Human pregnancy lasts for about 38 weeks from the time of fertilization, or conception, until delivery, or 40 weeks from the date of the last menstrual period.
During the first eight weeks after conception, the developing human is called an embryo, which means “grows from within.” This period, called the embryonic period , is characterized by the formation of most of the body’s systems.
Embryonic development: The first 4 weeks
Fertilization : From a biological point of view, “human development begins with conception,” when women and men each combine 23 of their own chromosomes by joining their reproductive cells.
A woman’s reproductive cell is called an “egg” and is also called an oocyte. The male reproductive cell is the sperm.
Following the release of an egg from a woman’s ovary through a process called ovulation, the egg and sperm unite within one of the uterine tubes that connect the ovaries to the uterus or womb, also known as the fallopian tubes.
The resulting single-celled embryo is called a zygote, which means “mated or united with another.”
DNA: The 46 chromosomes of the zygote represent the completely unique first edition of the complete genetic blueprint of a new individual. This master plane is found in tightly spiral molecules called DNA. They contain the instructions for the development of the whole body.
Cell division: approximately 24 to 30 hours after conception, the zygote completes its first cell division. Through the process of mitosis, a cell divides into two, two into four, and so on.
Early pregnancy factor: From 24 to 48 hours after conception, pregnancy can already be confirmed by detecting a hormone called “early pregnancy factor” in maternal blood.
Initial stages (morula and blastocyst) and stem cells: After 3 or 4 days from conception, the dividing cells of the embryo have assumed a spherical shape and the embryo is then called a morula.
By day 4 or 5, a cavity has formed within this ball of cells and then the embryo is called a blastocyst.
The cells inside the blastocyst are called the inner cell mass and give rise to the head, body, and other structures vital to human development.
The cells within the inner cell mass are called embryonic stem cells because they possess the ability to form each of the more than 200 types of cells in the human body.
Week 1 to 1½ of pregnancy
Implantation and human chorionic gonadotropin (HCG)
After going down the uterine tube, the embryo implants within the inner wall of the uterus. This process, called implantation, begins 6 days after conception and ends 10-12 days after conception.
The cells of the growing embryo begin to produce a hormone called human chorionic gonadotropin, or hCG, the substance detected in most pregnancy tests.
HCG signals the mother’s hormones to interrupt the normal menstrual cycle, to allow the pregnancy to continue.
The placenta and umbilical cord
After implantation, cells in the periphery of the blastocyst give rise to part of a structure called the placenta, which serves as an interface between the circulatory systems of the mother and the embryo.
The placenta delivers oxygen, nutrients, hormones, and medications from the mother to the developing embryo; removes all waste products and prevents maternal blood from mixing with the blood of the embryo or fetus.
The placenta also produces hormones and maintains the embryonic or fetal body temperature slightly above the mother’s temperature.
The placenta communicates with the developing human through the blood vessels of the umbilical cord. The life support capacity of the placenta equals that of modern hospital intensive care units.
Nutrition and protection
Within a week, the cells of the inner cell mass have formed two layers called the hypoblast and the epiblast.
The hypoblast gives rise to the yolk sac, which is one of the structures through which the mother provides nutrients to the early-stage embryo.
The cells of the epiblast form a membrane called the amnion, within which the embryo and then the fetus develop until birth.
Week 2 to 4 of pregnancy
Germ layers and organ formation
At about 2 1/2 weeks, the epiblast has formed 3 specialized tissues, or germ layers, called the ectoderm, endoderm, and mesoderm.
- The ectoderm gives rise to numerous structures such as the brain, spinal cord, nerves, skin, nails, and hair.
- The endoderm produces the mucosa of the respiratory and digestive tract, and generates portions of important organs such as the liver and pancreas.
- Mesoderm forms the heart, kidneys, bones, cartilage, muscles, blood cells, and other structures.
At three weeks the brain is dividing into three main sections called the forebrain, midbrain, and rhombencephalon.
The respiratory and digestive systems are also developing.
As the first blood cells appear in the yolk sac, blood vessels form throughout the embryo and the tubular heart emerges.
Almost immediately, the rapidly growing heart folds back on itself and the various chambers begin to form. The circulatory system is the first apparatus, or group of organs in the body, to begin to function.
The folding of the embryo
Between weeks 3-4 after conception, the body plane emerges and the brain, spinal cord, and heart of the embryo can be easily identified along with the yolk sac.
The rapid growth produces a fold of the relatively flat embryo. This process incorporates part of the yolk sac into the lining of the digestive system and forms the cavities of the chest and abdomen of the developing human.
Week 4 of pregnancy:
Amniotic fluid
At 4 weeks the transparent amnion already surrounds the embryo in a fluid-filled bag. This sterile fluid, called amniotic fluid, protects the embryo by preventing it from being injured.
Outlines of the limbs and skin
The development of the upper extremities begins with the appearance of the buttons at 4 weeks.
