Life in the Womb
Life in the Womb: Human Development from Conception to Birth invites you to travel on a journey through the nine months of pregnancy. Peer into a window of the womb as you learn about the stages and milestones that every developing child encounters before birth. Written in easy to understand language, this guide is perfect for learners of every educational level. You choose the level of detail to share as you follow a basic narrative about life before birth. For young and old, women and men, this tool will inspire true wonder in the development of human life in the womb.
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When a sperm penetrates an egg, the DNA from each parent combines to create a unique human being, known now as a zygote. This single cell begins to multiply immediately—from one to two to four to eight to sixteen cells, and so on.
As the cells multiply within the zygote, the earliest stage of the human embryo, it continues to grow. During this time, it is moving slowly through the fallopian tube toward the uterus, where it may implant into the uterine lining. If implantation occurs, the pregnancy will continue unless interrupted. The embryo will take approximately five days to reach the uterus, and it will be known as a blastocyst at this phase. The blastocyst is comprised of 70–100 cells. If implantation does not occur, the blastocyst will pass from the woman’s body during menstruation, resulting in an early pregnancy loss.
Essential Facts
• Under the right conditions and lighting, the human egg may be visible to the naked eye. The egg is about 25 times larger than a single sperm.
• Sperm, which are produced in the testicles, take about 70 days to reach maturity.
• The average number of sperm released during sexual intercourse ranges from 50 to 500 million.
• The tip of the sperm contains enzymes to break through the outer layer of the egg. Once the egg has received a single sperm, it immediately creates a barrier to prevent penetration by additional sperm.
• In addition to sperm, semen contains fructose, enzymes, citric acid, free amino acids, prostaglandin, potassium, and zinc.
Every human being possesses 46 chromosomes in his or her DNA (except for people born with chromosomal abnormalities). Though microscopic, DNA has a structure known as a double helix, as seen in this computergenerated image. The egg and sperm each contain 23 chromosomes—exactly one half of each parent’s DNA. At the moment of fertilization, the 23 chromosomes from the sperm combine with the 23 chromosomes from the egg to create a unique human being with 46 chromosomes. Every detail of the child’s development—sex, eye color, hair color, fingerprints, and more—is determined at this point.
Essential Facts
• The sex of the new human being formed at fertilization is determined solely by the sperm. Every egg carries a female sex chromosome, designated by the letter X. Each sperm carries either a female sex chromosome or a male sex chromosome, designated by the letter Y. If a “Y” sperm fertilizes an egg, the new person will be male—XY. If an “X” sperm fertilizes an egg, the new person will be female—XX.
• Every time that a sperm fertilizes an egg, the genetic material from the two cells combines in a completely new arrangement, which is why siblings with the same biological parents are unique (except for identical twins). However, because each sibling still possesses one half of each parent’s genetic code, there are often shared traits among brothers and sisters.
• Because every baby possesses exactly one half of the mother’s and father’s chromosomes, each baby possesses one quarter of each grandparent’s chromosomes.
• Identical twins are formed when one egg is fertilized by one sperm to create one zygote, which then divides into two separate embryos. These two embryos will develop into genetically identical people because they share the same genetic code.
• In the case of fraternal twins, two eggs must be released during ovulation. Both of these eggs must be fertilized and implant into the uterine wall. Although these twins will gestate and be born at the same time, they will not share the same genetic code. In fact, other than having the same birthday, these siblings will be no different from siblings who happen to be born several years apart.
• DNA is tightly coiled within each cell of the human body. If you were to uncoil all of the DNA in the cells of an adult human being and arrange it in a straight line, it would exceed 63 billion miles in length—long enough to travel back and forth to the sun 340 times!
Most organs and body structures have begun to form, including the brain and spinal cord, the heart, the stomach and intestines, bone tissues, eyes, and ears. Though she is already a mother, the woman does not yet suspect that she is pregnant.
At five weeks’ gestational age, or 21 days from conception, the embryo’s heart will begin beating at a rate near the mother’s, about 75–80 beats per minute (bpm). Within a month, the heart rate will have increased to a gestational peak of 185 bpm.
By the time this person has reached old age (80 years old), her heart will have beaten over 3.2 billion times!
