Prenatal development involves all of the changes that take place from fertilization until birth. Each week of a woman’s pregnancy, new milestones are achieved in development as the fetus begins to take shape.

During the first trimester the basic blueprint of human life – contained in the 46 chromosomes (23 from each partner) that determine a child’s traits – is put into action. A zygote formed after fertilization becomes a blastocyte, which then becomes an embryo composed of three layers: The ectoderm (where the brain, spinal cord and nerves, and backbone eventually develop); the mesoderm (where the heart, circulatory system, bones, muscles, kidneys and much of the reproductive system begins to develop); and the entoderm (where the lungs, intestines and bladder eventually develop).

Later during the first trimester, the embryo becomes a fetus. By the end of the first trimester, the fetus has the basics of human anatomy.

During the second trimester, the fetus’ growth rate accelerates significantly. The skin begins to form and fat stores develop. Sometime around the 19^th week of pregnancy, the mother should be able to feel the fetus’ movements. This is known as “quickening.” By the end of the 23^rd or 24^th week, some fetuses are potentially capable of surviving outside the womb, but only at very great risk.

The third trimester is the period when weight triples and length doubles as stores of protein, fat, iron and calcium increase substantially. The fetus’ movements may become constricted as space in the uterus begins to run out. Toward the end of the trimester, the fetus usually moves to a head-down position with legs tucked into the chest and knees against the nose. The fetus also will begin to move lower under the pelvis, a process known as “lightening.”

Prenatal development involves all of the changes that take place from the time of conception until birth. After conception, a zygote forms that rapidly changes into a blastocyte, embryo and finally a fetus. Every week of a pregnancy, the developing fetus undergoes dramatic changes as the body forms and takes shape.

During the prenatal development, certain milestones occur every week. Experts differ on exactly when these take place. For instance, some experts contend that the fetal period begins in the eighth week of pregnancy, whereas others contend that it starts in the ninth, 11^th or 12^th weeks. As a result, the timeframe used should serve as a rough estimate rather than an exact schedule of how prenatal development will unfold.

Though many pregnancies proceed smoothly, death or disease can occur at any time during prenatal development. About 25 percent of pregnancies end in a spontaneous abortion, according to the American Society for Reproductive Medicine. This typically occurs during the first trimester and is most often the result of problems related to chromosomes or some other abnormality.

Factors that can damage the fetus’ health include:

• Maternal use of alcohol and certain prescription and recreational drugs
  
  
• Maternal infection (including rubella and cytomegalovirus)
  
  
• Radiation from x-rays or radiation therapy
  
  
• Nutritional deficiencies

After fertilization, and even beforehand, the expectant mother can take steps to ensure normal development. These include taking prenatal vitamins and folic acid supplements, exercising regularly and maintaining a healthy weight, eating healthy meals, and trying to manage stress and health conditions. Pregnant women should also avoid smoking, drinking alcohol, using recreational drugs or taking certain over-the-counter or prescription medications.

The first trimester is a period of rapid growth in which the fetus’ major external features begin to take shape. Each week of the first trimester involves some significant new milestone.

The first trimester typically unfolds as follows:

• Week one: Conception typically occurs about two weeks after a woman’s period begins, with the due date following 40 weeks after the beginning of the woman’s last period. However, fertilization may occur at other times during the menstrual cycle depending on when the individual woman ovulated.
  
  
• Week two. A zygote is formed when the sperm and the egg come together in a woman’s fallopian tube. A zygote contains 23 chromosomes from each partner, for a total of 46. Chromosomes are the genetic information – also known as DNA – that determines a person’s traits (e.g., gender, eye and hair color, height). In most cases, just one zygote is formed, but multiple zygotes may develop. The zygote moves down the fallopian tube and toward the uterus. As it does so, it divides into a ball of cells.
  
