breasts will start to produce a yellowish, nutrient-rich substance called colostrum, which will feed her baby during its first few days of life.


Inside her womb, the rapid activity continues. Although the mother may not be able to feel it yet, the fetus now kicks and moves. Its head -- which just a few weeks ago dwarfed the rest of its body -- is now more proportional. A fine hair called lanugo covers its body. The kidneys and urinary tract start to produce urine and release it into the amniotic fluid. The liver begins secreting bile, and the spleen begins to aid in the production of red blood cells.

By week 15, the fetus is around five inches long and weighs about two ounces. Although its eyelids are fused shut, its eyes are now sensitive to light.

The mother will make her second prenatal doctor visit between weeks 12 and 16 of her pregnancy. At this visit, her Ob/Gyn may perform an ultrasound to look at the baby. Sometimes the doctor can determine the sex during this visit. The doctor will probably also measure the fundus -- the distance between the top of the mother's pubic bone and the top of her uterus. This measurement will allow the doctor to track the fetus' growth throughout the pregnancy.

Tests done during this trimester include an Alpha-Fetaprotein (AFP) or triple test -- a blood test that detects levels of alpha-fetoprotein (a protein produced in the fetus's liver) to identify Down syndrome or spina bifida. If the mother is over age 35, she might also have an amniocentesis, a procedure in which the doctor inserts a thin needle into her abdomen and removes a small sample of amniotic fluid to test for birth defects and chromosomal abnormalities. Between weeks 16 and 20, the fetus begins to hear and may even be able to hear the sound of its mother's voice. It can frown, squint, and make other facial expressions. The fetus' scalp is sprouting tiny hair buds, its skeleton is hardening, and millions of tiny neurons in its brain help its muscles move. Its tiny heart pumps about 25 quarts of blood each day. Its reproductive organs are forming. If the fetus is a girl, her ovaries are beginning to produce the millions of eggs that she will possess for her entire lifetime. During this period, the mother may start to experience "quickening" -- feeling her tiny fetus kick.

The mother's body continues to undergo numerous changes. Often, pregnant women appear to "glow." Although many women are truly suffused with the joy of pregnancy, the glow is often due to increased blood flow to the face. Many women also have a dark line running from their belly button to their pubic bone. This line, called linea nigra, occurs because of an increase in skin pigmentation, or melanin. It should disappear shortly after the baby is born. Some women also develop stretch marks -- thin pink or brown lines on their belly which often fade soon after the baby is born. Another common problem is edema, swelling of the ankles and feet due to fluid retention. Severe swelling in the hands and face could be a sign of a potentially dangerous condition called pre-eclampsia, which prevents the placenta from getting enough blood.


Second Trimester: Halfway There

The fetus at 20 weeks
Image courtesy A.D.A.M.

At week 20 -- halfway through the pregnancy -- the fetus is about six inches long and weighs about 10 ounces. Its digestive system produces meconium, a black, tar-like substance that will make up its first few bowel movements. The fetus is coated in a white greasy substance called vernix caseosa, which will protect its skin from the amniotic fluid in utero.

To train its digestive system and lungs, the fetus will swallow and "breathe" amniotic fluid. Its lungs produce a substance called surfactant, which will enable the air sacs to inflate for breathing once it is born.


At this point in the pregnancy, the mother may develop acne because of her skin's increased oil production. She may also develop varicose veins, which are caused when valves malfunction and allow blood to pool in the legs. Other changes include larger breasts, skin discoloration, heartburn and constipation. Nasal congestion is common as more blood flows through the mucous membranes, and the mother's legs may cramp as her uterus puts pressure on veins. Some mothers are more prone to bladder infections due to hormonal changes.

The fetus at 24 weeks
Image courtesy A.D.A.M.

Between 24 and 28 weeks, the Ob/Gyn will perform a glucose screening test to check the mother for gestational diabetes, a form of diabetes in which the mother does not produce enough insulin. If she does have gestational diabetes, she will have to control her blood sugar with diet and may need insulin injections.

Placenta previa may also become apparent during this time. Mothers with this condition have a placenta that hangs low in the uterus and blocks the cervical opening to the birth canal. If placenta previa continues into late pregnancy, the mother will have to deliver by caesarean section.

