Chapter 3 Flashcards
(51 cards)
The changes that transform a fertilized egg into a newborn human make up prenatal development.
Prenatal development takes an average of 38 weeks, which are divided into three stages: the period of the zygote, the period of the embryo, and the period of the fetus.
Period of the Zygote (Weeks 1-2)
begins with fertilization and lasts about 2 weeks. It ends when the fertilized egg, called a zygote, implants itself in the wall of the uterus. During these 2 weeks, the zygote grows rapidly through cell division and travels down the fallopian tube toward the uterus. Within hours, the zygote divides for the first time; then division occurs every 12 hours. Occasionally, the zygote separates into two clusters that develop into identical twins. Fraternal twins, which are more common, are created when two eggs are released and each is fertilized by a different sperm cell. After about 4 days, the zygote consists of about 100 cells, resembles a hollow ball, and is called a blastocyst
By the end of the first week, the zygote reaches the uterus. The next step is implantation: The blastocyst burrows into the uterine wall and establishes connections with the mother’s blood vessels. Implantation takes about a week to complete and triggers hormonal changes that prevent menstruation, letting the woman know she has conceived.
A small duster of cells near the center of the blastocyst, the germ disc, eventually develops into the baby.
The layer of cells closest to the uterus becomes the placenta, a structure for exchanging nutrients and wastes between the mother and the developing organism.
Implantation and differentiation of cells mark the end of the period of the zygote.
Period of the Embryo (Weeks 3-8)
After the blastocyst is completely embedded in the uterine wall, it is called an embryo. This new period typically begins the 3rd week after conception and lasts until the end of the 8th week. During the period of the embryo, body structures and internal organs develop. At the beginning of the period, three layers form in the embryo. The outer layer or ectoderm will become hair, the outer layer of skin, and the nervous system; the middle layer or mesoderm will form muscles, bones, and the circulatory system; the inner layer or endoderm will form the digestive system and the lungs.
The embryo rests in an amniotic sac, which is filled with amniotic fluid that cushions the embryo and maintains a constant temperature. The embryo is linked to the mother by two structures. The umbilical cord houses blood vessels that join the embryo to the placenta. In the placenta, the blood vessels from the umbilical cord run close to the mother’s blood vessels but aren’t actually connected to them. Instead, the blood flows through villi, finger-like projections from the umbilical blood vessels
With body structures and internal organs in place, another major milestone passes in prenatal development.
Period of the Fetus (Weeks 9-38)
The final and longest phase of prenatal development, the period of the fetus, extends from the 9th week after conception until birth.
During this period, the baby-to-be becomes much larger and its bodily systems begin to work.
At 4 weeks after conception, a flat set of cells curls to form a tube. One end of the tube swells to form the brain; the rest forms the spinal cord. By the start of the fetal period, the brain has distinct structures and has begun to regulate body functions. During the period of the fetus, all regions of the brain grow, par- ticularly the cerebral cortex, the wrinkled surface of the brain that regulates many important human behaviors.
Near the end of the embryonic period, male embryos develop testes and female embryos develop ovaries. In the 3rd month, the testes in a male fetus secrete a hormone that causes a set of cells to become a penis and scrotum; in a female fetus, this hormone is absent, so the same cells become a vagina and labia.
During the 5th and 6th months after conception, eyebrows, eyelashes, and scalp hair emerge. The skin thickens and is covered with a thick greasy substance, vernix, that protects the fetus during its long bath in amniotic fluid.
By about 6 months after conception, fetuses differ in their usual heart rates and in how much their heart rate changes in response to physiological stress.
With these and other rapid changes, by 22 to 28 weeks most systems function well enough that a fetus born at this time has a chance to survive, which is why this age range is called the age of viability. By this age, the fetus has a distinctly baby-like look
- However, babies born this early have trouble breathing because their lungs are not yet mature. Also, they cannot regulate their body temperature very well
There is fetal behaviour
Another sign of growing behavioral maturity is that the senses work.
- The fetus can hear the mother’s heart beating and can hear her food digesting. More important, the fetus can hear her speak and hear others speak to her
sensory experiences from pregnancy can have lasting effects.