  
• Week three: Further cell division creates an inner cluster of cells that has an outer shell. This combination is known as a blastocyst. The inner group of cells eventually will become an embryo while the outer group of cells becomes the membranes that nourish and protect the developing embryo. The blastocyst continues toward the uterus. Once the blastocyst reaches the uterus, it implants into the lining of the uterus known as the endometrium. The blastocyst receives nourishment from the mother’s bloodstream. The placenta, amniotic sac and umbilical cord begin to form. These structures provide nourishment and oxygen to the embryo and carry away waste.
  
  
• Week four: The embryo’s organs begin to form at this point, known as the embryonic period. The embryo becomes three layers. The neural tube that eventually will become the central nervous system (CNS) forms in the top layer of the embryo (ectoderm). It will house the brain, spinal cord and nerves, and backbone. A neural crest also forms, which will become the peripheral nervous system. The heart, circulatory system, bones, muscles, kidneys and much of the reproductive system begin to form in the middle layer of the embryo (mesoderm). The lungs, intestines and bladder develop in the inner layer of the embryo (endoderm).
  
  
• Week five: The heart and circulatory system continue to take shape, and the embryo’s heart begins to beat as the blood vessels come together. At this point, only ultrasound can detect the heartbeat. Circulation begins. The second month of development is a crucial time, as the embryo is especially vulnerable to damage from drugs, viruses or environmental factors (such as pesticides). The embryo now is 1/17 of an inch (.15 centimeters) long and more closely resembles a tadpole than a human at this stage of development.
  
  
• Week six: The embryo’s heart begins to beat with a regular rhythm, and facial features begin to take shape. These include an opening for the mouth and inner ear passages. The digestive and respiratory systems begin to form, and the beginnings of the embryo’s connective tissue, ribs and muscles emerge. Arms and legs appear as small buds. By this point, the embryo is about 1/8 inch (0.32 centimeters) long.
  
  
• Week seven: The umbilical cord has formed by this week. Though the embryo’s skull remains transparent, the passageway needed to circulate spinal fluid in the brain is already intact. The arms and face begin to be more clearly defined. By the end of the seventh week, virtually all essential organs have begun formation.
  
  
• Week eight: Webbed fingers and toes are present, as are clearly defined wrists, elbows, ankles, ears, upper lips and tip of the nose. The eyelids begin to form, and the embryo’s heart rate is about 150 beats per minute. By the end of the second month, all of the major body organs and systems have begun to develop.
  
  
• Week nine: Although expert opinions vary as to the exact timing, the ninth week can be seen as the approximate time when the embryo becomes a fetus. The fetus begins to take on more of a human appearance as the tail at the bottom of the spinal cord begins to disappear. Organs that have developed include the anus, bile ducts, gallbladder and pancreas. Nipples, hair follicles and internal reproductive organs (testes and ovaries) begin to form. The fetus sometimes begins moving at this point, although the mother cannot feel these movements.
  
  
• Week 10: The fetus’ tail has disappeared, as has the webbing between the fingers and toes. The skeleton begins to form at this point, and the brain begins to form neurons at the rate of 250,000 per minute. The eyelids lose their transparency, outer ears begin to assume their final form, and tooth buds first appear. Male fetuses begin to produce the hormone testosterone from their testes, which are still located in the abdomen at this stage.
  
  
• Week 11: Blood vessels in the placenta grow larger and begin to multiply to provide the fetus with the nutrients it needs. The ears move up toward the side of the head, and external genitalia (penis and labia majora) can be distinguished in an ultrasound at this point.
  
  
• Week 12: Fingernails and toenails develop, while the chin and nose become more defined. A fetus averages nearly 3 inches (7.62 centimeters) long at this point and weighs 4/5 ounce.

During the second trimester, the fetus’ growth rate accelerates significantly. The expectant mother will begin to appear more pregnant during the second trimester. She should continue the health regimen she established in the first trimester, with some exceptions. She should stop taking folic acid supplements and may have to modify her exercise regimen. In addition, women should avoid taking any type of medication unless a physician approves it.