By now, the embryo’s heartbeat can be clearly heard on sonogram, and blood cells are circulating throughout the body. All of the major organ systems are now forming. Basic facial features appear and arms and legs continue to grow. Though unfelt by the mother, the embryo’s body and limbs begin to move.
Essential Facts
• The embryo now possesses over 90 percent of the structures found in adults.
• The umbilical cord is now visible.
• There is reflexive response to touch and primitive brain activity can be measured.
• Ovaries and testicles begin to form.
• Lungs are present.
• Taste buds, tooth buds, and eyelids appear.
• The forehead is large and the external genital organs differentiate into female or male.
• The outer ears have begun to take shape.
For Further Discussion
This is a critical stage because the embryo’s developing organs are susceptible to toxins ingested by the mother. Once she knows she’s pregnant, it’s important for the mother to take good care of herself and her child by avoiding cigarettes and alcohol, eating nutritious foods, and taking prenatal vitamins when possible.
The embryo is now known as a fetus, which in Latin means “young one.” This young one will soon start sucking her thumb. All essential internal organs are formed and functioning
Essential Facts
• The kidneys are producing urine
• The head rounds out and comprises half the size of the fetus
• Bone formation is now underway in most bones
• Eyelids close to protect the eyes
• The genitals are differentiated
• The brain now controls muscle movement, as seen by yawning and sucking
For Further Discussion
One of the functions of the placenta, which is now completely developed, is to provide nutrient-rich, oxygenated blood to the child’s body. This nourishing blood circulates through the vein and arteries in the umbilical cord connecting the child to the placenta.
The baby’s legs and arms are exceptionally thin at this point because body fat has not yet begun to deposit there, but the limbs are moving independently, being controlled by the child’s brain. The umbilical cord, seen here behind the feet, is usually about 50 cm (20 inches) long.
The child can now make a fist with fingers that are fully differentiated. Fingernails can be seen and the child’s skin is almost transparent. His arms have lengthened to be in proportion to the rest of his body. In this photograph, oxygenated blood can be seen in the blood vessels running to the child’s fingertips.
The child’s movements can now be felt by the mother, as the child’s physical activity grows increasingly vigorous. By late in the pregnancy, not only can the child’s movements be felt externally, they can be seen as the growing child twists and turns in an increasingly confined space.
Essential Facts
• The child’s mouth makes sucking motions and she has begun to swallow amniotic fluid. • Tooth development is now underway. • The skin is still mostly transparent. • Swallowing and chest movements are present. • The liver and pancreas have begun to function. • The child’s head and body take their proper proportions. • The heart is now beating 110–180 beats per minute and pumping 23.7 liters (25 quarts) of blood every day
For Further Discussion
Millions of eggs are now growing in the fetal ovaries, and a uterus is present. When this little girl is born, her body will contain all of the eggs that it will ever produce, and they are being produced now—five months before she is even born!
Instead of being curled inward, the child’s head is now more erect than it has been. Her eyes have moved closer to the front of her face, and her ears are close to their final position.
Even though this fetus has been a unique human being since the moment of her conception, a significant outward form of that distinction can now be seen in her unique fingerprints and toe prints. Downy hair known as lanugo can be seen, and vernix, a waxy cream, coats and moisturizes the baby’s skin. The child will likely have established waking and sleeping cycles by this point, and she may have even found a favorite position in which to sleep.
Essential Facts
• Some studies have concluded that the fetus is capable of feeling pain at this stage.
• The skin becomes less transparent as fat begins to deposit.
• The child begins to punch and kick more vigorously, and the mother feels it!
• Eyebrows and eyelashes have appeared.
• Blinking and frowning reflexes have developed.
• The child can now suck her thumb.
• While the lungs have not developed enough to permit survival outside of the womb, breathing-like movements become regular.
• Buffered in amniotic fluid, the child can fully turn from side to side and front to back.
For Further Discussion
Now that the baby’s genitals are mostly formed, an ultrasound can reveal the baby’s sex. And since all body structures and systems are now in place, most of the baby’s energy will go toward gaining weight.
The child’s eyes, eyelids, and eyebrows are now fully formed, but her eyes still lack the pigment that will give them their color. Her skin is deeply wrinkled, and will be until it is filled out with a layer of fat.
