Life in the Womb
The First Week of Life
Fertilization
Biologically speaking, fertilization (or conception) is the beginning of human development. Fertilization normally occurs within several hours of ovulation (some authors report up to 24 hours) when a man’s sperm combines with a woman’s egg inside a woman’s uterine tube.
Fertilization begins with the spermatozoon contacting the cells surrounding the oocyte [egg] and ends with the mixing of the 23 male and 23 female chromosomes. The result is a single-cell embryo called a zygote, meaning “yoked or joined together,” and it is the first cell of the human body.
The zygote, like the oocyte, is encased by its protective covering, the zona pellucida and contains 46 unique chromosomes with the entire genetic blueprint of a new individual. Chromosomes contain tightly packed, tightly coiled molecules called DNA. Amazingly, DNA contains all the instructions needed for this single-cell embryo to develop into an adult.
The Second Week of Life
The primary event of the second week is implantation.
Implantation is the process whereby the early embryo embeds into the inner wall of the mother’s uterus. Implantation begins about 6 days after fertilization and is complete by about 12 days.
Implantation represents a significant obstacle to the developing embryo. It is estimated that up to one-half of all embryos fail to successfully implant and die – often without the mother realizing she is pregnant. Many of these embryos are thought to have severe genetic abnormalities incompatible with survival.
By the end of the first week, the embryo has traveled extensively, multiplied from 1 cell to several hundred, dramatically changed its shape and complexity, and begun the process of finding permanent housing.
The attachment phase of implantation is briefly discussed in Unit 1. Once superficial implantation occurs, the embryo begins receiving nourishment directly from the cells lining the mother’s uterus.1
HCG – The Pregnancy Hormone
Approximately 8 days after fertilization, cells from the growing embryo begin producing a hormone called human chorionic gonadotropin (human (ko-re-on’ik) (go’nad-o-tro’pin)) or hCG. This hormone is present in a pregnant woman’s blood and urine almost immediately and is the substance detected by most pregnancy tests. HCG acts directly to maintain the corpus luteum and enhance the corpus luteum’s production of estrogen and progesterone. Simply stated, HCG interrupts the normal menstrual cycle allowing pregnancy to continue.
Placenta Formation
Once implantation is complete, the embryo continues to grow inside the wall of the uterus. It’s hard to believe, but development progresses so quickly that just 10 days after implantation is complete, the embryo’s heart begins to beat
The Placenta and Umbilical Cord – The All-Purpose Supply Line
Even before a woman is noticeably pregnant, her intake of food and beverage passes through the developing placenta and umbilical cord to nourish (or harm) the growing embryo. The use of harmful substances during pregnancy is doubly damaging because both the woman and embryo / fetus may suffer significant harm.
Following implantation, maternal and embryonic tissues combine and begin to form the placenta (pla-sen’ta).6
Once the placental circulation is in place; nutrients, oxygen, vitamins, and water in the mother’s blood stream pass through the placenta and umbilical cord to the embryo (and later the fetus); while carbon dioxide and other waste flows back to the mother. The best intensive care units found in modern hospitals hardly rival the placenta’s life-support capabilities.
The yolk sac is another structure vitally important during early development. This highly vascular structure is surrounded by nutrient-rich fluid and absorbs these nutrients and delivers them for use by the embryo. This method of nutrition continues until the placenta becomes more fully developed and begins functioning. The yolk sac is also the site where the first blood cells form and where reproductive cells, or germ cells, originate. Additionally, a portion of the yolk sac is drawn into the embryo forming the lining of the digestive tract.
Week 3-4 of Life
Vital Systems – The Beating Heart
Only 3 weeks and 1 day after fertilization – the heart begins to beat. By 4 weeks, the heart typically beats between 105 and 121 times per minute.
The 4-week embryo is less than 1/100th of an inch long.
Week 4-5 of Life
The brain continues growing at an incredible rate. Between 4 and 5 weeks, the 3 primary vesicles divide into 5 secondary vesicles. During this time, the head makes up about one-third of the embryo’s entire size. An early form of the cerebellum appears by 4 to 4½ weeks; this area of the brain will later control muscle control and coordination.
The respiratory system is progressing as 2 primary lung buds form the beginning of the right and left lungs. By 4½ weeks, the right and left mainstem bronchi, the major airways to the right and left lungs respectively, are well established. They begin dividing into the lobar pattern seen in the adult – 3 lobes on the right and 2 on the left.