The second trimester usually unfolds as follows:

• Week 13: The fetus begins to flex its arms and kick its legs, although the expectant mother may not be able to feel this. The fetus may even begin to suck its thumb. The eyelids become fused together to protect the eyes as they develop, and bone cells begin to develop around the baby’s head and within the arms and legs. The ribs begin to become visible on ultrasound.
  
  
• Week 14: The roof of the mouth completes formation. In boys, the prostate gland develops. In girls, the ovaries shift from the abdomen to the pelvis. The fetus’ intestinal tract creates meconium, which is the material of the baby’s first bowel movement following birth.
  
  
• Week 15: Nearly transparent skin forms, and eyebrows and scalp hair begin to emerge. The hair follicles begin producing pigment in children who will have dark hair. Bone and marrow continue to develop, and the eyes and ears continue to take on a more human appearance. The ears, which have gradually been moving toward the side of the head for several weeks, are nearly in their final destination.
  
  
• Week 16: The fetus’ eyes become sensitive to light, and facial muscles allow facial expressions such as squinting and frowning. The fetus can make a fist at this point, and may begin to hiccup frequently. For girls, the eggs form in the ovaries.
  
  
• Week 17: Fat stores begin to develop. They keep the baby warm after birth and provide energy.
  
  
• Week 18: The fetus may begin to hear at this point, detecting sounds such as the mother’s heartbeat, stomach rumbles or the sound of blood moving through the umbilical cord. The fetus also can swallow by this point.
  
  
• Week 19: A pasty, white coating called vernix protects the baby’s skin. Underneath the vernix, fine hair called lanugo covers the fetus’s body. The kidneys produce urine, which is excreted into the amniotic sac protecting the fetus. Hearing continues to develop. The baby can soon make reflexive muscle movements due to all of the motor neurons developing in the brain. The mother should be able to feel movement – also known as “quickening” – soon. Once quickening is felt, it should be reported to the physician. Knowing the starting time of quickening helps determine the baby’s due date.
  
  
• Week 20: The fetus’ skin continues to thicken and develop layers. Thin eyebrows and scalp hair are apparent on ultrasound. The arms and legs are well-developed. Week 20 marks the halfway point of the pregnancy. The fetus is about 6 inches long (15.24 centimeters) and weighs 9 ounces (255 grams). The fetus’ heartbeat can be heard with a stethoscope.
  
  
• Week 21: The fetus begins to develop new sources of nourishment aside from the placenta. For example, swallowing amniotic fluid provides small amounts of glucose (blood sugar). Bone marrow begins to make blood cells, which previously had been accomplished by the liver and spleen.
  
  
• Week 22: Taste buds begin to form, and nerve endings provide the fetus with the ability to process the feeling of touch. The fetus may begin to test this sensation by feeling around for things. In boys, testes begin to descend from the abdomen, while the uterus and ovaries in girls are already in place.
  
  
• Week 23: The lungs begin to produce surfactant, which allows air sacs (alveoli) in the lungs to inflate and keeps the sacs from sticking together when they deflate. The skin becomes less transparent and the fetus starts to add fat at an increased rate. By the 23^rd week, the fetus is potentially capable of surviving outside the womb. However, there are substantial health threats for babies born at this point, including bleeding in the brain, vision impairment and lung damage.
  
  
• Week 24: Footprints and fingerprints begin to form, and the fetus develops a sense of balance as the inner ear becomes fully developed.
  
  
• Week 25: The fetus’ hands are fully developed, and the fetus may begin to explore structures inside the uterus.
  
  
• Week 26: Eyebrows and eyelashes are well-formed, and the fetus has increasing amounts of hair. The eyes are fully developed but may not open for another couple of weeks.
  
  
• Week 27: The lungs, liver and immune system continue to evolve.

During the third trimester, the fetus continues to grow in preparation for birth. Weight triples and length doubles during this period as stores of protein, fat, iron and calcium increase substantially. Pregnant women should follow the same list of precautions as they did in the second trimester. It is likely that they will spend a greater amount of time visiting their healthcare provider during this trimester.

