2 An egg can be fertilized during the first few days after ovulation If sperm is deposited in or near the vagina, it can move through the female reproductive system and fertilize an egg.If a sperm fertilizes an egg, pregnancy can occur.
3 Development before birth A fertilized egg, or zygote, is really small.However, it undergoes tremendous changes that result in the formation of a new human.After fertilization occurs, the zygote moves down the fallopian tube toward the uterus.During this four day trip, the zygote begins to divide.
4 The zygote, now composed of hundreds of cells, attaches to the uterus. From the two cell stage through the eighth week of development, the developing human is called an embryo.
5 Differentiation of the Embryo Middle layer: heart, bones, muscles, kidneys, testes, ovariesAt about three weeks, some cells in the embryo fold towards the center, forming three layers.The layer in which a cell is located determines how it will differentiate.As development progresses the three layer differentiated cells become more finely differentiated.Outer layer: skin, nervous system, pituitary gland, salivary glandsInner layer: pancreas, thyroid gland, liver, parts of digestive system.
6 Development of the fetus From about the ninth week of pregnancy until birth, the developing human is called a fetus.At four weeks, the heart beats in a regular rhythm. Eyes and ears begin to form
7 Development of the fetus At eight weeks, the heart has left and right chambers.Between the 4th and 6th month bones become more distinct. The arms and legs develop more completely. A heartbeat can be heard with a stethoscope.
8 Development of a fetusThe final three months prepare the fetus to survive outside the mother’s body. The brain develops grooves and ridges. The lungs become ready to carry out the exchange of oxygen and carbon dioxide. The eyelids can open.
9 Protection and Nourishment The membranes and other structures that form during development protect and nourish the developing embryo, and later the fetus.Amniotic sac: Filled with fluid, it cushions and protects the developing baby.Umbilical cord: Forms between the embryo and the placenta. It contains the veins and arteries that link the fetus to the mother.Placenta: Develops from fetal tissue. The embryo’s blood vessels are located next to the mother’s blood vessels.
10 Keeping the fetus healthy The placenta separates the two blood supplies, and protects the fetus from some diseases and chemicals.Unfortunately, some diseases, like HIV, and chemicals such as alcohol, tobacco and other drugs CAN pass through the barrier.This is why pregnant women should not smoke, drink alcohol or take any drug (legal or otherwise) without a doctor’s approval.
11 BirthAfter about 40 weeks of development inside the uterus, the baby is ready to be born.The birth of a baby takes place in three stages:Delivery: During delivery the baby is pushed completely out of the uterus, through the vagina and out of the mother’s body.Afterbirth: About 15 minutes after delivery, contractions push the placenta and other membranes out of the mother’s body.Labor: During the first stage of birth, strong muscular contractions of the uterus begin. The contractions cause the cervix to open. Labor may last from about 2 to more than 20 hours.
12 Growth and Development The changes that take place between infancy and adulthood include physical changes, such as an increase in size and coordination, and mental changes, such as the ability to communicate and solve complex problems.
13 InfancyDuring infancy, the first 2 years of life, a baby grows quickly.The baby’s nervous and muscular system become more coordinated.They learn to speak.By two years old, children can do many things for themselves, such as play with toys, feed themselves and understand simple directions
14 Childhood Begins about two years of age. Children grow taller and heavier as their bones and muscles increase in size.They become more coordinated.Children show growing curiosity and increased mental abilities (reading, solving problems, carry on conversations).
15 AdolescenceThe stage of development during which children mature into adults is called adolescence.Adolescents gradually become able to think as adults and take on adult responsibilities.Between 9 and 15, adolescents enter puberty, he stage of development during which the body becomes able to reproduce.Puberty is accompanied by profound physical and emotional changes.
16 Adulthood Mental and emotional growth continue. At about age 30, a process known as aging begins. (skin becomes wrinkled, muscle strength decreases, etc.)Aging is more noticeable between 40 and 65.The effects of aging can be slowed by following a sensible diet and good exercise plan.
17 Were you really listening? What three stages of development does a fertilized egg do through before birth?What is the general function of the membranes that surround a fetus?Why is it dangerous for a pregnant woman to drink alcohol or to smoke cigarettes?Compare and contrast the changes that happen to males and females during puberty.How many chromosomes does each reproductive cell (sperm or egg) have?In what ways are the functions of the ovaries and the testes similar? Different?What structure connects the fetus to the placenta?