FETAL BEHAVIOR
During the fetal period, the fetus actually starts to behave (Can kick)
When active, the fetus will move about once a minute
But these bursts of activity are followed by times when the fetus is still, as regular activity cycles emerge
Although movement is common in a healthy pregnancy, some fetuses are more active than others, and these differences predict infants’ behavior: An active fetus is more likely than an in- active fetus to be an unhappy, difficult baby
General Risk Factors
nutrition, stress, and a mother’s age.
NUTRITION
Most pregnant women need to increase
their intake of calories by about 10% to 20% to meet the needs of pre-natal development.
A woman should expect to gain between 25 and 35 pounds during pregnancy, assuming that her weight was normal before pregnancy.
A woman who was underweight before becoming pregnant may gain as much as 40 pounds
A woman who was over-weight should gain at least 15 pounds
Of this gain, about one-third reflects the weight of the baby, the placenta, and the fluid in the amniotic sac; another third comes from increases in a woman’s fat stores; yet another third comes from the increased volume of blood and increases in the size of her breasts and uterus
Proteins, vitamins, and minerals are essential for normal prenatal development. For example, folic acid, one of the B vitamins, is important for the nervous system to develop properly
- When mothers do not consume adequate amounts of folic acid, their babies are at risk for spina bifida, a disorder in which the embryo’s neural tube does not close properly during the 1st month of pregnancy
When a pregnant woman does not provide adequate nourishment, the infant is likely to be born prematurely and to be underweight.
Stress
When pregnant female animals experience constant stress such as repeated electric shock or intense over- crowding, their offspring are often smaller than average and prone to other physical and behavioral problems
Studies typically show that women who report greater anxiety during pregnancy more often give birth early or have babies who weigh less than average
when women are anxious throughout preg- nancy, their children are less able to pay attention as infants and more prone to behavioral problems as preschoolers
when a pregnant woman experiences stress, her body secretes hormones that reduce the flow of oxygen to the fetus while increasing its heart rate and activity level
stress can weaken a pregnant woman’s immune system, making her more susceptible to illness, which can, in turn, damage fetal development.
pregnant women under stress are more likely to smoke or drink alcohol and less likely to rest, exercise, and eat properly
MOTHER’S AGE
Traditionally, the 20s were thought to be the prime childbearing years. Teenage women as well as women who were 30 or older were considered less fit for the rigors of pregnancy.
Compared to women in their 20s, teenage women are more likely to have problems during pregnancy, labor, and delivery. This is largely because pregnant teenagers are more likely to be economically disadvantaged and do not get good prenatal care, because they are unaware of the need and wouldn’t be able to afford it if they did.
Children of teenage mothers generally do less well in school and more often have behavioral problems
Women in their 20s are twice as fertile as women in their 30s. And past 35 years of age, the risks of miscarriage and stillbirth increase rapidly. Among 40- to 45-year-olds, for example, nearly half of all pregnancies result in miscarriage.
What’s more, women in their 40s are more liable to give birth to babies
with Down syndrome.
social selection and social influence
According to the second mechanism, called social selection, some teenage girls are more likely than others to become pregnant, and those same factors that cause girls to become pregnant may put their children at risk.
One mechanism, called social influence, refers to events set in motion when a teenage girl gives birth, events that make it harder for her to provide an environment that’s positive for her child’s development.
Teratogens: Diseases, Drugs, and Environmental Hazards
In the late 1950s, many pregnant women in Germany took thalidomide, a drug to help them sleep. Soon, however, came reports that many of these women were giving birth to babies with deformed arms, legs, hands, or fingers. Thalidomide was a powerful teratogen, an agent that causes abnormal prenatal development. Ultimately, more than 10,000 babies worldwide were harmed before thalidomide was withdrawn from the market
three primary types of teratogens: diseases, drugs, and environmental hazards
DISEASES
Sometimes women become ill while pregnant. Most diseases, such as colds and many strains of flu, do not affect the developing organism. However, several bacterial and viral infections can be very harmful and, in some cases, fatal to the embryo or fetus
Some of these diseases pass from the mother through the placenta to attack the embryo or fetus directly. They include cytomegalovirus (a type of herpes), rubella, and syphilis. Other diseases attack at birth: The virus is present in the lining of the birth canal, and the baby is infected as it passes through to be born. Genital herpes is transmitted this way. AIDS is transmitted both ways - through the placenta and during passage through the birth canal.
