Chapter 1. Periods of Prenatal Development

Synopsis

Human Development Video Activity

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You must read each slide, and complete any questions on the slide, in sequence.

age of viability

The age at which a fetus can survive outside of the womb with advanced medical care. The age of viability is 22 weeks.

blastocyst

A cell mass that begins developing from the zygote about 5-6 days after fertilization during the germinal period before implantation. As the cells of the blastocyst continue to divide and multiply, the inner cell mass develops into the embryo while the outer layer of cells develops into the placenta.

embryo

The term for a developing human organism after being called a zygote and before being called a fetus from about the third through the eighth week after conception.

embryonic period

The prenatal stage of development from approximately 3 weeks to 8 weeks after conception during which the embryonic cells first resemble a human being. By the end of the embryonic period, most basic organs have developed.

fetus

The term for a developing organism from the ninth week after conception until birth.

fetal period

The prenatal period from the 9th week after conception until birth during which the baby grows from approximately an inch to 20 inches in length and organs grow and mature in preparation for life outside the womb.

germinal period

The earliest stage of prenatal development that begins with conception and lasts until implantation in the uterus at anywhere from 6 to 12 days after fertilization. Often the germinal period is used to refer to the first 2 weeks after fertilization.

implantation

The process of the blastocyst attaching to the uterine lining. Implantation occurs anywhere from 6 to 12 days after fertilization. Upon successful implantation, the mother’s hormones trigger her menstrual cycle to stop.

placenta

The vascular organ that builds up on the mother’s uterine wall to surround and nourish the embryo and fetus during pregnancy. It is expelled out of the mother after the baby’s birth.

Periods of Prenatal Development

A close-up of the head of a fetus in the womb

Author

Tom E. Ludwig, Hope College
Catherine F. Robertson, Grossmont College
S. Stavros Valenti, Hofstra University

Synopsis

This activity presents the germinal, embryonic, and fetal periods of prenatal development. For the earlier germinal and embryonic periods, you will learn about the emerging organism in the journey from fertilization to implantation at the end of the eighth week. For the fetal period, you will see the amazing pace at which the tiny being develops after week eight before exiting from the protection of the mother’s womb with fully developed organ systems and a complete little body.

REFERENCES

The Visible Embryo. Available at http://www.visembryo.com/baby/index.html

Nilsson, L., & Hamberger, L. (1990). A Child is Born: The Completely New Edition, New York: Dell Publishing.

The Germinal Period: Conception to Implantation

The successful union of a woman’s ovum and a man’s sperm that occurs in the woman’s fallopian tube generates a zygote, the first cell of a new individual. This fusion of ovum and sperm is called fertilization, or conception. Cell divisions commence immediately. After three days, the new organism begins a journey through the fallopian tube toward the uterus. As it travels, the new organism transforms into a hollow ball of cells known as a blastocyst.

Once inside the uterus, the blastocyst implants in the uterine wall. The inner cell mass of the blastocyst will develop into the new organism while the outer layer of cells will develop into membranes, including the placenta, umbilical cord, and amniotic sac, that will surround, protect, and deliver nutrients to the baby.

Implantation may occur anywhere between 6 to 12 days after conception. It marks the end of the germinal period. For the sake of simplicity, we will refer to the germinal period as the first two weeks following conception.

Play the video to observe the steps of the germinal period from the moment of conception until the implantation of the blastocyst.

The Embryonic Period

^{This animation demonstrates the development of the embryo during the fourth and fifth weeks. Note how the embryo becomes longer and changes into a C-shape. The eyes and ears begin to form. The neural tube closes, and the heart begins to beat. Hands begin to form, and cells begin to differentiate to form the internal organs.}

The embryonic period begins at implantation and lasts until the end of the eighth week of development. During this critical period, the embryo’s basic support system forms, and the organs of the body take shape.

At four weeks, the embryo is curved and now bears a slight resemblance to a shrimp. The top of the neural tube will form the brain. The primitive heart is circulating blood. The facial structure begins to emerge. The eyes and ears begin to form. The limb buds and digestive system are starting to develop.

Rapid brain development occurs during the fifth week as it grows and differentiates. The sensory systems, the gastrointestinal system, the respiratory system, and the circulatory system, which includes the heart, become more sophisticated during this period. The arm buds and leg buds continue to elongate and take shape.

Play the animation that shows development during the fourth and fifth weeks.

The Embryonic Period: Weeks 6 and 7

During the sixth week, the embryo develops a straighter form. As the brain continues to grow, the head becomes the dominant feature. The facial structure becomes more recognizable. The umbilical cord takes shape as intestines form within it. Hormonal and lymph systems begin to function.

During week seven, cartilage begins to form making the embryo more substantial. Muscles start to develop and to differentiate into their final forms in order to assume their functions. The head becomes erect and has an overall cube-like appearance. The brain is producing nerve cells, and brain waves begin to function. Circulatory, respiratory, and gastrointestinal systems continue to mature. Gonads (ovaries in girls, testicles in boys) appear, and the genital bud emerges.

Play the animation that shows development during weeks six and seven.

The Embryonic Period: Week 8

In this final week of the embryonic period, all of the essential structures of the human being are in place. The controlling systems (circulatory, respiratory, gastrointestinal, and so on) have emerged, and the brain, now linked to nerves and muscles, is taking control. The embryo’s tail has disappeared. Although the head is still quite large relative to the body, it takes on a more familiar rounded shape. Cartilage begins to transform into bone. Gender development continues as the internal and external sex organs are now in place. From this point on, development will consist primarily of the growth and refinement of existing features.