Week 27 marks the end of the second trimester. By now, the fetus has grown to about 14 inches long, and weighs about 2 pounds. It is already starting to look like a newborn baby. If born at this point, the baby may survive, but would face potentially serious problems.

We'll look at the changes that take place during the third trimester next.


Third Trimester

The fetus at 32 weeks
mage courtesy A.D.A.M.

During the next 12 weeks, the fetus will finish its development and prepare for the difficult birth process. It is now about 15 inches long and weighs between two and three pounds. Its eyes are a definite color (although they may change after it is born) and are fringed with lashes. Its body is rounding out as fat deposits under its skin. This fat will help its body regulate temperature once it is born. Its brain is becoming larger and more defined, and its skull is growing to accommodate it. If it is a boy, his testicles are descending into his scrotum. If the fetus is a girl, her clitoris is developed.

As the mother's belly swells, she may be in more discomfort, especially near the end of her pregnancy. She might experience hemorrhoids and insomnia. The pressure of the growing uterus on her diaphragm may also make her feel short of breath. It can press down on nerves, causing pain in her lower back and legs, and constrict her bladder, making her run to the bathroom constantly. Many women feel tired and have difficulty sleeping because of their increasing girth.


Sometime during the third trimester, the muscles of the mother's uterus begin contracting. These Braxton-Hicks contractions are practice to help the mother's body prepare for labor. Frequent or painful contractions could be a sign of premature labor.

By week 32, the fetus can move its eyes back and forth and tell the difference between light and dark. It is gaining weight quickly -- about a half pound per week. At this stage in the pregnancy, the fetus weighs just over four pounds and measures 15 to 17 inches long. It may have a full head of hair as well as fingernails and toenails.

The mother's uterus is getting very crowded. Although the fetus continues to move, its movements won't be as fervent as they were earlier in the pregnancy.

At week 35, the fetus measures between 15 and 18 inches long and weighs five-and-a-half to 6 pounds. The lanugo starts to fall off. The vernix caseosa, which protected its body in the amniotic fluid, also disappears. Its organs are almost fully developed.

From this point on, the mother will probably visit her doctor every week until the baby is born. The doctor may do a culture of the mother's vagina to test for bacteria called Group B streptococci, or Group B strep. Mothers who test positive for this bacteria will get intravenous antibiotics during labor to reduce their risk of passing the infection to their baby during labor.

By this point in the pregnancy, the fetus has probably rotated so that it is head down in preparation for birth. Babies that are turned the wrong way are called breech deliveries, but there are ways of turning the baby before labor begins. As her baby drops lower in her pelvis, the mother may feel relief from the breathlessness and other symptoms that have plagued her for the last few weeks.

Although delivery is still a few weeks away, at week 37 the fetus is considered full term. This means that if it were delivered now, it would be able to function outside the womb. It should weigh at least 6 pounds and measure between 19 and 20 inches. At any time between now and the onset of labor, the mucus plug which had blocked the entrance to the mother's cervix to prevent bacteria from entering will come out. This is a sign that labor is on its way.

In the last few weeks of pregnancy, the mother's doctor will check to see how far she has "effaced" or "ripened." Effacement is the process by which the cervix softens and thins to prepare for delivery. When the mother is 100-percent effaced, she is almost ready for labor.

Week 40 officially marks the end of the pregnancy, but only a small percentage of women actually deliver on their due date -- many are either early or late. If the baby has still not been delivered by week 41 or 42, the doctor will probably induce labor.

Next, we'll find out how labor progresses.



In movies, pregnant women experience a dramatic rush of fluid as their water breaks. But in reality, very few women will have their water "break" (which is actually the breaking of the amniotic sac). Most of the time, the nurse or midwife will break the amniotic sac once labor has already begun.

Signs of labor include:


  • Contractions that increase in frequency, duration, and intensity
  • Lower back pain that doesn't go away
  • Cervical dilation (opening up), revealed during a pelvic examination

A mother has many options when it comes to giving birth. She can deliver in a hospital or birthing center with the help of an obstetrician or midwife, or at home with the help of a midwife. She can also hire a doula -- a trained professional who offers support during the birthing process. Doctors recommend that women with high-risk pregnancies deliver in a hospital, because medical facilities are best equipped to handle emergencies should they occur.

