Prenatal Development
Fertilization: the process by which a sperm penetrates an egg
Ovum: an unfertilized egg
Ovulation: the release of a mature egg from an ovary
The prenatal journey begins when a follicle in a woman's ovary matures and releases an ovum (egg) during her monthly menstrual cycle in the process called ovulation. Prenatal development is divided into three stages of very different lengths. The germinal stage lasts from conception to 2 weeks, the embryonic stage lasts from 2 weeks to 2 months, and the last stage, called the fetal stage, is the longest, lasting from 2 months until birth. The length of time since conception is called the gestational age.
Gestational age: the length of time since the conception of the developing organism
The Germinal Stage
The prenatal stage that lasts from conception to 2 weeks post conception. It begins when the sperm penetrates the egg. Once fertilization occurs and a zygote (fertilized egg) has been created, the outside of the egg thickens so that no other sperm will be able to enter the egg. The newly created zygote continues its journey through the fallopian tube, and the process of cell division begins. It takes about 15 hours for that single cell to become two cells. During the woman's menstrual cycle, her hormones have prepared the lining for the purpose of holding the zygote. The solid ball becomes a hollow ball called a blastocyst which now has a solid group of cells at one end and an outer ring of cells. Cells in the trophoblast secrete an enzyme that digests some of the lining in the uterus so that the blastocyst cam securely embed itself there.
Blastocyst: a hollow ball of cells that consists of the inner cell mass (which becomes the embryo) and an outer ring of cells (which becomes the placenta and chorion)
Inner cell mass: a solid clump of cells in the blastocyst, which later develops into the embryo
Trophoblast: the outer ring of cells in the blastocyst that later develops into the support system for the pregnancy
Fertilization: the process by which a sperm penetrates an egg
Ovum: an unfertilized egg
Ovulation: the release of a mature egg from an ovary
The prenatal journey begins when a follicle in a woman's ovary matures and releases an ovum (egg) during her monthly menstrual cycle in the process called ovulation. Prenatal development is divided into three stages of very different lengths. The germinal stage lasts from conception to 2 weeks, the embryonic stage lasts from 2 weeks to 2 months, and the last stage, called the fetal stage, is the longest, lasting from 2 months until birth. The length of time since conception is called the gestational age.
Gestational age: the length of time since the conception of the developing organism
The Germinal Stage
The prenatal stage that lasts from conception to 2 weeks post conception. It begins when the sperm penetrates the egg. Once fertilization occurs and a zygote (fertilized egg) has been created, the outside of the egg thickens so that no other sperm will be able to enter the egg. The newly created zygote continues its journey through the fallopian tube, and the process of cell division begins. It takes about 15 hours for that single cell to become two cells. During the woman's menstrual cycle, her hormones have prepared the lining for the purpose of holding the zygote. The solid ball becomes a hollow ball called a blastocyst which now has a solid group of cells at one end and an outer ring of cells. Cells in the trophoblast secrete an enzyme that digests some of the lining in the uterus so that the blastocyst cam securely embed itself there.
Blastocyst: a hollow ball of cells that consists of the inner cell mass (which becomes the embryo) and an outer ring of cells (which becomes the placenta and chorion)
Inner cell mass: a solid clump of cells in the blastocyst, which later develops into the embryo
Trophoblast: the outer ring of cells in the blastocyst that later develops into the support system for the pregnancy
The Embryonic Stage
The prenatal stage that lasts from 2 weeks to 2 months post conception. The conception is called an embryo at this point. The support system for the pregnancy includes two fetal membranes as well as the placenta and umbilical cord. The chorion is the outer one and the inner one is called the amnion. The placenta performs the essential function of bringing oxygen and nutrients to the developing embryo. Maternal blood flows through the placenta, where it fills up empty space. During the embryonic stage, the inner cell mass differentiates into three layers, each of which go on to become different organs and structures. These include the ectoderm , endoderm, and the mesoderm. During this stage, all of the major organ systems of the body are laid down.
Embryo: the developing organism from conception to the end of the second month of pregnancy
Chorion: the outer fetal membrane that surrounds the fetus and gives rise to the placenta
Amnion: the inner fetal membrane that surrounds the fetus and is filled with amniotic fluid
Ectoderm: the outermost layer of the inner cell mass that later becomes the skin, sense organs, brain, and spinal cord
Endoderm: the innermost layer of the inner cell mass that later becomes the respiratory system, digestive system, liver, and pancreas
Mesoderm: the middle layer of the inner cell mass that later becomes the muscles, bones, blood, heart, kidneys, and gonads
Throughout the prenatal period, development is from the head region down through the body.
Cephalocaudal development: a principle whereby development proceeds from the head region down through the body
Critical period: a period of time during which development is occurring rapidly and the organism is especially sensitive to damage, which is often severe and irreversible
The Fetal Stage
The prenatal stage that lasts from 2 months post conception until birth. This stage is characterized by the continued growth of the fetus and a remarkable increase in size and weight. All of the organ systems need to complete their development and become functional so that the newborn will be capable of surviving independently of the mother after birth. The transformation of the genitalia occurs during this period determining if the fetus will be male or female. At about 10 weeks, fetal breathing movements begin. Fetuses breathe in and then expel amniotic fluid. Although the fetus is protected from extreme stimulation within its uterine environment, it is not isolated from the sensory world. All senses are functional by the time the baby is born. There is a great deal of prenatal development and the fetus will become functional. This time prepares the newborn to begin interacting with and responding to the environment.
Three Trimesters of Pregnancy
The nine months of pregnancy are divided. Each three month period is called a trimester, and each has its own characteristics. During the first trimester it may not be apparent to other people that the womb is pregnant, but changes in the level of her hormones may cause certain effects, including fatigue, breast tenderness, and morning sickness. In the second trimester, the pregnancy begins to become apparent as the fetus grows larger. The woman is now able to feel the fetus moving inside of her. In the third trimester, the fetus continues to grow, and the woman becomes more tired and uncomfortable.