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Fetal Period: The Ninth Week to Birth
Development during the fetal period is concerned primarily with body growth and differentiation of tissues, organs, and systems. Rudimentary organ systems were formed during the embryonic period. The rate of body growth during the fetal period is rapid, and fetal weight gain is phenomenal during the terminal weeks ( Table 7.1 ). Ultrasonographic measurements of the crown – rump length (CRL) can be used to determine fetal size and probable age ( Fig. 7.1 ). The intrauterine period may be divided into days, weeks, or months ( Table 7.2 ), but confusion arises if it is not stated whether the age is calculated from the last normal menstrual period (LNMP) or from the fertilization age . Unless otherwise stated, fetal age in this book is calculated from the estimated time of fertilization and months refer to calendar months. Clinically, the gestational period is divided into three trimesters , each lasting 3 months. Various measurements and external characteristics are useful for estimating fetal age (see Table 7.1 ). Measurement of the CRL is the method of choice for estimating fetal age until the end of the first trimester.
Table 7.1
Criteria for Estimating Fertilization Age During the Fetal Period
| Age (Weeks) | Crown–Rump Length (mm) * | Foot Length (mm) * | Fetal Weight (g) † | Main External Characteristics |
|---|---|---|---|---|
| Previable Fetus | ||||
| 9 | 50 | 7 | 8 | Eyelids are closing or have closed. Head is rounded. External genitalia are still not distinguishable as male or female. Intestinal herniation is present. |
| 10 | 61 | 9 | 14 | Intestine is in the abdomen. Early fingernail development. |
| 12 | 87 | 14 | 45 | Sex is distinguishable externally. Well-defined neck. |
| 14 | 120 | 20 | 110 | Head is erect. Lower limbs are well developed. Early toenail development. |
| 16 | 140 | 27 | 200 | Auricles of the ears stand out from the head. |
| 18 | 160 | 33 | 320 | Vernix caseosa covers the skin. Fetal movement (quickening) is felt by the mother. |
| 20 | 190 | 39 | 460 | Head and body hair (lanugo) are visible. |
| Viable Fetus ‡ | ||||
| 22 | 210 | 45 | 630 | Skin is wrinkled and red. |
| 24 | 230 | 50 | 820 | Fingernails are present. Lean body. |
| 26 | 250 | 55 | 1000 | Eyes are partially open. Eyelashes are present. |
| 28 | 270 | 59 | 1300 | Eyes are open. Most fetuses have scalp hair. Skin is slightly wrinkled. |
| 30 | 280 | 63 | 1700 | Toenails are present. Body is filling out. Testes are descending. |
| 32 | 300 | 68 | 2100 | Fingernails extend to fingertips. Skin is smooth. |
| 36 | 340 | 79 | 2900 | Body is usually plump. Lanugo is almost absent. Toenails extend to the toe tips. Flexed limb; firm grasp. |
| 38 | 360 | 83 | 3400 | Prominent chest; breasts protrude. Testes in the scrotum or palpable in the inguinal canals. Fingernails extend beyond fingertips. |
* These measurements are averages, and dimensional variations increase with age.
† These weights refer to fetuses that have been fixed for approximately 2 weeks in 10% formalin. Fresh specimens usually weigh approximately 5% less.
‡ There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable or beyond which survival is ensured, but experience has shown that it is uncommon for an infant to survive if its weight is less than 500 g or if its fertilization age or developmental age is less than 22 weeks.
Endovaginal scan of a 9-week fetus with a crown–rump length of 41.7 mm (calipers). Chorionic cavity (CC) has low-level echoes normally, whereas the amniotic cavity (AC) is echo free.
(Courtesy E. A. Lyons, MD, Professor of Radiology, and Obstetrics and Gynecology, and Anatomy, University of Manitoba, Health Sciences Centre, Winnipeg, Manitoba, Canada.)
Table 7.2
Comparison of Gestational Time Units
| Reference Point | Calendar | Lunar | ||
|---|---|---|---|---|
| Days | Weeks | Months | Months | |
| Fertilization | 266 | 38 | 8.75 | 9.5 |
| Last normal menstrual period | 280 | 40 | 9.25 | 10 |
Highlights of Fetal Period
There is no formal staging system for the fetal period; however, it is helpful to consider the main changes that occur during the period of 9 to 38 weeks.