Play the animation that shows development during the entire embryonic period, which ends after week 8.

The Fetal Period Begins

By the end of week eight, embryos weigh about one gram and are about two and a half centimeters (one inch) long. Now floating in a fluid-filled amniotic sac, the developing organism is so small at this point that some women are unaware that they are even pregnant. Nevertheless, the peanut-sized embryo has now become recognizably human. It has all its basic organ systems, and its facial features resemble those of a human being. The skeletal system has also developed, and bone cells are beginning to form.

At the end of eight weeks, the embryonic period ends, and the developing organism is now called a fetus (from a Latin word meaning “offspring” or “young one”). The fetal period lasts from nine weeks after conception until birth at about 38 weeks. With the formation of all organ systems in their rudimentary forms, the fetus now has two primary tasks to accomplish during this final phase of prenatal development.

Task #1 − Continued development of organ systems − Each system must mature and eventually begin functioning in preparation for birth.
Task #2 − Continued overall body growth − As shown in this illustration, the size of the fetus increases by twentyfold during these few months. Arms and legs grow dramatically, and before long, the head no longer seems so disproportionate in relation to the rest of the body.

Watch the video for an overview of development during the fetal period.

First, Second, and Third Trimester

^{Observe prenatal development from beginning to end.}

Traditionally, mothers and medical personnel divide the nine months of pregnancy into three trimesters, each containing three months.

First trimester (Weeks 1−13) − The baby moves through the germinal and embryonic periods and into the first four weeks of the fetal period. By 13 weeks of gestation, the limbs are fully formed down to the fingers and tiny fingernails. The intestines have moved from the umbilical cord to the abdomen. The external genitals are distinct for boys and girls.

Second trimester (Weeks 14−26) − Morning sickness fades for most mothers. There is a surge in fetal brain growth, and fetal movements are large enough to be felt by the mother. Sometimes, these movements are in response to sound, which now can be heard by the fetus because of developments in the inner ear. There is continued development of the lungs, but they are not yet ready to function.

Third trimester (Weeks 27 until birth) − The fetus will reach the age of viability, which is the age at which the fetus may survive outside the mother’s uterus if specialized medical care is available (about 28 weeks gestation). At this point, a typical fetus weighs about 1300 grams (almost three pounds). From 28 weeks until birth, the fetus more than doubles its weight, and the nervous and respiratory systems rapidly mature. Although the fetus can still change positions, gravity pulls the heavy head down. Eventually, most fetuses get wedged into position with the head pressing against the mother’s cervix, which is, in fact, the most favorable position for birth.

Fetal Period: Week 38 and Beyond

At 38 weeks, the fetus is at full-term. The fetus is now wedged firmly in place and is ready for the birth process to begin. An average 38-week fetus weighs a little over 3000 grams (7 pounds) and is about 50 centimeters (20 inches) in length.

As the time for birth approaches, the fetus’s rate of growth slows down. The lungs are finally reaching maturity, and the inner surface of the lungs is now coated with a substance that will facilitate actual breathing when the infant exits the mother’s womb. The heart is now pumping about 300 quarts of blood each day.

The bones of the skeleton are growing more rigid, but fortunately for both infant and mother, the skull is not yet fully solid. Five separate bony plates, known as fontanels, allow the skull to compress into a bullet shape with a reduced diameter during labor making delivery less difficult. After delivery, the baby’s head returns to a rounded shape.

Typically, preparations for the big day begin about 366 days after conception. At this time, the fetus’s brain secretes a hormone that triggers the fetus’s lungs to prepare for breathing air and the mother’s uterus to prepare for labor. On average, labor lasts about 8 to 12 hours. With help from the mother, the contracting uterus sends the fetus on its journey into the world.

Assessment: Check Your Understanding

Question 1.1

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Correct! The implantation of the blastocyst marks the end of the germinal period.

Sorry. The implantation of the blastocyst marks the end of the germinal period.

Assessment: Check Your Understanding

Question 1.2

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Correct! The embryonic period begins with the implantation of the blastocyst in the uterine wall, and it lasts for six weeks.

Sorry. The embryonic period begins with the implantation of the blastocyst in the uterine wall, and it lasts for six weeks.

Assessment: Check Your Understanding

Question 1.3

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Your response should mention that: - the support systems for the future baby are formed during the embryonic period; - all of the basic organs of the body take shape during the embryonic period; - all of the body systems begin working during the embryonic period; - development after the eighth week, which marks the end of the embryonic period, will be primarily growth and refinement of the features that are formed during the embryonic period.

Assessment: Check Your Understanding

Question 1.4

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Correct! As the time for birth approaches, the fetus’s rate of growth does slow down, but during the entire fetal period, the size of the fetus increases by up to twentyfold.

Sorry. As the time for birth approaches, the fetus’s rate of growth does slow down, but during the entire fetal period, the size of the fetus increases by up to twentyfold.

Assessment: Check Your Understanding

Question 1.5

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Your response should mention at least two of these three main themes: - dramatic growth in body size as the fetus is approximately one inch long at week nine and approximately 20 inches at birth; - shift in proportions as the lower portion of the body begins to catch up with the early and rapid development of the head that occurred during the embryonic period; - functional development of the various organ systems as each system gradually begins working and then becomes more efficient in preparation for birth.

Congratulations! You have completed this activity.Total Score: x out of x points (x%) You have received a provisional score for your essay answers, which have been submitted to your instructor.