By 5 weeks, the embryo’s liver is producing blood cells.
Development of the stomach, esophagus, pancreas, and the small and large intestines is all underway.
The permanent kidneys appear by 5 weeks.
Next to the kidneys, the gonads, or reproductive organs, are developing. These will eventually become ovaries in the female and testes in the male.
The embryo’s endocrine system is also developing. This system of glands regulates the release of hormones throughout a person’s life. The pituitary (pi-tu’i-tar-e) gland forms at the base of the brain during week 5 and begins secreting growth hormone and the hormone ACTH which stimulates further growth of the adrenal glands.
The limb buds continue to grow and by five weeks the embryo develops hand plates.
Week 5-6 of Life
Cartilage formation begins by 5½ weeks.
By 6 weeks a portion of the brain called the cerebral cortex appears. Nerve cells, or neurons, in the spinal cord now begin to develop specialized connections. These connections, where neurons meet and communicate with one another, are called synapses.
Though a pregnant woman does not feel movement for at least another 8 to 10 weeks, the embryo begins to move between 5 and 6 weeks. The embryo’s first movements are both spontaneous and reflexive. A light touch to the mouth area causes the embryo to reflexively withdraw its head, while the embryo’s trunk will twist spontaneously. Movements are essential for the normal development of bones and joints.
By 5½ weeks, nipples appear near the embryo’s underarm. We will see their location change as they reach their final position on the chest wall.
The diaphragm (di’a-fram), the primary muscle used in breathing, is largely formed by 6 weeks.
Week 6-7 of Life
The 6-week embryo measures less than ¼ of an inch long from head to rump.
The embryo has brainwaves by 6 weeks, 2 days!
From 6 to 6½ weeks, the cerebral vesicles will double in size. Individualized brainwaves recorded via electroencephalogram, or EEG, have been reported as early as 6 weeks, 2 days.
Also by 7 weeks, cell groupings resembling taste buds appear on the tongue and hiccups begin. Nasal plugs are prominent at this time and will persist for another 6 weeks or so.
By 7 weeks, the heart has 4 chambers.
By 6½ weeks, the elbows are distinct and the embryo begins moving both hands. The fingers are also starting to separate.
The foot plate and ankle also emerge while toes and metatarsal (met’a-tar’sal) bones begin to form in the feet. Joint development is underway and the onset of primary muscle fiber formation indicates the embryo’s muscles are growing.
Babies continue to fascinate themselves and us by moving their hands – an ability they have been practicing long before birth.
The hands and feet change dramatically between 6 and 7 weeks as separate fingers and toes begin to emerge. At 6 weeks the hand plates develop a subtle flattening between the digital rays. By 6 weeks, 2 days the hand takes on a polygon shape; prominent notches appear between the digital rays by 7 weeks and individual fingers are fully separated by 7½ weeks.
Also, by 7 weeks, the embryo moves the legs and exhibits a startle response.
The immune system is maturing, as evidenced by the presence of B-lymphocytes in the liver. After birth and relocation away from the protection of the womb, these lymphocytes will produce proteins called antibodies to fight infection.
By 7 weeks, the ovaries appear in the female embryo. In the male embryo, a gene on the Y-chromosome produces a substance causing the testes to begin to differentiate.
Did you know the oldest bones in your body are your collar bone and jaw? These bones begin to develop in the womb by 7 weeks after fertilization.
Week 7-8 of Life
From 7 to 7½ weeks, tendons attach leg muscles to bones, and knee joints appear. Also by 7½ weeks, the hands can be brought together, as can the feet. The embryo also kicks, and will jump if startled.
Also by 7 to 7½ weeks, nephrons, the basic filtration units in the kidneys, begin to form.
By 8 weeks the brain is highly developed and makes up approximately 43 percent of the embryo’s total weight. Growth continues at an extraordinary rate. One of the major control centers for the body – the hypothalamus – begins to take form. The hypothalamus eventually controls body temperature, heart rate, blood pressure, fluid balance, and the secretion of vitally important hormones by the pituitary gland.
Our body’s temperature is regulated by the hypothalamus – an important structure which begins developing within the 8-week embryo’s brain.
