When we study the course of prenatal development, we pay special attention to support for healthy growth as well as damaging influences that threaten the child’s health and survival.
In the course of prenatal development, there are three main stages. The first two weeks after conception are known as the germinal stage. The third through the eighth week is known as the embryonic period, and the time from the ninth week until birth is known as the fetal period.
- The Germinal Stage: Fertilization To 2 Weeks
- The Embryonic Stage: 2 Weeks To 8 Weeks
- The Fetal Stage: 8 Weeks To Birth
It all start with Conception
In the middle of a woman’s menstrual cycle, an ovum bursts from one of her ovaries, two walnut-sized organs located deep inside her abdomen, and is drawn into one of two fallopian tubes—long, thin structures that lead to the hollow, softly lined uterus.
The male produces sperm in vast numbers—an average of 300 million a day—in the testes, two glands located in the scrotum, sacs that lie just behind the penis. Each sperm develops a tail that permits it to spine long distances, upstream in the female reproductive tract, through the cervix (opening of the uterus) and into the fallopian tube, where fertilization usually takes place. With conception, the story of prenatal development begins to unfold.
1. Germinal Stage of the Prenatal Development.
The germinal period lasts about two weeks, from fertilization and formation of the zygote until the tiny mass of cells drifts down and out of the fallopian tube and attaches itself to the wall of the uterus.
The zygote’s first cell duplication is long and drawn out; which is why it is not complete until about 30 hours after conception. Gradually, new cells are added at a faster rate. By the fourth day, 60 to 70 cells exist that form a hollow, fluid-filled ball called a blastocyst. The cells on the inside of the blastocyst, called the embryonic disk, will become the new organism; the outer ring of cells, termed the trophoblast, will become the structures that provide protective covering and nourishment. Where the blastocyst becomes implanted in the uterus’s wall, which is rich in nutrients.
Between seventh and the ninth days implantation occurs. Trophoblast forms a membrane, called the amnion, that encloses the developing organism in amniotic fluid, which helps keep the temperature of the prenatal world constant and provides a cushion against any jolts caused by the women’s movement. This stage is characterized by methodical cell division, which gets off to a quick start: three days after fertilization, the organism consists of some 32 cells, and by the next day the number doubles. Within a week, it is made up of 100 to 150 cells, and the number rises with increasing rapidity. By the end of the germinal period, the developing organism has found food and shelter. These dramatic beginnings take place before most mothers know they are pregnant.
2. Embryonic Period
The period of the embryo lasts from implantation through the eighth week of pregnancy. The organism is firmly secured to the wall of the mother’s uterus. The most rapid prenatal changes take place as the groundwork is laid for all body structures and internal organs, during these brief six weeks.
In the first week of this period, the embryonic disk forms three layers of cells:
- The ectoderm, which will become the nervous system and skin;
- The mesoderm, from which will develop the muscles, skeleton, circulatory system, and other internal organs; and
- The endoderm, which will become the digestive system, lungs, urinary tract, and glands.
These three layers give rise to all parts of the body.
The head & brain undergo rapid growth during the embryonic period. The head begins to represent a significant proportion of the embryo’s size, encompassing about 50 percent of its total length. The ectoderm folds over to form the neural tube, or primitive spinal cord.
In the second month, growth continues rapidly. The eyes, ears, nose, jaw, and neck form. Tiny buds become arms, legs, fingers, and toes. Changing body proportions cause the embryo’s posture to become more upright.
By the end of this period, the embryo—about 1 inch long and 1∕7 ounce in weight—can already sense its world. It responds to touch, particularly in the mouth area and on the soles of the feet. Although its tiny flutters are still too light to be felt by the mother, it can move .
3. Fetal Period of the Prenatal Development.
The fetal stage, where the developing child becomes easily recognizable.
In the third month, the organs, muscles, and nervous system start to become organized and connected. When the brain signals, the fetus kicks, bends its arms, forms a fist, curls its toes, turns its head, opens its mouth, and even sucks its thumb, stretches, and yawns. The tiny lungs begin to expand and contract in an early rehearsal of breathing movements.
By the twelfth week, the external genitals are well-formed, and the sex of the fetus can be detected with ultrasound.
Prenatal development is sometimes divided into trimesters, or three equal time periods. At the end of the third month, the first trimester is complete.
By the middle of the second trimester, A white, cheese-like substance called vernix emerges on the skin, protecting it from chapping during the long months spent bathing in the amniotic fluid. White, downy hair called lanugo also appears over the entire body, helping the vernix stick to the skin.
During the final trimester, a fetus born early has a chance for survival. The point at which the baby can first survive, called the age of viability, occurs sometime between 22 and 26 weeks. Between 30 and 34 weeks, fetuses show rhythmic alternations between sleep and wakefulness that gradually increase in organization. Around 36 weeks, synchrony between fetal heart rate and motor activity peaks. By the end of pregnancy, the fetus takes on the beginnings of a personality. Fetal activity is linked to infant temperament.
The third trimester brings greater responsiveness to external stimulation.
To read more about the developmental stages and periods of development, click here.
- Robert. S. Feldman. (2017). Development Across the Lifespan. (8th ed.). Pearson Education.
- Laura. E. Berk. (2018). Development Through the Lifespan (7th ed.). Pearson Education.