Introduction

• Four themes are emphasized regarding the prenatal period of development:
1. Genes are only one part of a complex developmental system.
2. Development involves differentiation, the process by which parts of an organism progressively take on specialized forms and functions.
3. Development involves repeated reorganization and qualitative change.
4. New structures and capacities emerge in an orderly way from those that existed before.

Genetic Processes

• Genes are segments of DNA that contain the code for producing a particular protein.
• Chromosomes, threadlike structures in the nucleus of cells, are composed of long molecules of DNA.
• A single chromosome may have as many as 20,000 genes.
• Mechanisms of cell division
1. Body cells, called somatic cells, contain 23 pairs of chromosomes.
2. These cells were formed by means of cell division called mitosis:
• DNA in the original cell duplicates itself, producing double-stranded chromosomes
• Double-stranded chromosomes line up along the cell's center; each one splits into two single-stranded chromosomes.
• Single-stranded chromosomes separate and move to opposite sides of the cell.
• The original cell divides down the middle, producing two new cells that are exact copies of the original, with 46 chromosomes.
3. Two chromosomes in a pair are homologues, similar in size and shape, containing the same type of genes. One member of this pair is from the sperm, the other is from the egg. Germ cells (from which egg and sperm are produced) contain 23 chromosomes.
4. Germ cells divide by a process called meiosis:
• The DNA in the germ cells duplicates itself.
• The homologous chromosomes arrange themselves in pairs, one from each parent.
• These chromosomes then separate and move to opposite ends of the cell.
• The cell divides; each of the two resulting cells has 23 double-stranded chromosomes (first meiotic division).
• The double-stranded chromosomes in these two cells split
• These cells divide, resulting in four cells, each with a single set of 23 chromosomes (second meiotic division). For males, all four cells become mature sperm (gamete). For females, only one becomes a mature egg (gamete).
5. During the second meiotic division, crossing over occurs, producing new genetic combinations (homologues exchange genes), and random assortment of mother's or father's chromosomes may occur when they move to opposite ends of the cell. Thus, the chance that a couple will produce two identical children from separate sperm-egg combinations is 1 in 64 trillion.
• How Genes Influence Development
1. The environment can influence how a gene is expressed. Genes also are influenced by one's developmental history.
2. The Interaction of Genes and Environment: The case for Physical Gender Development
• Gender is shaped by the sex chromosomes (the 23^rd pair). Two X chromosomes result in a female, one X and one Y chromosome result in a male. Y chromosomes contain less genetic material.
• Differentiation into testes or ovaries occurs during week seven after conception, its critical period.
• After the critical period, gender development is shaped by hormones.
• Testes secrete male sex hormones (androgens), causing differentiation of the male reproductive tract. The absence of androgens allows development of the female reproductive tract.
• Prenatal hormones during the critical period can alter genital development (e.g., genetic females can develop male genitals with excessive exposure to androgens).
• Genes and environment interact to guide development.
• How Genes Affect One Another
1. Genes come in several forms. Alternate forms of genes for the same trait (e.g., blood type) are alleles. Some alleles are homozygous (identical alleles) while some are heterozygous (different alleles).
2. Sometimes one allele is dominant and the other recessive. The dominant will be expressed. Sometimes alleles are codominant where both are expressed.
3. Phenotypes (observable traits) do not always match genotypes (genetic makeup) as dominant alleles mask the presence of recessive alleles.
4. Some recessive traits are sex-linked traits; they are carried on the X chromosome and are commonly expressed in males (e.g., red-green color blindness, hemophilia). A male has no dominant allele to cancel out the recessive allele's effects. A female needs homozygous alleles to express the trait. Otherwise, she is a carrier of the trait.
5. Many human characteristics are polygenic, influenced by a number of gene pairs (e.g., intelligence, weight).
• Summing Up the Influence of Genes
• Phenylketonuria (PKU) is a classic example of the interaction of genes and environment on development. If a low-phenylalanine diet is started in the first 6 weeks of life, few harmful effects will be suffered. If after this time, IQ scores are generally below 70.