Slowly or rapidly, singularly or repetitively, spontaneously or reflexively, the embryo continues to practice the movements begun earlier and to move in new ways. Frequently, hands will touch the face and the head will turn. The many muscles of the face are now largely well developed in preparation for the complex facial expressions to follow. Touching the embryo can produce squinting, jaw movement, grasping motions, and toe pointing.
The earliest sign of right- or left-handedness begins around eight weeks, with 75 percent of embryos already exhibiting right arm dominance. Left hand dominance and no preference comprise the other 25 percent.
The diaphragm muscle is completely formed by eight weeks and intermittent breathing motions begin.
8 weeks marks the end of the embryonic period. During this time, the human embryo has grown from a single cell into nearly 1 billion cells forming over 4000 distinct anatomic structures. The embryo now possesses more than 90 percent of the structures found in the adult.
Week 8-9 of Life
By 8 weeks the developing human measures about a ½ inch from head to rump.
By the beginning of the fetal period, synapses, or connections between nerve cells, are found within the cerebral cortex. The fetal period continues until birth.
By 9 weeks thumb sucking begins and the fetus may swallow amniotic fluid. The fetus can also grasp an object, move the head forward and back, open and close the jaw, move the tongue, sigh, and stretch.
By 9 weeks the fetus will grasp an object, an action we continue to see in newborns and small children exploring the world about them.
By 9 weeks, the nerve receptors in the face, palms of the hands, and soles of the feet can sense and respond to light touch. Following a light touch on the sole of the foot, the fetus will bend the hip and knee and may curl the toes.
Similar to a child having his foot tickled, the 9-week fetus will withdraw if the sole of the foot is lightly touched.
Long before birth, the female fetus is already developing reproductive cells which may one day help form yet another generation.
The eyelids fuse completely shut by 9 weeks.
The peristalsis previously restricted to the large intestine begins in the small intestine.
In female fetuses the uterus is identifiable by 9 weeks Outside the body, the genitalia begin to differentiate as either male or female.
Week 9-10 of Life
The fetus experiences a huge burst of growth between 9 and 10 weeks as weight increases approximately 77%.
The corpus callosum, which connects the left and right sides of the brain, begins to develop at this time.
Upon stimulation of the upper eyelid, the eyes roll downward. This marks the first sign of eye movement. Vocal cords are developing. The fetus yawns and often opens and closes the jaw as early as 9½ weeks. Most fetuses suck their thumbs (usually preferring the right thumb).
At the ends of the fingers and toes, nails begin to emerge, as do fingerprints. These patterns can be used for identification throughout life.
Week 10-11 of Life
By 11 weeks, the nose and lips are completely formed. The fetus can now produce complex facial expressions and, according to one group of researchers, is capable of smiling. As with every other body part, facial appearance will change at each stage of the life cycle
Though gender is determined at fertilization, genitalia now distinguish male from female.
Week 11-12 of Life
Between 11 and 12 weeks, a second huge burst of growth occurs as weight increases by roughly 58%. 12 weeks also marks the end of the first trimester of pregnancy.
The taste bud cells that appeared by 7 weeks have matured into discrete taste buds, but are still scattered throughout the mouth. By birth, the taste buds will be confined to the tongue and palate or roof of the mouth.
By 12 weeks, the arms have grown to approximate their final proportion relative to body size. As usual, leg development is slower than arm development and the legs still have not achieved their final proportion.
The whole body surface, except the top of the head and the back, responds to light touch.
Months 3-4 of Life
The 14-week fetus weighs about 2 ounces and measures slightly less than 5 inches from head to heel.
A division between the oral and nasal cavities forms this week as the opening in the roof of the mouth fuses shut. Fetuses often touch their mouths, sometimes up to 50 times per hour. The jaw opens reflexively in response to pressure at the base of the thumb.
Gender dependent developmental differences appear for the first time starting at 14 weeks. For instance, female fetuses exhibit mouth movement more frequently than males and this difference increases with advancing age. In contrast to the withdrawal response seen earlier, stimulation near the mouth now evokes a turn toward the stimulus while the fetus opens the mouth. This reflex mimics the rooting reflex which persists after birth to help the newborn find his or her mother’s nipple during breastfeeding.
Although movement begins in the six week embryo, a pregnant woman first senses fetal movement between 14 and 18 weeks. Traditionally, this event has been called quickening.