Disease - Potential Consequences
AIDS
- Frequent infections, neurological disorders, death
Cytomegalovirus
- Deafness, blindness, abnormally small head, mental retardation
Genital herpes
- Encephalitis, enlarged spleen, improper blood clotting
Rubella (German measles)
- Mental retardation; damage to eyes, ears, and heart
Syphilis
- Damage to the central nervous system, teeth, and bones
DRUGS
Notice that most of the drugs in the list are substances that you may use routinely - alcohol, aspirin, caffeine, and nicotine. Nevertheless, when consumed by pregnant women, they present special dangers
Cigarette smoking is typical of the potential harm from teratogenic drugs. The nicotine in cigarette smoke constricts blood vessels and thus reduces the oxygen and nutrients that can reach the fetus through the placenta. Therefore, pregnant women who smoke are more likely to miscarry (abort the fetus spontaneously) and to bear children who are smaller than average at birth
- Finally, even secondhand smoke harms the fetus: When pregnant women don’t smoke but fathers do, babies tend to be smaller at birth
Alcohol also carries serious risk. Pregnant women who consume large quantities of alcoholic beverages often give birth to babies with fetal alcohol syndrome (FAS). Children with FAS usually grow more slowly than normal and have heart problems and misshapen faces.
- youngsters with FAS often have a small head, a thin upper lip, a short nose, and widely spaced eyes. FAS is the leading cause of mental retardation in the United States, and children with FAS have serious attentional, cognitive, and behavioral problems
When women drink moderately throughout pregnancy, their children are often afflicted with alcohol-related neurodevelopmental disorder (ARND). Children with ARND are normal in appearance but have deficits in attention, memory, and intelligence
Alcohol
- Fetal alcohol syndrome, cognitive deficits, heart damage, retarded growth
Aspirin
- Deficits in intelligence, attention, and motor skills
Caffeine
- Lower birth weight, decreased muscle tone
Cocaine and heroin
- Retarded growth, irritability in newborns
Marijuana
- Lower birth weight, less motor control
Nicotine
- Retarded growth, possible cognitive impairments
ENVIRONMENTAL HAZARDS
As a by-product of life in an industrialized world, people are often exposed to toxins in food they eat, fluids they drink, and air they breathe. Chemicals associated with industrial waste are the most common environmental teratogens, and the quantities involved are usually minute.
Polychlorinated biphenyls (PCBs) illustrate the danger of environmental ter- atogens. These were used in electrical transformers and paints, until the U.S. government banned them in the 1970s. However, like many industrial by-products, they seeped into the waterways, where they contaminated fish and wildlife. The amount of PCBs in a typical contaminated fish does not affect adults, but when pregnant women ate large numbers of PCB-contaminated fish, their children’s cognitive skills and reading achievement were impaired
major studies have examined the impact of exposure to the electromagnetic fields that are generated by VDTs, and they have found no negative results.
Environmental teratogens are treacherous because people are often unaware of their presence in the environment.
Try to avoid convenience foods, which often contain many chemical additives. Stay away from air that’s been contaminated by household products such as cleansers, paint strippers, and fertilizers. Women in jobs that require contact with potential teratogens (e.g., housecleaners, hairdressers) should switch to less potent chemicals.
Lead
- Mental retardation
Mercury
- Retarded growth, mental retardation, cerebral palsy
PCBs
- Impaired memory and verbal skills
X-rays
- Retarded growth, leukemia, mental retardation
How Teratogens Influence Prenatal Development
- The impact of a teratogen depends on the genotype of the organism.
- Thalidomide was harmless to rats and rabbits but not to people.
- some women who took thalidomide gave birth to babies with normal limbs, yet others who took comparable doses at the same time in their pregnancies gave birth to babies with deformities. - The impact of teratogens changes over the course of prenatal development.