The child’s ears can now perceive sounds from outside of the womb, and loud noises may even startle her. And though the mother can’t hear her, the child’s vocal cords are now active. If you were to peek into the womb through ultrasound, you’d be able to see her squinting, smiling, and frowning.
Essential Facts
• Eyes are fully functional.
• Eyebrows and eyelashes are almost fully formed.
• Rapid brain growth continues.
• Lungs are developing rapidly.
• Rapid eye movement, which is associated with dreaming, can be measured.
For Further Discussion
The child has now reached a point where she could survive outside of the womb if given intensive care. The youngest children known to survive preterm birth were not even 22 weeks old; they were born barely halfway through pregnancy!
These are the first pictures taken of a female named Beatrix. These three-dimensional (3D) images were taken with an advanced ultrasound system, which allows mothers and fathers to see their children months before they are born. As she has grown, Beatrix has started to grow cramped in the womb, seen by the toes at her mouth, knees at the chin, and arms crossed over her face.
If you put your ear to a pregnant woman’s abdomen, you may be able to hear the baby’s heart beating. And even though the child’s lungs are not yet fully developed, she would have a good chance of survival if she were born at this stage.
Essential Facts
• Brain-wave patterns are similar to those of a full-term baby.
• Though she won’t cry until she’s born, her eyes can now produce tears.
The child’s brain now controls her “breathing” movements and body temperature.
• Eyelids are opening and closing and the child’s eyes can perceive light.
• The child is getting her exercise through acrobatic kicks and stretches.
• More fat is deposited and the child’s skin is smoothing out, losing its wrinkles.
For Further Discussion
Many babies are in breech position at this stage of pregnancy, which means that they are positioned feet- or bottom-first, instead of the typical head-down birth position. There is still plenty of time for the baby to change position, however, and most babies will rotate within the next few weeks.
The baby’s movements will soon grow less acrobatic as she settles into a head-down birth position. Most of the skin wrinkles will have disappeared from her face. By the time she is born eight weeks from now, her present weight will have more than doubled!
Essential Facts
• Though the lungs are not yet mature, rhythmic “breathing” is occurring.
• Even though the bones are fully developed, they are still pliable and soft.
• The baby’s skin has thickened.
• Nails have grown to the tips of the fingers and toes.
• The baby’s head may now be covered with hair.
For Further Discussion
Some women may notice Braxton Hicks or “practice” contractions at this stage, which are intermittent contractions every 10 to 20 minutes. These contractions may be accompanied by mild discomfort or pain.
The child has probably rotated into birth position by now, with her head pointed downward in the mother’s pelvis, where she’ll stay until she’s born. This shift will likely allow the mother to breathe more easily, but because the baby has settled lower onto the woman’s bladder, she may need to urinate more often.
Essential Facts
• The eyes are open during alert times, and closed during sleep.
• The child will notice light and turn toward a light source. • The child’s outer ears are now fully formed.
• Breast buds are present on baby girls and boys.
• The child can now grasp firmly.
For Further Discussion
The extra fat that the baby is putting on—about half a pound a week—will help her to regulate her body temperature after she is born. Her body weight at birth will be about 15 percent fat.
The journey from conception to birth has been about nine months long, and the baby will be born sometime between weeks 38 and 42. Weight gain will continue rapidly throughout these weeks, and hair and fingernails will continue to grow. Since the moment of her conception, the child has been a being of staggering complexity. For nine months, a unique genetic code, defined when the sperm met the egg, has guided the development of this person. Now she is ready to draw her first breath. Welcome to the world, young one!
Essential Facts
• Breast milk will supply important antibodies to replace those supplied by the placenta before birth.
• The newborn baby’s skull bones have not yet fused, which allows them to compress in the birth canal.
• Boys tend to weigh more than girls at birth.
• The newborn’s eyes are fully formed, but the nerves inside her eyes will continue to develop in the weeks after her birth.
For Further Discussion
A newborn baby has about 300 bones, far more than the 206 bones of an adult. Some of these bones are made of cartilage that will turn to bone and fuse together over the next few years. Bone growth is complete by age 25, after which point the bones are as big as they will ever be.