Nine to Twelve Weeks
At the beginning of the ninth week, the head constitutes half of the CRL of the fetus (see Fig. 7.1 ). Subsequently, growth in body length accelerates rapidly so that, by the end of 12 weeks, the CRL has almost doubled (see Table 7.1 ).
At 9 weeks, the face is broad, the eyes are widely separated, the ears are low set, and the eyelids are fused. Early in the ninth week, the legs are short and the thighs are relatively small. By the end of 12 weeks, the upper limbs have almost reached their final relative lengths, but the lower limbs are still slightly shorter than their final relative lengths.
The external genitalia of males and females are not fully developed until the end of the 12th week. Intestinal coils are clearly visible in the proximal end of the umbilical cord until the middle of the 10th week. By the 11th week, the intestines have returned to the abdomen ( Fig. 7.2 ). Urine formation begins between the 9th and 12th weeks, and urine is discharged through the urethra into the amniotic fluid. The fetus reabsorbs some of this fluid after swallowing it. Fetal waste products in blood are transferred to the maternal circulation by passing across the placental membrane (see Chapter 8 ).
Thirteen to Sixteen Weeks
Growth is very rapid during this period ( Figs. 7.3 and 7.4 ; see Table 7.1 ). By 16 weeks, the head is relatively small compared with that of the 12-week fetus and the lower limbs have lengthened. Limb movements, which first occur at the end of the embryonic period, become coordinated by the 14th week, but they are too slight to be felt by the mother. However, these movements are visible during ultrasonographic examinations.
By the beginning of the 16th week, the developing bones are clearly visible on ultrasound images. Slow eye movements occur at 14 weeks. Scalp hair patterning is also determined during this period. By 16 weeks, the ovaries are differentiated and contain primordial ovarian follicles that have oogonia (primordial germ cells). The eyes face anteriorly rather than anterolaterally.
Seventeen to Twenty Weeks
Growth slows down during this period, but the fetus still increases its CRL by approximately 50 mm (see Figs. 7.3 and 7.5 ; see Table 7.1 ). Fetal movements — quickening —are commonly felt by the mother. The skin is now covered with a greasy material— vernix caseosa . It consists of dead epidermal cells and a fatty secretion from the fetal sebaceous glands. The vernix caseosa protects the delicate fetal skin from abrasions, chapping, and hardening that could result from exposure to the amniotic fluid. Fetuses are usually completely covered with fine, downy hair— lanugo —that helps hold the vernix on the skin.
Eyebrows and head hair are also visible. Brown fat forms during weeks 17 through 20 and is the site of heat production, particularly in the neonate. This specialized adipose tissue, found chiefly at the neck, posterior to the sternum, produces heat by oxidizing fatty acids.
By 18 weeks, the fetal uterus is formed and canalization of the vagina has begun. By 20 weeks, the testes have begun to descend but they are still located on the posterior abdominal wall.
Twenty-One to Twenty-Five Weeks
Substantial weight gain occurs during this period, and the fetus is better proportioned. The skin is usually wrinkled and more translucent. The skin is pink to red because blood is visible in the capillaries. At 21 weeks, rapid eye movements begin, and blink-startle responses have been reported at 22 to 23 weeks. Fingernails are present by 24 weeks. Also by 24 weeks, the secretory epithelial cells (type II pneumocytes) in the interalveolar walls of the lung have begun to secrete surfactant, a surface-active lipid that maintains the patency of the developing alveoli of the lungs (see Chapter 11 ). A 22- to 25-week fetus born prematurely may survive initially if given intensive care support; however, the fetus may die because its respiratory system is still immature. Fetuses born before 26 weeks of gestation have a high risk of neurodevelopmental (functional) disability.
Twenty-Six to Twenty-Nine Weeks
During this period, fetuses usually survive if born prematurely and given intensive care because the lungs have developed sufficiently to provide adequate gas exchange. In addition, the central nervous system has matured to the stage at which it can direct rhythmic breathing movements and control body temperature. The highest neonatal mortality occurs in low-birth-weight infants weighing 2500 g or less. The eyelids are open at 26 weeks, and lanugo and head hair are well developed. Toenails are visible and considerable subcutaneous fat is now present, smoothing out many of the skin wrinkles.
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