Several terms are used to define pregnancy duration, and thus fetal age (Fig. 7-1). Gestational age or menstrual age is the time elapsed since the first day of the last menstrual period, a time that actually precedes conception. This starting time, which is usually about 2 weeks before ovulation and fertilization and nearly 3 weeks before blastocyst implantation, has tradition- ally been used because most women know their last period.
Embryologists describe embryo-fetal development in ovulation age, or the time in days or weeks from ovulation. Another term is postconceptional age, nearly identical to ovulation age.
Clinicians customarily calculate gestational age as menstrual age. Approximately 280 days, or 40 weeks, elapse on average between the first day of the last menstrual period and the birth.
Embryogenesis and Fetal Morphological Development
CHAPTER 7
GESTATIONAL AGE VARIOUSLY DEFINED. . . . 127 EMBRYO-FETAL GROWTH AND DEVELOPMENT. . . . 127 PLACENTAL PHYSIOLOGY AND FETAL GROWTH. . . . 131 FETAL NUTRITION . . . . 133 FETAL ORGAN SYSTEM DEVELOPMENT . . . . 135 DEVELOPMENT OF GENITALIA. . . . 144
128 Placentation, Embryogenesis, and Fetal Development
SECTION 3
inner cell mass—and the remaining 53 cells form the placental ss trophoblast. Details of implantation and early development of the blastocyst and placenta are described in Chapter 5 (p. 88).
■ Embryonic Period
The conceptus is termed an embryo at the beginning of the third week after ovulation and fertilization. Primitive chori- onic villi form, and this coincides with the expected day of menses. The embryonic period lasts 8 weeks, during which organogenesis takes place. The embryonic disc is well defined, and most pregnancy tests that measure human chorionic
gonadotropin (hCG) become positive by this time. As shown inFigures 7-3 and 7-4, the body stalk is now differentiated, and the chorionic sac is approximately 1 cm in diameter.
There are villous cores in which angioblastic chorionic meso- derm can be distinguished and a true intervillous space that contains maternal blood. During the third week, fetal blood vessels in the chorionic villi appear. In the fourth week, a cardiovascular system has formed, and thereby, a true circula- tion is established both within the embryo and between the embryo and the chorionic villi. By the end of the fourth week, the chorionic sac is 2 to 3 cm in diameter, and the embryo is 4 to 5 mm in length (Figs. 7-5and 7-6). Partitioning of the primitive heart begins in the middle of the fourth week.
Arm and leg buds are present, and the amnion is beginning to unsheathe the body stalk, which thereafter becomes the umbilical cord.
At the end of the sixth week, the embryo is 22 to 24 mm long, and the head is large compared with the trunk. The earli- est synapses in the spinal cord develop at 6 to 7 weeks (Kadic, 2012). The heart is completely formed. Fingers and toes are present, and the arms bend at the elbows. The upper lip is complete, and the external ears form definitive elevations on either side of the head. Three-dimensional images and videos of human embryos from the MultiDimensional Human Embryo project are found at: http://embryo.soad.umich.edu/.
Embryonic Period
(Organogenesis) Fetal Period (Growth)
Crown-rump length (cm)
Period: Implantation
2 3
1
Weight (g)
Brain
Weeks 4 5 6 7 8 9 12
6-7
Neural tube
12 16 21 25 28 32
110 320 630 1100 1700 2500
16
5 6 20 24 28 32 36 38
Hemispheres, cerebellum,
ventricles, choroid plexus Temporal lobbe, sulci, gyri, cellular migration, myelinizationn
Lips, tongue, palate, cavitation, fusionn
Canals, cochlea, inner earsoo , ossicles Face
Eyes Ears Pinnae Diaphragm Lungs Heart
Intestines Urinary tract Genitalia
Axial skeleton Limbs Skin
Optic cups, lens, optic nerves, eyelids
Pinnae
Transverse septum, diaphragm Tracheoesophageal septum, bronchi, lobes Primitive tube, great vessels, valves, chambers
Foregut, liver, pancreas, midgut
Glomeruli
Genital folds, phallus, labioscrotal swelling
Vertebral cartilage, ossification centers Buds, rays, webs, separate digits
Vernix Brows
Canaliculi Terminal sacss
Eyes opene
Abdominaal waa all, gut rotationaa
Mesonephric duct Metanephric duct collecting sytem
Fingernails Penissss, urethra, scrotum
Clitooris, labiaoo
Lanugo hair
FIGURE 7-2 Embryo-fetal development according to gestational age determined by the first day of the last menses. Times are approximate.