- During the period of the zygote, exposure to teratogens usually results in spontaneous abortion of the fertilized egg
- During the embryonic period, exposure produces major defects in body structure.
- During the fetal period, exposure to teratogens either produces minor defects in body structure or causes body systems to function improperly. - Each teratogen affects a specific aspect (or aspects) of prenatal development.
- teratogens do not harm all body systems; instead, damage is selective. - The impact of teratogens depends on the dose.
- Damage from teratogens is not always evident at birth but may appear later in life.
Prenatal Diagnosis and Treatment
A counselor constructs a fam- ily tree for each prospective parent to check for heritable disorders. If it turns out that one (or both) carries a disorder, further tests can determine the person’s genotype. With this more detailed information, a genetic counselor can discuss choices with the prospective parents. They may choose to go ahead and conceive “naturally;’ taking their chances that the child will be healthy. Or they could decide to use sperm or eggs from other people. Yet another choice would be to adopt a child.
A standard part of prenatal care in North America is ultrasound, a procedure using sound waves to generate a picture of the fetus.
Ultrasound can be used as early as 4 or 5 weeks after conception; before this time the fetus is not large enough to generate an interpretable image. Ultrasound pictures are useful for determining the date of conception, which enables the physician to predict the due date more accurately. Ultrasound pictures are also valuable in showing the position of the fetus and placenta in the uterus, and they can be used to identify gross physical deformities, such as abnormal growth of the head.
When a genetic disorder is suspected, two other techniques are particularly valuable because they provide a sample
of fetal cells that can be analyzed.
In amniocentesis, a needle is inserted through the mother’s abdomen to obtain a sample of the amniotic fluid that surrounds the fetus. Amniocentesis is typically performed at approximately 16 weeks after conception. The fluid contains skin cells that can be grown in a laboratory dish and then analyzed to determine the genotype of the fetus.
In chorionic villus sampling (CVS), a sample of tissue is obtained from the chorion (a part of the placenta) and analyzed. CVS is often preferred over amniocentesis because it can be done about 10 to 12 weeks after conception, nearly 4 to 6 weeks earlier than amniocentesis.
Results are returned from the lab in about 2 weeks following amniocentesis and in 7 to 10 days following CVS.
With samples obtained from either amniocentesis or CVS, about 200 different genetic disorders can be detected.
These procedures are virtually error-free, but they have a price: Miscarriages are slightly more likely after amniocentesis or CVS
A whole new field called fetal medicine is concerned with treating prenatal problems before birth.
- One approach is to treat disorders medically, by administering drugs or hormones to the fetus.
- Another way to correct prenatal problems is fetal surgery. Surgeons cut through the mother’s abdominal wall to expose the fetus
- Yet another approach to treating prenatal problems is genetic engineering - replacing defective genes with synthetic normal genes.
Labor and Delivery
a woman goes into labor about 38 weeks after conception. The timing of labor depends on the flow of hormonal signals between the placenta and the brain and adrenal glands of the fetus. When estrogen and other hormones reach critical levels, the muscles in the uterus begin to contract, the first sign of labor
The first stage begins when the muscles of the uterus start to contract. These contractions force amniotic fluid up against the cervix, the opening at the bottom of the uterus that is the entryway to the birth canal. The wavelike motion of the amniotic fluid with each contraction causes the cervix to enlarge gradually.
In the early phase ofStage 1, the contractions are weak and spaced irregularly.
By the end of the early phase, the cervix is
about 5 centimeters (2 inches) in diameter. In the late phase of Stage 1, contractions are stronger and occur at regular intervals. By the end of the late phase, the cervix is about 7 to 8 centimeters (3 inches) in diameter. In the transition phase of Stage 1, contractions are intense and sometimes occur without interruption. Women report that the transition phase is the most painful part of labor. At the end of transition, the cervix is about 10 centimeters (4 inches) in diameter.
- Stage 1 lasts from 12 to 24 hours for the birth of a first child, and most of the time is spent in the relative tranquility of the early phase. Stage 1 is usually shorter for subsequent births, with 3 to 8 hours being common.
