2nd week 8-14 days:
The umbilical cord, composed of two arteries and one larger vein, surrounded by a thick jelly, has formed and becomes a lifeline transporting nutrients and wastes to and from the embryo and placenta.
3rd week 15-21 days
The heart is formed at 20 days and starts to beat at 21 to 22 days. A primitive intestine a neural tube is also visible at this age. The circulation system for the brain has begun. By the end of the third week, the backbone, spinal column, and nervous system are forming.
4th week 22-28 days:
The embryo may float freely in the uterus for about 48 hours before implanting. Upon implantation, complex connections between the mother and embryo develop to form the placenta. The embryo produces hormones, which stop the mother's menstrual cycle.
The gut track is visual by the end of the fourth week, and a recognized mouth is visible. The brain is forming at a rapid rate. In the fourth week cells of the neural crest migrate throughout the body and form an astounding array of structures, including the sensory and autonomic nerves, pigment cells, and most of the bones and connective tissue and neck. In the head, the earliest recognizable traces of the future eyes and inner ear are readily distinguishable. By the end of the fourth week the unborn baby has a highly functional circulation with three sets of blood vessels.
By day 21, the embryo's tiny heart has begun beating. At 22 days of age the major blood vessels that enter and leave the heart are visualized.
5th week 29-35 days:
The fifth week is characterized by profound changes in almost all organ systems of the human being. The brain becomes subdivided in 5 parts, corresponding to the major divisions of the adult brain, and nerve cells are forming. The eyes have formed a lens, and the nerves in the retina are taking shape. An olfactory placode, the precursor of the organ of smell in the nose is prominent.
At this stage, the embryo is the size of a raisin. The neural tube enlarges into three parts, soon to become a very complex brain. The placenta begins functioning. The spine and spinal cord grows faster than the rest of the body at this stage and give the appearance of a tail. This disappears as the child continues to grow.
At Five weeks, the eye socket is formed. Within five weeks, the hand begins to form as a flipperlike growth from the trunk. The head is disproportionately large at this time. The umbilical cord joins the embryo to the placenta and to the yolk sac, which manufacturer blood cells during the five weekï¿½s of the embryoï¿½s life. A future brain is also detected, as the front of the primitive neural tube is enlarged into three parts that will soon become five.
During the sixth week after fertilization the unborn child can respond to local tactile stimulation by reflex movements. At the end of the sixth week, the unborn child is clearly recognizable as a human being by gross morphological observation. The spread of cutaneous receptors is preceded by the development of synapses between sensory fibers and interneurons in the dorsal horn of the spinal cord, which first appear during the sixth week of gestation. Spontaneous movement begins somewhere in the 6th to 7th week.
Arms and legs are extremely short but starting to take shape.
Brain waves can be detected, recorded and read at approximately 40 days.
7th week 43-49 days:
In the seventh week, facial features are visible, including a mouth and tongue and the eyes have a retina and lens. The major muscle system is developed, and the unborn child practices moving. The child has its own blood type, distinct from the mother's. The liver produces these blood cells now instead of the yolk sac.
By the seventh week a lens forms over the eye as well as a cornea and an iris. You can often see a black circle in the head which is the retina seen through the future lens of the eye. The hand will grow into a web shape. Fingers take shape, grow and will lengthen. Nose, Mouth, and Ears take form. The embryo is about three-quarters of an inch long, 40,000 times larger than where it began.
The liver is developing by this stage and will often show as a dark mass. The yolk sac gradually gives over to the liver the task of forming blood cells. The yolk sac and the little tube that joined it to the embryo will disappear. Now the fetus will produce itï¿½s own blood. It has a network of arteries and veins, oxygen is provided though the blood coming and going between the embryo and the placenta. The circulation system is now perfectly formed.
Somewhere between the second and third month, the fetusï¿½ external genital organs develop and become obvious.
Spontaneous movements are seen shortly after the completion of the seventh week. Cutaneous sensory receptors appear in the perioral area of the human fetus in the 7th week of gestation.
8th week 50-56 days:
During the eighth week the heartbeat is approximately 160 beats per minute. The unborn baby has the ability to open his jaw, move his tongue, and even hiccup (8-10weeks). Movement by the fetus away from cutaneous stimulation and brain activity has been observed in the 8th week. Also at 8 weeks local stimuli may produce the following movements; squinting, opening the mouth, partial finger closure, and plantar flexion of the toes. A basic sensory loop is clearly present.
At eight weeks almost all the organs are almost formed and in place. Outer cells are becoming nerves and deep within the embryo the cells are becoming organs, bones, muscles, and blood vessels. A simple intestinal tube is forming. A rudimentary heart is now formed as well as an eye.
By eight weeks skin begins to fold over the eye creating a lid.
"Years ago, while giving an anesthetic for a ruptured tubal pregnancy (at two months) I was handed what I believed
to be the smallest human being ever seen. The embryo sac was intact and transparent. Within the sac was a tiny human
male, swimming extremely vigorously in the amnionic fluid, while attached to the wall by the umbilical cord. The
tiny human was perfectly developed, with long, tapering fingers, feet and toes. It was almost transparent as
regards to the skin, and the delicate arteries and veins were prominent to the ends of the fingers. The baby
was extremely alive and did not look at all like the photos and drawings of 'embryos' which I have seen."
- Paul E. Rockwell
9th week 57-63 days:
The first detectable brain activity in response to noxious ( or pain stimuli) is elicited in the thalamus of the brain between the 9th and 10th weeks. By nine weeks, a developing fetus can hiccup and react to loud noises.
The fetusï¿½ genitals show a clear sexual differentiation and already contain primitive egg or sperm cells. The fetus now sleeps and awakes, drinks the amniotic fluid for nutrition, and excretes waste.
Around the ninth week of pregnancy, the baby starts making her first movements. Those movements are probably visible with an ultrasound, even though they can't be felt for several more weeks.
10th week 64-70 days:
The heart is almost completely developed and very much resembles that of a newborn baby. An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child's blood is oxygenated through the placenta. Twenty tiny baby teeth are forming in the gums.
Neuronal multiplication occurs mainly from the 10-12 weeks, after which no new nerve cells are formed, though neuronal arborization and the information reorganization of synapses continue with adulthood. General fetal movements appear in the 8th week. More complex movements appear such as: sucking, swallowing, and breathing appear in the 10th to 12th week.
"As early as eight to 10 weeks' gestation,
and definitely by thirteen and a half weeks,
the human fetus experiences organic pain."
Dr. Vincent J. Collins,
a diplomat of the American Board of Anesthesiologists
The embryo is now about 1 inch in length. Facial features, limbs, hands, feet, fingers, and toes become apparent. The nervous system is responsive and many of the internal organs begin to function.
11th week 71-77 days:
The fetus measures about 3 inches. It does not use its lungs yet, oxygen is supplied by the placenta. The umbilical cord, which attaches the fetus to the mother, delivers about one-half pint of blood every minute.
Cutaneous sensory receptors spread to the rest of the face, the palms of the hands, and the soles of the feet by the 11th week
12th week 78-84 days:
Before the end of the first trimester (3-months) the hands are clearly formed and visible. By the third or fourth month the hand can grab and the foot can make small kicks for the mother to feel against the womb.