A fetus can feel pain! By 16 weeks, the fetus produces many of the same hormones found in adults. Painful procedures trigger a hormonal stress response. As in newborns and adults, pain is followed by the release of cortisol (kor’ti’sol), ß-endorphins (ba’ta en’dor-finz), and norepinephrine (nor’ep’i-nef’rin) into the blood stream.
By 15 weeks stem cells arrive and multiply in the bone marrow. Most blood cell formation will eventually occur here. In the respiratory system, the bronchial tree is now nearly complete. All airways contain smooth muscle and nerve bundles.
Month 4-5 of Life
The 16-week fetus weighs about 4 ounces and measures slightly less than 7 inches from head to heel.
By 19 weeks, more than 20 million heartbeats have occurred.
The 18-week fetus weighs around 6 ounces and measures about 8 inches from head to heel.
The 20-week fetus weighs about 9 ounces and measures about 10 inches from head to heel.
In the eye, the retina now has discrete layers. The vast majority of neuron multiplication in the brain is complete by 16 weeks. In the teeth, enamel begins to develop between 16 and 20 weeks. Beginning at 18 weeks, ultrasound scans show a distinct type of motion in the fetal voice box, or larynx (lar’ingks), similar to movements made during speaking.
From 18 to 20 weeks, fetal movement, breathing activity, and heart rate begin to follow daily cycles called circadian (ser-ka’de-an) rhythms; the same rhythms that characterize various biological activities throughout life.
A protective white substance, called vernix caseosa (ver’niks caseo’sa), now encases the fetus, protecting the skin from exposure to amniotic fluid.
By about 19 weeks, the number of oogonia within the ovaries of a female fetus peaks at approximately 7 million. From this point, not only does oogonia production end forever, but their numbers decrease to about 2 million by birth. These oogonia give rise to several thousand primary oocytes.
The inner ear, fully formed by 20 weeks, contributes to the body’s ability to maintain balance.
The cochlea is the frequency analyzer of the ear and converts sound waves of varying frequencies into electrical impulses which are then communicated to the brain. By 20 weeks it reaches adult size within the fully developed inner ear.
From now on, the fetus will respond to a growing medley of sounds.
Month 5-6 of Life
With specialized medical care some fetuses can survive outside the womb by 22 weeks with survival rates reported as high as 40% in some medical centers.
The 22-week fetus weighs slightly less than 1 pound and measures about 11 inches from head to heel.
In the growing fetus, the 2 sides of the brain begin to differentiate asymmetrically as early as 20 weeks.
The structure of the fetal gastrointestinal tract has developed to approximate that of the newborn although full function is still weeks away.
Rapid eye movement, or REM, a term describing the distinct pattern of eye motions seen during the stage of sleep where dreaming occurs,3 begins between 18 and 21 weeks.
By 24 weeks, the eyelids reopen and the fetus exhibits a blink-startle response. This reaction to sudden, loud noises typically develops earlier in the female fetus. This response is very much like what adults and children exhibit in the same situation.
When exposed to a loud noise, the fetal heart rate increases, as does the rate of movement. Excessive fetal swallowing following exposure to loud noise may lead to a loss of amniotic fluid. Possible long-term consequences for fetuses are the same as consequences for children and adults: hearing loss and deafness.1
The fetus also responds to pressure, movement, pain, hot and cold, taste, and light.
Month 6-7 of Life
The 24-week fetus weighs about 1¼ pounds and measures about 12 inches from head to heel.
By 24 weeks more than 30 million heartbeats have occurred.
24-week fetuses have impressive lung development as primitive gas exchange now becomes possible in the event of premature birth. Terminal sacs now appear which will eventually become alveoli. Alveoli become the site for the oxygen/carbon dioxide exchange and are essential for survival outside of the womb.
By 24 weeks the lung cells begin to produce a substance called surfactant, a substance absolutely necessary for successful gas exchange. The absence of this substance is often a limiting factor in the viability of premature newborns, as its absence precludes successful breathing. Neonatologists, or doctors specializing in the care of newborns, can introduce a drug form of surfactant to the lungs of premature babies, stretching viability, or the age at which survival outside the womb becomes possible, even farther back in pregnancy.
By 25 weeks, breathing motions may occur up to 44 times per minute.