The skin is transparent at this point because it is only one cell thick.
As the skin thickens, it will no longer be transparent, which means that we will only be able to observe the development of the organs for one more month.
Week 5 of pregnancy
Cerebral hemispheres : Between weeks 4 and 5, the brain continues to grow rapidly and divides into 5 distinct sections. The head is roughly 1/3 of the total size of the embryo.
The cerebral hemispheres appear, gradually becoming the largest parts of the brain.
Functions controlled by the brain hemispheres include thinking, learning, memory, speech, sight, hearing, voluntary movement, and problem solving.
Main airways : In the respiratory system, the right and left bronchus are present, which will later connect the trachea with the lungs.
Liver and kidneys : You can see the huge liver that fills the abdomen next to the beating heart. Permanent kidneys appear at 5 weeks.
Yolk sac and germ cells : The yolk sac contains primitive reproductive cells called germ cells. By five weeks, these germ cells have migrated to the reproductive organs adjacent to the kidneys.
Plates of the hands and cartilage : Also at five weeks, the embryo has developed paddles that will be the hands, and at 5 1/2 weeks cartilage formation has begun.
The left hand and wrist paddle is seen here at 5 weeks and 6 days.
Week 6 of pregnancy
Movement and sensation : The embryo begins to make spontaneous movements and reflexes. This movement is necessary to promote normal neuromuscular development. Touching the mouth area causes the embryo to reflexively withdraw its head.
Formation of the outer ear : The ear begins to take shape.
Blood cells: Blood cells are already forming in the liver where lymphocytes are already present. This type of white blood cell plays a key role in the developing immune system.
Diaphragm and intestines: The diaphragm, the main muscle used in respiration, is practically formed by 6 weeks. A portion of the intestine for now protrudes into the umbilical cord. This normal process, called a physiological hernia, creates space for the other organs that develop in the abdomen.
Hand Plates and Brain Waves : At 6 weeks the hand paddles flatten slightly. Primitive brain waves are recorded at 6 weeks and 2 days.
Nipple formation : The nipples appear on the sides of the trunk a little before reaching their final location on the front of the chest.
Limb development : At 6 1/2 weeks, elbows can be seen, fingers begin to separate, and hand movements can be observed.
Bone formation, called ossification, begins within the clavicle, a shoulder bone, and the bones of the upper and lower jaws.
Week 7 of pregnancy
Hiccups and startle reflex : Hiccups have been observed at 7 weeks. Leg movements and startle reflex are observed.
Heart activity : The 4-chamber heart is largely complete. On average, the heart now beats 167 times per minute.
Cardiac electrical activity recorded at 7 1/2 weeks reveals a waveform similar to that of the adult.
Ovaries and eyes : In women, the ovaries are already identified at 7 weeks.
At 7 1/2 weeks, the pigmented retina of the eye can be easily seen, and the eyelids are beginning a period of rapid growth.
Fingers and toes : The fingers of the hands are already separated and those of the feet are only joined at the base. Now you can put your hands together as well as your feet. The knee joints have also formed.
Week 8 of pregnancy
Brain development : At 8 weeks the embryo’s brain is highly complex, constituting almost half of its total body weight. Growth continues at an extraordinary pace.
Right-handed and left-handed? At 8 weeks, 75% of the embryos prove to be right-handed. Of the rest, half prove to be left-handed and the other half show no preference. This is the first evidence of left- or right-handed behavior.
Roll over : Pediatric textbooks say that the ability to “roll over” appears 10 to 20 weeks after birth. However, this remarkable coordination is seen much earlier in the low-gravity environment of the fluid-filled amniotic sac. Only not having the strength to overcome the greater force of gravity outside the uterus prevents newborns from turning over.
The embryo becomes more physically active during this stage. The movements can be slow or fast, single or repeated, spontaneous or reflex. He turns his head, extends his neck, and touches his face more often.
If the embryo is touched, it squints its eyes, moves its jaw, makes grasping motions, and stretches its toes.
Eyelid fusion: Between weeks 7 and 8, the lower and upper eyelids quickly cover the eyes and partially unite.
Respiratory movement and urination: Although there is no air in the uterus, the embryo makes intermittent respiratory movements for 8 weeks. At this time, the kidneys produce urine, which is excreted in the amniotic fluid. In male embryos, the developing testes begin to produce and release testosterone.
Extremities and skin: The bones, joints, muscles, nerves, and blood vessels of the extremities closely resemble those of adults.
At 8 weeks the epidermis, the outer part of the skin, has transformed into a multi-layered membrane and has lost much of its transparency. Eyebrows grow and hair appears around the mouth.
Via | EHD
Images | Colin Behrens mitosis image on Pixabay, 3-4 week lunar caustic (CC) embryo image on Flickr, Wikipedia 5 week embryo, 6 week embryo The Multi-Dimensional Human Embryo, 7 week EHD embryo. Cover photo – 6-7 week caustic lunar embryo (CC) on Flickr