Conception

• Conception depends on appropriate timing of a chain of events. An ovum ripens in one of the woman's ovaries over a period of about 28 days. When it is ready for fertilization, ovulation occurs; the ovum is released into the fallopian tube and travels to the uterus over the course of several days. Without fertilization within 24 hours, the ovum disintegrates. If conception occurs, a zygote is produced.
• More than one ovum may be released and fertilized by different sperm, resulting in dizygotic (di = two, zygotic = fertilized egg) twins if two ova were released.
• If a fertilized egg splits into two, identical twins are formed (monozygotic twins; mono = one, zygotic = fertilized egg)

Prenatal Development

• Prenatal development, conception to birth at about 38 weeks gestational age, occurs along a predetermined schedule and involves incredible differentiation and rapid growth.
• Stages of Prenatal Development: Germinal period , Embryonic period, Fetal period
• The Germinal Period: Conception Through Week 2
1. The single celled zygote divides into two cells about 30 hours after conception - the beginning of mitosis.
2. After 60 hours, there are four cells and at the end of a week, there are hundreds of cells clustered together into a blastocyst; cell differentiation has begun.
3. The embryoblast, inner cell mass, develops into the embryo.
4. The trophoblast, outer cell mass, forms the embryo's life support system.
5. Implantation occurs about the sixth day after fertilization; uterine lining is rich with blood. Attachment has occurred by the end of the second week.
• The Embryonic Period: Weeks 3 Through 8
1. Once the zygote is firmly implanted, it is referred to as an embryo.
2. Period of organogenesis when all of the vital organs and other major body structures are formed.
3. The Embryo's Life Support System
• The placenta forms partly from cells of the uterine lining and partly from cells of the trophoblast. Oxygen and nutrients are transferred and waste removed through cell membranes. The placenta offers protection from many bacteria, for instance.
• The umbilical cord links the placenta to the embryo and to the mother's uterine wall via separate sets of blood vessels
• Amniotic sac is a fluid-filled protective environment that cushions and regulates temperature.
4. Embryonic Cell Differentiation
• In week 3, cells on the surface migrate toward an indentation (beginning of mouth and digestive tract) and form a central layer between an inner and outer layer: endoderm (internal organs), mesoderm (bones, muscles, and blood) and ectoderm (central nervous system, sensory organs, and skin).
• Embryonic inductions - caused by chemical substances that create critical tissue interactions triggering developmental changes. The mesoderm induces the changes in the developing embryo. Another example of embryonic induction pertains to the formation of the lens of the eye.
• There are critical periods in embryonic cell differentiation.
5. The Timetable for Embryonic Development
• There are critical periods in development during the embryonic and fetal periods, for which there is a predictable timetable.
• Cephalocaudal development - head to tail.
• Proximodistal development - near to far from the center of the body.
• The Fetal Period: Week 9 to Birth
1. Major body parts grow rapidly and become refined in structure, which is distinct from the type of development during the embryonic period.
2. Growth in body length reaches its maximum rate early in fetal development, while weight gains are greatest as delivery approaches.
3. The fetus is more responsive than the embryo. The fetus, by week 10, will flex if touched. The responses become more specialized with development, reflecting qualitative changes in the central nervous system.
4. Behaviors become increasingly regular and integrated.

Mother's Experience of Pregnancy

• Trimesters - three, three-month periods of the pregnancy.
1. First trimester - fatigue and drowsiness are common as are breast swelling, frequent urination, and morning sickness.
2. Second trimester - fatigue and nausea usually disappear. Quickening, first fetal movements, is usually experienced by the fourth month. Bulging abdomen.
3. Third trimester - the increase in the size of the fetus and the uterus puts pressure on other organs. Fetal kicking can cause discomfort.