When the cervix is fully enlarged, the second stage oflabor begins. Most women feel a strong urge to push the baby out, using their abdominal muscles. This pushing, along with uterine contractions, propels the baby down the birth canal. Soon the top of the baby’s head appears, an event known as crowning. In about an hour for first births and less for later births, the baby passes through the birth canal and emerges from the mother’s body. Most babies arrive head first, but a small percentage come out feet or bottom first, which is known as a breech presentation.
With the baby born, you might think that labor is over, but it’s not. There is a third stage, in which the placenta (also called, appropriately, the afterbirth) is expelled from the uterus. The placenta becomes detached from the wall of the uterus and con- tractions force it out through the birth canal. This stage is quite brief, typically lasting 10 to 15 minutes.
Approaches to Childbirth
Birth is more likely to be problem free and rewarding when mothers and fathers understand what’s happening during pregnancy, labor, and delivery
- prepared childbirth means going to classes to learn basic facts about pregnancy and childbirth
Natural methods of dealing with pain are emphasized over medication. Why? When a woman is anesthetized either with general anesthesia or regional anesthesia (in which only the lower body is numbed), she can’t use her abdominal muscles to help push the baby through the birth canal. Without this pushing, the obstetrician may have to use mechanical devices to pull the baby through the birth canal, which involves some risk. Also, drugs that reduce the pain of childbirth cross the placenta and can affect the baby.
- Consequently, when a woman receives large doses of pain-relieving medication, her baby is often withdrawn or irritable for days or even weeks
- Relaxation is the key to reducing birth pain without drugs. Because pain often feels greater when a person is tense, pregnant women learn to relax during labor by deep breathing
Involve a supportive “coach.” The father-to-be, a relative, or close friend attends childbirth classes with the mother-to-be. The coach learns the techniques for coping with pain and, like the men in the top photo, practices them with the preg- nant woman. During labor and delivery, the coach is present to help the woman use the techniques she has learned and to offer support and encouragement. Sometimes the coach is accompanied by a doula, a person familiar with childbirth who is not part of the medical staff but instead provides emotional and physical support throughout labor and delivery.
Birth need not always take place in a hospital. Nearly all babies in the United States are born in hospitals; only 1% are born at home
- Yet around the world-in Europe, South America, and Asia-many children are born at home, reflecting a cultural view that the best place to welcome a new family member is at home, surrounded by family members.
Freestanding birth center. Birthing centers are typically small, independent clinics. A woman, her coach, and other family members and friends are assigned a birthing room that is often decorated to look homelike rather than institutional. A doctor or nurse-midwife assists in labor and delivery, which takes place entirely in the birthing room, where it can be observed by all. Like home deliveries, birthing centers are best for deliveries that are expected to be trouble-free.
Adjusting to Parenthood
A woman experiences many physical changes after birth. Her breasts begin to produce milk and her uterus gradually becomes smaller, returning to its normal size in 5 or 6 weeks. And levels of female hormones (e.g., estrogen) drop.
contact in the first few days after birth-although beneficial for babies and pleasurable for babies and parents alike-is not essential for normal development
half of all new mothers find that their initial excitement gives way to irritation, resentment, and crying spells-the so-called “baby blues:’ These feelings usually last a week or two and probably reflect both the stress of caring for a new baby and the physiological changes that take place as a woman’s body returns to a nonpregnant state
For 10% to 15% of new mothers, however, irritability continues for months and is often accompanied by feelings of low self-worth, disturbed sleep, poor appetite, and apathy - a condition known as postpartum depression. Postpartum depression does not strike randomly. Biology contributes: Particularly high levels of hormones during the later phases of pregnancy place women at risk for postpartum depression (Harris et aI., 1994). Experience also contributes: Women are more likely to experience postpartum depression when they were depressed before pregnancy, are coping with other life stresses (e.g., death of a loved one or moving to a new residence), did not plan to become pregnant, and lack other adults (e.g., the father) to support their adjustment to motherhood
- children of depressed mothers are more likely to become depressed themselves and are also at risk for other behavior problem
Birth Complications
Cephalopelvic disproportion
- The infant’s head is larger than the pelvis, making it impossible for the baby to pass through the birth canal.