The labor process typically consists of several stages.

During early labor, the mother will feel the first contractions. Her cervix will gradually efface and dilate in preparation for delivery. The contractions will be more frequent and become more painful. For many women, the early stage of labor can last for hours, and doesn't necessarily require an immediate trip to the hospital. The contractions come more frequently during active labor. They are also more painful -- sometimes too painful for the mother to talk through. Once the contractions start coming about every five minutes for an hour, the mother should call her doctor and get ready to go to the hospital. The mother's cervix continues to ripen -- it must stretch from about three centimeters to the full 10 centimeters before delivery can begin. Once she reaches the hospital and is dilated sufficiently, the mother can have medication for her pain if she chooses to do so. Options include an epidural (anesthesia injected into the woman's spinal cord, which blocks sensation in her lower body) and an intravenous pain reliever.

In the transition stage, the cervix reaches its full dilation as contractions become stronger and even more frequent. They may come every three minutes and last up to a minute each. The baby is also descending into the birth canal in preparation for delivery. As the baby moves down, the mother may feel pressure (as if she needs to have a bowel movement), and an urge to push. This stage may last anywhere from a few minutes to a few hours. Labor typically progresses more slowly for first-time moms.

Next, we'll look at delivery.



Once her cervix is completely dilated, the mother is ready to start pushing. The combined force of her pushes and uterine contractions move the baby down the birth canal. Eventually the baby's head will make its way down to the mother's perineum (the tissue that stretches between the vagina and rectum). Pressure on the perineum can be intense, and feel like burning or stinging when the baby "crowns," or when the widest part of its head becomes visible.

A procedure called an episiotomy involves cutting the perineum to make more room for the baby and prevent the vagina from tearing. Although this procedure used to be routine, it's not done very often anymore. Sometimes the vagina tears by itself as the baby passes through.


When the baby's head finally emerges completely, the doctor or midwife will suction out its nose and mouth and check to make sure that the umbilical cord is not wrapped around its neck. With another push or two, the baby's body should be out. The doctor, nurse or midwife will dry the baby off with a towel, finish suctioning out its mouth, and clamp the umbilical cord. Sometimes doctors allow the mother's partner or another family member or friend to cut the cord.

Click on the play button to see the progression of vaginal delivery.

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Although the baby is out, labor is not quite over. The mother still needs to deliver the placenta, but it usually takes no more than one quick push. After the placenta is out, her uterus will continue to contract to seal off the blood vessels where the placenta was attached.

If the birth is not progressing well, or if either the mother or baby's health is in jeopardy, the doctor may perform a cesarean section, or c-section. An anesthesiologist numbs the mother from the waist down with an epidural or spinal anesthesia, and the doctor makes an incision in the mother's abdomen, just above the pubic bone. He opens the uterus, drains the amniotic fluid and removes the baby. Unless there are complications, the baby is treated in much the same way as he is following a vaginal delivery.

Typically after a few days in the hospital, the parents can bring home their new bundle of joy!

For more information on pregnancy and related topics, check out the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links


  • Alpha-Fetoprotein (AFP) in Blood. WebMDHealth.
  • Childbirth. Women's Health.
  • "Chorionic villus sampling." Medline Plus Medical Encyclopedia.
  • "C-section." MedLine Plus Encyclopedia.
  • During Your Pregnancy: Amniocentesis. March of Dimes.
  • Episiotomy. BabyCenter.
  • "Fetal Development: How your baby grows." BabyCenter.
  • "Fetal development: What happens during the first trimester?" Mayo Clinic.
  • Frequently Asked Questions about Infertility: American Society for Reproductive Medicine.
  • Miscarriage. March of Dimes.
  • The National Infertility Association.
  • Placenta Previa. BabyCenter.
  • Planning for Pregnancy: Ohio State University Medical Center. healthinformation/diseasesandconditions/maternity/planning/
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  • Wollcott, Trish. "Debunking Some Pregnancy Myths." Pregnancy Today.
  • Your Developing Baby: American Pregnancy Association.