• Congenital defect (birth defect) - abnormality present at birth. 25% are from genetic defects, 3% are from the prenatal environment, 25% are caused by a combination of genes and environment and in over 40% of the cases, the causes are unknown.
• Single-gene disorders (Mendelian disorders) - most are the result of inheriting a recessive allele from each parent (such as with PKU, sickle-cell anemia, Tay-Sachs disease).
• A few disorders are due to inheritance of a dominant allele (e.g., Huntington's disease).
• Other genetic defects involve chromosomal abnormalities, due to errors in meiosis (extra, missing, or damaged chromosomes). 1 in 160 newborns have some type of chromosomal abnormality.
1. Best known example is Down syndrome (trisomy 21): physical features are abnormal as are many of their organs, and they present with mental retardation. 1 in 800 births, with the number increasing with maternal age.
2. Sex chromosome abnormalities - errors in meiosis,1 in 500 births. Examples: Klinefelter syndrome where males have an extra X, XYY syndrome, Turner syndrome (females with only one X), trisomy X. Some result in developmental problems, many do not.
• Environmental influences
1. Teratogens - substances in the environment that can cause abnormalities during prenatal development. Teratology - study of the effects of these substances.
2. Usually cause abnormalities by preventing or modifying normal cell division and differentiation, generally posing the greatest danger during the embryonic period.
3. Alcohol, Tobacco, and Illicit Drugs
• Can impact development at least three ways:
1. By passing through the placenta and interfering directly with the developing embryo or fetus.
2. By affecting the mother physically and psychologically in ways that can have an indirect impact on the embryo or fetus.
3. By causing the baby to be born with a physical addiction, which is greatest in likelihood for narcotics.
• Alcohol
1. 20 % of newborns in the U.S. have prenatal exposure and nearly 1% of newborns show some degree of neurological problems as a result.
2. Alcohol severely disrupts brain development in a variety of ways, up to the time of birth.
3. Heavy drinkers have a 1 in 6 chance of a stillbirth and a 1 in 2 chance of having a baby with a birth defect. Neurological problems are quite likely with binge drinking. Those who drink heavily late in pregnancy have babies with the most severe retardation in physical growth.
4. 1 in 3 babies born to heavy drinkers have fetal alcohol syndrome (FAS) - 2 in every 1000 births; have a distinctive pattern of unusual facial characteristics, poor growth, and central nervous system problems.
5. Those exposed to lower levels of alcohol prenatally can have neurological and behavioral problems.
• Tobacco
1. About 25% of unborn children are exposed to maternal tobacco use in the U.S. Babies are more likely to be delivered prematurely and to have low birth weight. Later cognitive functioning, language development, and reading skills may be impacted with heavy smoking.
2. Smoking directly exposes the fetus to nicotine and indirectly impacts the mother's body, decreasing oxygen levels in the fetus and increasing fetal heart rate.
• Cocaine
1. Cocaine-exposed infants are estimated to account for over half of the newborns affected by illicit drugs in the U.S. Cocaine use increases miscarriage and stillbirths as well as premature delivery.
2. Exposed newborns show a variety of behavioral effects, depending on degree of exposure: impaired arousal, irritability, social interaction problems and some cognitive deficits.
• Heroin and Methadone
1. Babies of addicted mothers often are addicted and suffer classic withdrawal symptoms.
2. Babies are often underweight, have small head circumference, and remain small. Are hypersensitive to touch, sound, and position changes. They tend to avoid stimulation. Later may show hyperactivity and delays in cognitive and language skills.
• Medications
1. Thalidomide was used as a remedy for morning sickness in the late 1960's. Babies had a variety of malformations, which ones and how severe depended on the critical periods during which the drug was taken. This drug is a classic example of the fact that animal testing is no guarantee of safety, that prolonged exposure is not needed to have a negative impact, and that prenatal development has a precise timetable.
2. Hormones can cause birth defects. Classic example came from use of diethylstilbestrol (DES) to prevent miscarriages. Years later, many of the female offspring developed higher than normal rates of cervical and vaginal cancers and male and female offspring had reproductive tract abnormalities.
3. Diseases can affect prenatal development in three ways. 1) some have direct effects on the developing embryo or fetus. 2) some are directly transmitted from mother to child before or during delivery. 3) some affect the unborn child indirectly by impacting the mother's body chemistry
• Rubella - if exposed early in pregnancy, there is a high incidence of blindness, deafness, mental retardation, and heart defects in the child. Critical periods of exposure.
• HIV - can cross the placenta and infect the fetus. Exposure can come during delivery or during breastfeeding. An estimated 13.8 million women worldwide were infected with HIV in 1998. In the U.S. 15 to 25% of infected mothers transmit the disease to their infants; the rate is 25 to 35% in Africa. AZT during pregnancy reduces the risk of infection. HIV is one of the top 10 causes of death for children under the age of 14 in the U.S. Nearly half who show signs of infection by in the first year will die by age 3. Initial signs are failure to grow and repeated infections, with later problems in motor and cognitive development.
4. Maternal Stress
• High levels of maternal stress are correlated with low birth weight, prematurity, and newborn irritability. Animal models of maternal stress indicate that offspring have abnormal physiological responses to stress. Stress effects are hormone related, which can pass directly to the infant and can cause maternal health problems indirectly affecting the infant.
5. Maternal Nutrition
• Poor maternal nutrition during the last few months of pregnancy lowers brain cell numbers. Specific vitamin and mineral deficits have also been studied.
6. Maternal Age
• Rates of miscarriage, stillbirths, and infant deaths in the U.S. are highest for adolescent mothers (immaturity in reproductive organs) and for mothers over age 35 (greater cumulative insults to egg cells). Birth defects related to chromosomal abnormalities are clearly age linked.
• Detection and Treatment of Fetal Disorders
1. Ultrasound - technique that produces a computer image of the fetus by bouncing sound waves off it. Allows detection of structural abnormalities. Good diagnostic tool .
2. Amniocentesis - amniotic fluid is withdrawn via a needle inserted through the woman's abdomen (ultrasound helps guide this process). Cells are analyzed for chromosomal abnormalities. Performed at 16 to 18 weeks. Can detect neural tube defects.
3. Chorionic Villus Sampling (CVS) - cells are suctioned from the chorion via a small tube passed through the vagina and cervix or the abdominal wall. Cells are examined for genetic defects. Performed at 8 to 12 weeks.
4. Genetic counseling - worthwhile for those with family histories of genetic disorders.