By 26 weeks sudden, loud noises may trigger a blink-startle response, which may increase movement, heart rate, and swallowing.
The 26-week fetus weighs almost 2 pounds and measures about 14 inches from head to heel.
At this point, the brain takes off on a growth spurt of its own. This brain growth consumes more than 50 percent of the energy used by the fetus and results in a brain weight increase between 400 and 500 percent.
The fetus has developed a relatively plump, less wrinkled appearance due to increased deposits of body fat underneath the skin.
Increasingly sensitive to different sound frequencies, the fetus is now able to hear many new noises with greater accuracy.
A sweet substance placed in the amniotic fluid increases the rate of fetal swallowing. In contrast, decreased fetal swallowing follows the introduction of a bitter substance.11 Altered facial expressions often follow. A pregnant woman’s dietary intake can reach the fetus rather quickly. For instance, amniotic fluid assumes the odor of garlic within 45 minutes of ingestion by pregnant women.
Month 7-8 of Life
By 27 weeks the thigh bones and the foot bones are each about two inches long (about 5 cm).
By 28 weeks the sense of smell is functioning and eyes produce tears.
The 28-week fetus weighs more than 2½ pounds and measures about 15 inches from head to heel.
Each day, the adrenal glands produce large amounts of steroid products. The adrenal glands have doubled in size since week 20, and will double again before the end of pregnancy.
Breathing movements become more common, occurring 30 – 40% of the time in an average fetus.
The 30-week fetus weighs about 3¼ pounds and measures about 16 inches from head to heel.
By 31 weeks more than 40 million heartbeats have occurred. Wrinkles in the skin are disappearing as more and more fat deposits are formed.
The 32-week fetus weighs about 4 pounds and measures about 17 inches from head to heel.
Starting at 32 weeks, true alveoli (al-ve’o-li), or air “pocket” cells, begin forming from alveolar ducts. Development of alveoli continues through birth and until about 8 years of age.
Studies suggest that towards the end of prenatal development, the fetus has been developing preferences and tastes based on prenatal experience. For instance, fetuses whose mothers consumed anise, the substance which gives licorice candy its flavor, showed a preference for anise after birth. Newborns without this fetal exposure disliked anise.
Are you a black jelly bean fan? If you like the licorice-flavored sweets, chances are your mother probably ate some kind of anise during pregnancy.
Month 8-9 of Life
The 34-week fetus weighs about 5 pounds and measures about 18 inches from head to heel.
The fetus hears numerous sounds before birth, with the mother’s voice and heartbeat dominating other sounds. Studies show that after months of listening to the mother’s voice, the newborn prefers her voice to any other. The newborn also prefers female voices to male voices and familiar lullabies heard before birth to new lullabies after birth. Newborns can distinguish prose passages heard during the last 6 weeks of pregnancy from new passages, providing additional evidence of in utero memory formation and learning.
The digestive system further develops as the lower esophageal (e-sof’a-je-al) sphincter (sfingk’ter), a valve leading to the stomach, begins functioning by 32 weeks.
Blood-filtering groups of capillaries called glomeruli (glo-mar’yu-li) have completed their formation in the kidneys.
At 35 weeks, the fetus has a firm hand grasp.
Month 9 – Delivery
During the last 11 weeks of pregnancy, the fetus typically doubles in overall weight, while brain weight doubles in the last 9 weeks of pregnancy.
At term, the umbilical cord is typically 20 to 24 inches long.
At full-term birth, newborn babies typically weigh between 6 and 9 pounds and measure between 18 and 21 inches from head to heel.
The eye of the full-term fetus or newborn has achieved roughly 75% of its adult size and about two-thirds its adult weight.
Labor is initiated by the fetus, ideally around 40 weeks, leading to childbirth. Powerful contractions of the uterus result in childbirth and the expulsion of the placenta, which now weighs about 1/6th as much as the newborn. At birth, the oxygen received through the placenta is suddenly replaced by the onset of air breathing
From fertilization to birth and beyond, human development is dynamic, continuous, and complex. New discoveries about this fascinating process increasingly show the vital impact of fetal development on lifelong health. As our understanding of early human development advances, so too will our ability to enhance health – both before and after birth.
Information obtained from The Endowment for Human Development : http://www.ehd.org/index.php