At the beginning of the ninth week of development, the developing offspring has a distinctively human appearance and is now referred to as a fetus.
|Weeks of Development||Changes|
|5-8 Weeks||Recognizable human shape; head as large as the body; eyes far apart; upper and lower limbs present with digits; heart possesses four chambers; tail disappears; formed elements produced by liver; organ systems present in rudimentary form|
|9-12 Weeks||Head is half the length of the body; nails form on digits; brain enlarges; eyes nearly fully developed but still far apart and eyelids still fused; ears formed but low set; nose bridge forms; upper limbs almost fully lengthened; heartbeat detectable; gender distinguishable by external genitalia; heartbeat detectable but not with stethoscope; ossification begins; fetus moves but not detectable by mother|
|13-16 Weeks||Body is larger than head; eyes and ears reach characteristic positions; facial features well developed; lips exhibit sucking movements; bones distinct; lower limbs lengthen; kidneys well formed; meconium (fetal feces) begins to form in intestines; heartbeat detectable by stethoscope|
|17-20 Weeks||Head more proportional to body; lanugo and vernix caseosa cover body; eyebrows, eyelashes, and head hair present; lower limbs fully lengthened; brown fat forms for future heat production; fetus assumes fetal position due to limited space; fetal movements felt by mother|
|21-25 Weeks||Rapid increase in weight gain; skin pink and wrinkled; surfactant formed by lungs|
|26-29 Weeks||Eyes open; body is lean; head and body proportional; skin still wrinkled and pinkish; subcutaneous tissue begins to form; testes begin to descend towards scrotum in males; red bone marrow begins formed element production; organ systems continue to develop|
|30-34 Weeks||Skin pinkish and smoother due to subcutaneous tissue deposition; testes continue to descend in males; bones still continuing to ossify; fetus assumes “head down’ position|
|35-38 Weeks||More subcutaneous tissue is deposited; skin smoother and pinkish because melanin is not produced until skin is exposed to light; testes located within scrotum in males; lanugo is shed; vernix caseosa still present; skull bones largely ossified except at fontanelles; body larger than head|
At the start of the fetal stage, the head is as large as the body and all rudimentary organs are present. The ossification of bones begins during weeks nine through twelve and organs continue to develop. By the twelfth week, the fetus weighs about 45 grams and is about the size of a candy bar.
During the thirteenth through sixteenth weeks, the eyes and ears reach their final positions and a heartbeat may be detected with a stethoscope. During the seventeenth through twentieth weeks, fine hair (lanugo) covers the body and hair appears on the scalp. Sebum from sebaceous glands and dead epidermal cells form the vernix caseosa, which protects the skin from the digestive and urinary wastes in the surrounding amniotic fluid. Movements of the fetus may now be detectable by the mother. By the end of the twentieth week, the fetus weighs about 460 grams, or approximately one pound.
Continued growth of organ systems during the twenty-first through twenty-ninth weeks enables the head, body, and limbs to attain infant proportions. However, the fetus is still lean, with wrinkled, rather translucent skin. By the twenty-ninth week, the fetus weighs about 1,300 grams, or approximately 3 pounds.
Deposition of adipose tissue in the subcutaneous tis sue plus continued growth and development of organ systems, as well as the descent of the testes in males, occur in the eighth and ninth months. By the time the fetus is full term, the lanugo has been shed, the skin is pinkish with ample subcutaneous tissue, hair covers the scalp, and all organ systems are ready for birth. The fetus weighs about 3,400 grams or about 7.5 pounds.