• The fetus prepares for birth by moving into a head-down position, the uterus contracts, helping the baby move through the birth canal. The first sign often is the breaking of the amniotic sac.
• Stages of labor
1. Stage 1 - regular, strong contractions begin 15 to 20 minutes apart and become stronger and more frequent as labor proceeds. This stage continues until the cervix becomes fully dilated (may be one hour or more than 24 hours).
2. Stage 2 - with normal deliveries, this stage begins with crowning, top of the head pushes through the cervix into the vagina and ends when the baby is delivered. Lasts between 30 minutes and 2 hours.
3. Stage 3 - begins when the baby is delivered and ends with the delivery of the afterbirth, including the placenta and other membranes.
• Birth Complications
1. Apgar scale - score (from zero to 10) at one and five minutes after birth is the sum of the ratings (0, 1, 2) for heart rate, respiration, reflexes, muscle tone, and skin color. Scores below 5 indicate the need for immediate medical attention.
2. Anoxia - a long disruption in the oxygen supply; can cause brain damage. Can occur if the umbilical cord becomes pinched during delivery (more common with breech births) or can occur if the baby fails to begin breathing immediately after birth.
3. Low birthweight and prematurity
• Any baby born less than 35 weeks after conception is considered premature.
• Any baby weighing under 5 pounds at birth is considered low birth weight. May be due to premature delivery, retarded growth or both. Those born after 7 months gestational age usually suffer no lasting ill effects. Those born small for their gestational age, even if carried full term, remain at risk for later difficulties.
• Low birth weight affects 7% of babies born in the U.S., and is most common with people living in poverty. Have an increased risk of incidence of neurological problems, physical abnormalities, and lung ailments. Mortality rate for low birth weight babies is twice that of normal weight babies. Most weighing less than 1 pound at birth do not survive; those who do survive have an increased long-term risk for impairments and need early intervention.
• Cultural Variation in Childbirth
1. Babies in North American and many other industrialized countries are delivered by physicians in hospital rooms. However, in Holland, most births take place at home with a midwife. Lay midwives deliver many babies in developing countries.
2. The medical approach in the U.S. has changed somewhat over the past 30 years, unlike in Russia.
3. Childbirth preparation classes (e.g., Lamaze method) can decrease anxiety about labor and delivery, leading to less reliance on painkilling drugs during labor.
4. Common drugs administered: anesthetics, analgesics, and sedatives. Tend to lower mother's blood pressure and oxygen content in her blood, reducing this supply to the baby. Most of these drugs cross the placenta and can directly affect the baby both during and after birth.
5. Cesarean section - surgical incision in the abdomen and uterus; used when heart-rate monitors indicate fetal distress, when the pelvis is too small for the baby's head to pass through, or when vaginal delivery is not going well. The procedure is overused in the U.S. (5 to 23% increase from 1965 to 1985), but the rate of use is decreasing.

• Genes are guidelines, but they guide within environmental contexts that include present and past events/developmental changes.
• Some genes are activated and others deactivated in response to environmental signals, many of which stem from interactions with other cells.
• The environmental context that shapes development always has several layers.