Scan Image Of A Normal Fetus At 12 Weeks

Chorion Frondosum

Figure 6.1 A 9-week fetus. Note the large size of the head compared with that of the rest of the body. The yolk sac and long vitelline duct are visible in the chorionic cavity. Note the umbilical cord and herniation of intestinal loops. One side of the chorion has many villi (chorion frondosum), while the other side is almost smooth (chorion laeve).

Figure 6.1 A 9-week fetus. Note the large size of the head compared with that of the rest of the body. The yolk sac and long vitelline duct are visible in the chorionic cavity. Note the umbilical cord and herniation of intestinal loops. One side of the chorion has many villi (chorion frondosum), while the other side is almost smooth (chorion laeve).

approximately half of the CRL (Figs. 6.1 and 6.2). By the beginning of the fifth month, the size of the head is about one-third of the CHL, and at birth it is approximately one-fourth of the CHL (Fig. 6.2). Hence, over time, growth of the body accelerates but that of the head slows down.

During the third month the face becomes more human looking (Figs. 6.3 and 6.4). The eyes, initially directed laterally, move to the ventral aspect of the

Normal Head Size For Week Fetus

3rd month 5th month At birth

Figure 6.2 Size of the head in relation to the rest of the body at various stages of development.

3rd month 5th month At birth

Figure 6.2 Size of the head in relation to the rest of the body at various stages of development.

face, and the ears come to lie close to their definitive position at the side of the head (Fig. 6.3). The limbs reach their relative length in comparison with the rest of the body, although the lower limbs are still a little shorter and less well developed than the upper extremities. Primary ossification centers are present in the long bones and skull by the 12th week. Also by the 12th week, external genitalia develop to such a degree that the sex of the fetus can be determined by external examination (ultrasound). During the 6th week intestinal loops cause a large swelling (herniation) in the umbilical cord, but by the 12th week the loops withdraw into the abdominal cavity At the end of the third month, reflex activity can be evoked in aborted fetuses, indicating muscular activity.

During the fourth and fifth months the fetus lengthens rapidly (Fig. 6.5 and Table 6.1), and at the end of the first half of intrauterine life its CRL is approximately 15 cm, that is, about half the total length of the newborn. The weight of the fetus increases little during this period and by the end of the fifth month is still less than 500 g. The fetus is covered with fine hair, called lanugo hair; eyebrows and head hair are also visible. During the fifth month movements of the fetus can be felt by the mother.

During the second half of intrauterine life, weight increases considerably, particularly during the last 2.5 months, when 50% of the full-term weight (approximately 3200 g) is added. During the sixth month, the skin of the fetus is reddish and has a wrinkled appearance because of the lack of underlying connective tissue. A fetus born early in the sixth month has great difficulty

Sixth Month Fetus
Figure 6.3 An 11-week fetus. The umbilical cord still shows a swelling at its base, caused by herniated intestinal loops. Toes are developed, and the sex of the fetus can be seen. The skull of this fetus lacks the normal smooth contours.

surviving. Although several organ systems are able to function, the respiratory system and the central nervous system have not differentiated sufficiently, and coordination between the two systems is not yet well established. By 6.5 to 7 months, the fetus has a length of about 25 cm and weighs approximately 1100 g. If born at this time, the infant has a 90% chance of surviving. Some developmental events occurring during the first 7 months are indicated in Table 6.2.

During the last 2 months, the fetus obtains well-rounded contours as the result of deposition of subcutaneous fat (Fig. 6.6). By the end of intrauterine life, the skin is covered by a whitish, fatty substance (vernix caseosa) composed of secretory products from sebaceous glands.

Week Fetus
Figure 6.4 A 12-week fetus in utero. Note the extremely thin skin and underlying blood vessels. The face has all of the human characteristics, but the ears are still primitive. Movements begin at this time but are usually not felt by the mother.

At the end of the ninth month the skull has the largest circumference of all parts of the body, an important fact with regard to its passage through the birth canal. At the time of birth the weight of a normal fetus is 3000 to 3400 g; its CRL is about 36 cm; and its CHL is about 50 cm. Sexual characteristics are pronounced, and the testes should be in the scrotum.

table 6.2 Developmental Horizons during Fetal Life

Event Age (Weeks)

Tastebuds appear 7

Swallowing 10

Respiratory movements 14-16 Sucking movements 24

Some sounds can be heard 24-26 Eyes sensitive to light* 28

•Recognition of form and color occurs postnatally.

•Recognition of form and color occurs postnatally.

Weight And Pics Week Fetus
Figure 6.5 An 18-week-old fetus connected to the placenta by its umbilical cord. The skin of the fetus is thin because of lack of subcutaneous fat. Note the placenta with its cotyledons and the amnion.

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  • Sian
    Is a thin head normal for 12 week fetus?
    8 years ago

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