There are several distinguishing features of the transition of an embryo to a fetus. Both of these words are medical terms for a certain stage of development in the womb. There is no abrupt change from the embryo to the fetus. Instead there are cumulative changes that are evolving day to day.
The embryo changes from an amorphous shape to more of a tiny human with all parts. Also all of the major body organs have either appeared or the precursor of them has developed. As the embryo survives the second month in the womb, and is alive at the beginning of the third month, the tiny human is less vulnerable to the effects of disease in the mother, or radiation of the mother, or medicines the mother may have taken. Of course women who are pregnant should never drink alcohol or use drugs! At any stage of the pregnancy the baby in the womb is harmed by alcohol, and many drugs have teratogenic effects.
The term fetus is appropriate after the major organs are either formed entirely or the precursor of them is present e.g. for the kidney. The fetus is still vulnerable but not as much so, since the tiny human has a larger body and therefore it would take larger doses of medicines through the placenta to harm the tiny being, and the tiny baby also has more developed organs to filter the medicines that might pass to the baby through the placenta. At the end of the first month after fertilization the human embryo is about a quarter of an inch in length from the top of the flat bulbous like head to the end of the emerging legs that look at first like a tail. Most of the organs are beginning to take shape as the cells continuously divide and differentiate.
The tiny human develops by a 'blue print' that is built into the chromosones and genes that the tiny human received from the mother and from the father. Twenty-three chromosones come from each parent in most instances. In Down's syndrome the chromosones are different. The baby's brain develops first since it is needed first. The wisdom of GOD in creating humans is obvious if you really study human anatomy and physiology much. The outlines of eyes appear and a primitive mouth and by 30 days after fertilization the three primary parts of the brain are present , and the eyes, ears, and nasal passages have begun to develop. The beginnings of the backbone is present but that first month the tiny embryo is sort of hunched together in a C shape. There is no discernible arms or legs yet but the "tail" will become legs. The heart will have started beating even though it is not completely formed. The tiny human has a closed circulatory system that includes the baby's own blood vessels and also the umbilical cord and placenta to the mother. The digestive tract opens from the mouth downward for the first time on about the 28 th day after fertilization by the male sperm. The liver appears by about the 21 st day after fertilization. The thyroid gland begins to form. The stomach, intestines,liver and pancreas are there but there is no anus yet. The lungs are not developed yet. Lung buds begin about day 27 and the trachea [windpipe ] appears a few days after that.
By the time the tiny being would be called a fetus he or she would have gained weight to one gram or about one- thirtieth of an ounce, which is not as much as an aspirin tablet but for the tiny human it is significant progress since it means a 50-fold gain in body mass gained during the second month over again what the tiny human weighed at the end of four weeks after fertilization. By day 31 muscles appear in the pelvis and arms and legs begin to form and look sort of like a bud on a tree in bloom.The valve separating the chambers of the heart appear on day 31 or soon after. The germ cells [sex determining] move along the mesoderm toward the genital ridges and will form either ovaries or testicles. By day 32 the caudal muscles appear [in what becomes the baby's behind] and the last pair of gill- like arches that will develop into lungs appears. By day 33 the cerebral cortex in the brain develops. By day 34 more of the muscles that will become the thighs and legs develop and the stalk that separates the baby from the yolk sac lengthens. By day 35 the primitive germ cells have arrived in large numbers near the ridge that develops into the kidney. And the olfactory part of the brain that regulates small is present in the brain. By the fifth week the tiny human is 8 millimeters or one- third of an inch long from crown to rump and weighs about 1/1000 of an ounce. The jaws are forming and the face is beginning to look like a human face. The umbilical cord is present and is the only part connected to the placenta. The pituitary gland is forming in the middle of the brain. The pharynx branches into the two parts that will go to each lung. The membranes [peritoneum] that separate the intestines form the chest have formed and separate the lungs,heart and trachea from the stomach and intestine. By day 36 the arm and legs buds appear, and the 'tail' starts to shrink and the baby may start moving about. By day 37 the intestines have grown enough that they are beginning to curve into their adult shape. The brain stem that controls respiration is recognizable. By day 38 the jaws of upper and lower begin to look symmetrical and with an mouth between them and the eye muscles have begun to form in the orbits. Hands that look more like paddles form and then when the fingers elongate they look more like hands. The insides of the baby have grown so fast that they sort of bulge and could be seen easily through the almost transparent skin of the baby. By day 39 the nerve fibers are present to connect the sense of smell of the olfactory nerves. By day 40 the pigment [color] of the eyes could be seen through the transparent skin also. The jaws are well formed and the teeth, and facial muscles are forming also. The gill arches disappear and the diaphragm appears that moves up and down in respirations though the baby does not breathe in the womb but later sort of practices that effort. The liver starts producing red blood cells and takes over from the yolk sac. By day 42 the reflexes being and fingers and toes are more visible and the beginnings of gonads are visible. By the sixth week the fetus is 13 millimeters or half an inch long from crown to rump. Cartilage that will become bone appears. The head seems huge in relation to the rest of the body and bends forward onto the chest since baby is still sort of a C shape. Fingers appear. And the mammary glands begin. The stomach and intestines rotate into position, muscles lengthen and both cartilage and bone are visible in the forming skeleton. The kidney has begun with the pronephros the first but non-functioning kidneys. By day 44 the retinas of the eyes form, and also the palate of the mouth and and the canals of the ear. By day 46 the gonads are formed either ovaries or testicles so the sex of the baby. Viola! by day 48 after fertilization the tiny human looks like a human! And is definitely a fetus.
/s/ Gloria Poole, R.N. with reference from medical textbook "From Conception to Birth' by Roberts Rugh and Landrum B Shettles MDs.
You can read more about me Gloria Poole RN & artist at this about me page:
Gloria Poole, RN & artist, writer; blogger; citizen journalist;photographer [for the glory of GOD] This document is owned by, and written by me Gloria Poole,RN for Publishing Life. I am a white, single, woman, and Christian since age seven.
Also, for the record: I have the original sketches that I drew to illustrate medical facts of stages of human development. I had added them to a blog I owned [in about 2008-09]. that blog with my content on it was broken into by cyber-criminals who stole the photos I took of my sketches I drew, and they stole my domains on that hosting account & tried to ruin my life. FYI.
Gloria Poole,RN 7 artist; at my apt in Missouri; 19-Jan-2013; /@ 1:10pm.
Also, the woman with the hat that I think shows to public on this blog and on other blogs of mine is me Gloria Poole. FYI