The third trimester unfolds as follows:

• Week 28: The eyes begin to open and close. The fetus sleeps for 20 to 30 minutes at a time. By this point, the average fetus is 15 inches (38.10 centimeters) long and weighs about 2 to 3 pounds (0.91 or 1.36 kilograms).
  
  
• Week 29: The fetus begins to store minerals such as iron, calcium and phosphorus. Bones are formed but soft at this point. The fetus may make more vigorous movements that occasionally may feel uncomfortable for the expectant mother.
  
  
• Week 30: The fetus practices breathing by moving the diaphragm rhythmically. When the fetus gets the hiccups, the expectant mother may feel small spasms of the uterus. The average fetus weighs about 3 pounds (1.36 kilograms) and will add a half-pound (0.23 kilograms) a week through the 37^th week.
  
  
• Week 31. Testicles in boys move through the groin toward the scrotum. The clitoris is prominent in girls. The lungs are better developed but are still not fully mature, and a fetus born at this point cannot breathe without the assistance of a ventilator.
  
  
• Week 32: The lanugo begins to be shed from the baby’s skin. The fetus’ movements may become constricted as space in the uterus begins to run out. However, a physician should be informed if the mother notices fewer than 10 movements in a two-hour period.
  
  
• Week 33: The pupils in the eyes are able to constrict, dilate and detect light. Lungs are more completely developed.
  
  
• Week 34: The vernix that coats and protects the skin begins to thicken. The lanugo has nearly disappeared.
  
  
• Week 35: The fetus continues to grow at a rapid pace. The punches and kicks that formerly were felt in the uterus have been replaced by stretches and wiggles.
  
  
• Week 36: The face begins to round out because of increased fat deposits. The fetus usually begins to descend in a head-down position with legs tucked into the chest and knees against the nose. This prepares the fetus for birth. The fetus also will begin to move lower under the pelvis, a process known as “lightening.” At this point, the average fetus is between 16 and 19 inches (40.64 to 48.26 centimeters) long and weighs about 6 to 6.5 pounds (2.95 kilograms).
  
  The obstetrician or other practitioner can usually determine the fetal position with a physical examination of the woman’s abdomen. Tests such as ultrasound may be used as confirmation. Sometimes the fetus is in an atypical position, such as a head-up breech presentation. Manual techniques such as external version may in some cases be recommended to try to move a fetus from the breech to head-down position. If the fetus is not in a head-down position near term, a Caesarean section may be needed for delivery.
  
  
• Week 37: Fat deposits continue to accumulate, giving the fetus a rounder look. A baby born after week 37 is considered to be full-term.
  
  
• Week 38: The fetus’ brain and nervous system continue to develop. The average fetus weighs about 7 pounds (3.18 kilograms).
  
  
• Week 39: The fetus now has enough fat under the skin to keep a constant body temperature.
  
  
• Week 40. This is the average gestation period in humans. However, only a small percentage of women deliver on this date. Many deliveries occur a week before or a week after the due date. (According to new analysis by the March of Dimes, the typical U.S. pregnancy is 39 weeks.)  

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to prenatal development:

1. What steps can I take to help my fetus develop normally?
   
   
2. Should I modify my diet or exercise regimen during my pregnancy?
   
   
3. What medications and supplements are safe me to take during pregnancy? Which should I avoid?
   
   
4. What behaviors should I avoid during my pregnancy?
   
   
5. Will it be obvious to me when I first feel the baby moving?
   
   
6. Will these movements cause me any pain?
   
   
7. What signs or symptoms might indicate that my baby is not developing normally?
   
   
8. How soon will I find out the gender of my baby?
   
   
9. How often will I need to consult you to help ensure the baby’s normal development?
   
   
10. Will I feel anything significant when the baby turns prior to delivery?
    
    
11. What are the options if my baby does not move to the normal head-down position?
    
    
12. What signs indicate that my baby is about to be born?