Irregular position
- In shoulder presentation, the baby is lying crosswise in the uterus and the shoulder appears first; in breech presentation, the buttocks appear first.
Preeclampsia
- A pregnant woman has high blood pressure, protein in her urine, and swelling in her extremities (due to fluid retention).
Prolapsed umbilical cord
- The umbilical cord precedes the baby through the birth canal and is squeezed shut, cutting off oxygen to the baby.
Some of these complications, such as a prolapsed umbilical cord, are dangerous because they can disrupt the flow of blood through the umbilical cord. If this flow of blood is disrupted, infants do not receive adequate oxygen, a condition known as hypoxia.
PREMATURITY AND LOW BIRTH WEIGHT.
- Normally, gestation takes 38 weeks from conception to birth. Premature infants are born less than 38 weeks after conception. Small-for-date infants are substantially smaller than would be expected based on the length of time since conception.
Assessing the Newborn
The Apgar score, a measure devised by Virginia Apgar, is used to evaluate the newborn baby’s condition. Health professionals look for five vital signs including breathing, heartbeat, muscle tone, presence of reflexes (e.g., coughing), and skin tone.
- Each of the five vital signs receives a score of 0, 1, or 2, with 2 being optimal.
- The five scores are added together, with a score of 7 or more indicating a baby in good physical condition.
- A score of 4 to 6 means the newborn will need special attention and care.
- A score of 3 or less signals a life-threatening situation that requires emergency medical care
- The Apgar score provides a quick, approximate assessment of the newborn’s status by focusing on the body systems needed to sustain life.
For a comprehensive evaluation ofthe newborn’s well-being, pediatricians and child-development specialists use the Neonatal Behavioral Assessment Scale, or NBAS
- The NBAS is used with newborns to 2-month-olds to provide a detailed portrait of the baby’s behavioral repertoire. The scale includes 28 behavioral items along with 18 items that test reflexes
- The baby’s performance is used to evaluate function- ing of four systems:
* Autonomic. The newborn’s ability to control body functions such as breath- ing and temperature regulation
* Motor.The newborn’s ability to control body movements and activity level
* State. The newborn’s ability to maintain a state (e.g., staying alert or staying asleep)
* Social. The newborn’s ability to interact with people
The Newborn’s Reflexes
An important part of this preparation is a rich set of reflexes, unlearned responses that are triggered by a specific form of stimulation.
Some reflexes pave the way for newborns to get the nutrients they need to grow: Rooting and sucking ensure that the newborn is well prepared to begin a new diet of life-sustaining milk. Other reflexes protect the newborn from danger in the environment. The blink and withdrawal reflexes, for example, help newborns avoid unpleas- ant stimulation.
Yet other reflexes serve as the foundation for larger, voluntary patterns of motor activity.
Reflexes indicate whether the newborn’s nervous system is working properly. For example, infants with damage to their sciatic nerve, which is found in the spinal cord, do not show the withdrawal reflex; and infants who have problems with the lower part of the spine do not show the Babinski reflex.
Newborn States
Newborns spend most of their day alternating among four states
- Alert inactivity. The baby is calm with eyes open and attentive; the baby looks as if he is deliberately inspecting his environment.
- Waking activity. The baby’s eyes are open, but they seem unfocused; the baby moves her arms or legs in bursts of uncoordinated motion.
- Crying. The baby cries vigorously, usually accompanying this with agitated but uncoordinated motion.
- Sleeping. The baby’s eyes are closed and the baby drifts back and forth from periods of regular breathing and stillness to periods of irregular breathing and gentle arm and leg motion.
CRYING
Newborns spend 2 to 3 hours each day crying or on the verge of crying.
A basic cry starts softly, then gradually becomes more intense and usually occurs when a baby is hungry or tired; a mad cry is a more intense version of a basic cry; and a pain cry begins with a sudden, long burst of crying, followed by a long pause and gasping.
Another useful technique is swaddling, in which an infant is wrapped tightly in a blanket. Swaddling, shown in the photo, is used in many cultures around the world, including Turkey and Peru as well as countries in Asia. Swaddling provides warmth and tactile stimulation that usually works well to soothe a baby
