Prenatal Development and Birth

 Prenatal Development and Birth 

 

Prenatal development

Definition: Prenatal development refers to the process in which a baby develops from a single cell after conception into an embryo and later a fetus.

Conception occurs when a single sperm cell from the male unites with an ovum (egg) in the female’s fallopian tube in a process called fertilization.

 

THE COURSE OF PRENATAL DEVELOPMENT

Typical prenatal development begins with fertilization and ends with birth, lasting between 266 and 280 days (from 38 to 40 weeks). It can be divided into three periods: germinal, embryonic, and fetal.

The Germinal Period

The germinal period is the period of prenatal development that takes place in the first two weeks after conception. It includes the creation of the fertilized egg, called a zygote, cell division, and the attachment of the zygote to the uterine wall.

Rapid cell division by the zygote continues throughout the germinal period that this cell division occurs through a process called mitosis. By approximately one week after conception, the differentiation of these cells—their specialization for different tasks—has already begun. At this stage, the group of cells, now called the blastocyst (The inner layer of cells that develops during the germinal period. These cells later develop into the embryo), consists of an inner mass of cells that will eventually develop into the embryo, and the trophoblast (The outer layer of cells that develops in the germinal period. These cells provide nutrition and support for the embryo), an outer layer of cells that later provides nutrition and support for the embryo. Implantation, the attachment of the zygote to the uterine wall, takes place about 11 to 15 days after conception. Figure 3.1 illustrates some of the most significant developments during the germinal period.

The Embryonic Period

The embryonic period is the period of prenatal development that occurs from two to eight weeks after conception. During the embryonic period, the rate of cell differentiation intensifies, support systems for cells form, and organs appear.

This period begins as the blastocyst attaches to the uterine wall. The mass of cells is now called an embryo, and three layers of cells form. The embryo’s endoderm is the inner layer of cells, which will develop into the digestive and respiratory systems. The mesoderm is the middle layer, which will become the circulatory system, bones, muscles, excretory system, and reproductive system. The ectoderm is the outermost layer, which will become the nervous system and brain, sensory receptors (ears, nose, and eyes, for example), and skin parts (hair and nails, for example). Every body part eventually develops from these three layers. The endoderm primarily produces internal body parts, the mesoderm primarily produces parts that surround the internal areas, and the ectoderm primarily produces surface parts.

As the embryo’s three layers form, life-support systems for the embryo develop rapidly. These life-support systems include the amnion, the umbilical cord (both of which develop from the fertilized egg, not the mother’s body), and the placenta. The amnion is like a bag or an envelope and contains a clear fluid in which the developing embryo floats. The amniotic fluid provides an environment that is temperature and humidity controlled, as well as shockproof. The umbilical cord contains two arteries and one vein, and connects the baby to the placenta. The placenta consists of a disk-shaped group of tissues in which small blood vessels from the mother and the offspring intertwine but do not join.

Figure 3.2 illustrates the placenta, the umbilical cord, and the blood flow in the expectant mother and developing organism. Very small molecules—oxygen, water, salt, food from the mother’s blood, as well as carbon dioxide and digestive wastes from the offspring’s blood—pass back and forth between the mother and embryo or fetus (Wick & others, 2010). Large molecules cannot pass through the placental wall; these include red blood cells and harmful substances, such as most bacteria and maternal wastes. The mechanisms that govern the transfer of substances across the placental barrier are complex and are still not entirely understood (Barta & Drugan, 2010).

By the time most women know they are pregnant, the major organs have begun to form. Organogenesis is the name given to the process of organ formation during the first two months of prenatal development. While they are being formed, the organs are especially vulnerable to environmental changes (Rojas & others, 2010; Torchinsky & Toder, 2010). In the third week after conception, the neural tube that eventually becomes the spinal cord forms. At about 21 days, eyes begin to appear, and at 24 days the cells for the heart begin to differentiate. During the fourth week, the urogenital system becomes apparent, and arm and leg buds emerge. Four chambers of the heart take shape, and blood vessels appear. From the fifth to the eighth week, arms and legs differentiate further; at this time, the face starts to form but still is not very recognizable. The intestinal tract develops and the facial structures fuse. At eight weeks, the developing organism weighs about 1/30 ounce and is just over 1 inch long.

The Fetal Period

The fetal period, lasting about seven months, is the prenatal period between two months after conception and birth in typical pregnancies. Growth and development continue their dramatic course during this time.

Three months after conception, the fetus is about 3 inches long and weighs about 3 ounces. It has become active, moving its arms and legs, opening and closing its mouth, and moving its head. The face, forehead, eyelids, nose, and chin are distinguishable, as are the upper arms, lower arms, hands, and lower limbs. In most cases, the genitals can be identified as male or female. By the end of the fourth month of pregnancy, the fetus has grown to 6 inches in length and weighs 4 to 7 ounces. At this time, a growth spurt occurs in the body’s lower parts. For the first time, the mother can feel arm and leg movements.

By the end of the fifth month, the fetus is about 12 inches long and weighs close to a pound. Structures of the skin have formed—toenails and fingernails, for example. The fetus is more active, showing a preference for a particular position in the womb. By the end of the sixth month, the fetus is about 14 inches long and has gained another half pound to a pound. The eyes and eyelids are completely formed, and a fine layer of hair covers the head. A grasping reflex is present and irregular breathing movements occur.

As early as six months of pregnancy (about 24 to 25 weeks after conception), the fetus for the first time has a chance of surviving outside of the womb—that is, it is viable. Infants are born early, or between 24 and 37 weeks of pregnancy, usually need help breathing because their lungs are not yet fully mature. By the end of the seventh month, the fetus is about 16 inches long and now weighs about 3 pounds.

During the last two months of prenatal development, fatty tissues develop, and the functioning of various organ systems—heart and kidneys, for example—steps up. During the eighth and ninth months, the fetus grows longer and gains substantial weight—about another 4 pounds. At birth, the average American baby weighs 7½ pounds and is about 20 inches long.

Figure 3.3 gives an overview of the main events during prenatal development. Notice that instead of describing development in terms of germinal, embryonic, and fetal periods, Figure 3.3 divides prenatal development into equal periods of three months, called trimesters. Remember that the three trimesters are not the same as the three prenatal periods we have discussed. The germinal and embryonic periods occur in the first trimester. The fetal period begins toward the end of the first trimester and continues through the second and third trimesters. Viability (the chances of surviving outside the womb) occurs at the very end of the second trimester.