Menses Ovulation Fertilization Implantation
Gestational age Menstrual age
40 weeks (280 days) Ovulation age
2 weeks
3 weeks
Postconceptional age
FIGURE 7-1 Terminology used to describe the pregnancy duration.
CHAPTER 7
Yolk sac Y Y
br Emb yo
mnion Am
hor C ion
Dev loping villie Boddy stalk
Allaantois
A C
Yolk Y Y sac
Allaantois
Boddy stalk Chorion
Amnion Developing neural groove
B
FIGURE 7-3 Early human embryos. Ovulation ages: A. 19 days (presomite). B. 21 days (7 somites). C. 22 days (17 somites).
(After drawings and models in the Carnegie Institute.)
Chorionic villus
S ti l t h bl t Chorionic villus
Syncytial trophoblast
Primitive streak Amnion
Amnionic cavity Embryonic disc Chorionic membrane Body stalk with
allantoic diverticulum Villus
Extraembryonic coelom
Yolk sac
FIGURE 7-4 Drawing of an 18-day Mateer-Streeter embryo shows the amnionic cavity and its relations to chorion and yolk sac. (Redrawn from Streeter, 1920.)
■ Fetal Period
Transition from the embryonic period to the fetal period is arbitrarily designated by most embryologists to begin 8 weeks after fertilization—or 10 weeks after onset of last menses. At this time, the embryo-fetus is nearly 4 cm long (see Fig. 7-6C).
Development during the fetal period consists of growth and maturation of structures that were formed during the embry- onic period. Crown-to-rump measurements, which correspond to the sitting height, are most accurate for dating (Table 7-1).
12 Gestational Weeks
The uterus usually is just palpable above the symphysis pubis, and the fetal crown-rump length is 6 to 7 cm. Centers of ossi- fication have appeared in most fetal bones, and the fingers and toes have become differentiated. Skin and nails have developed, and scattered rudiments of hair appear. The external genitalia are beginning to show definitive signs of male or female gender.
The fetus begins to make spontaneous movements.
16 Gestational Weeks
The fetal crown-rump length is 12 cm, and the weight is 110 g.
By 14 weeks, gender can be determined by experienced observers by inspection of the external genitalia. Eye move- ments begin at 16 to 18 weeks, coinciding with midbrain maturation.
20 Gestational Weeks
This is the midpoint of pregnancy as estimated from the begin- ning of the last menses. The fetus now weighs somewhat more than 300 g, and weight increases in a linear manner. From this point onward, the fetus moves about every minute and is active 10 to 30 percent of the time (DiPietro, 2005). The fetal skin has become less transparent, a downy lanugo covers its entire body, and some scalp hair has developed. Cochlear function develops between 22 and 25 weeks, and its maturation contin- ues for six months after delivery.
24 Gestational Weeks
The fetus now weighs approximately 630 g. The skin is char- acteristically wrinkled, and fat deposition begins. The head is still comparatively large, and eyebrows and eyelashes are usu- ally recognizable. The canalicular period of lung development, during which the bronchi and bronchioles enlarge and alveolar ducts develop, is nearly completed. A fetus born at this time will attempt to breathe, but many will die because the terminal sacs, required for gas exchange, have not yet formed. By 26 weeks, nociceptors are present over all the body, and the neural pain system is developed (Kadic, 2012).
28 Gestational Weeks
The crown-rump length is approximately 25 cm, and the fetus weighs about 1100 g. The thin skin is red and cov- ered with vernix caseosa. The pupillary membrane has just disappeared from the eyes. Isolated eye blinking peaks at 28 weeks. The otherwise normal neonate born at this age has a 90-percent chance of survival without physical or neurological impairment.
130 Placentation, Embryogenesis, and Fetal Development
SECTION 3
Otic pit Lens placode
Arm bud
Leg bud
E Third
branchial arch Hyoid
arch y Otic pit
Arm bud Leg bud
D D Somites Mandibular arch Heart prominence
C C
Neural fold Rostral neuropore
Neural tube Somites
Caudal neuropore B
Neural fold
Rostral neuropore Neural groove
Neural tube Caudal neuropore Somites
A
Mandibular arch
FIGURE 7-5 Three- to four-week-old embryos. A, B.Dorsal views of embryos during 22 to 23 days of development showing 8 and 12 somites, respectively. C–E.Lateral views of embryos during 24 to 28 days, showing 16, 27, and 33 somites, respectively. (Redrawn from Moore, 1988.)
A B C
FIGURE 7-6 Embryo photographs. A. Dorsal view of an embryo at 24 to 26 days and corresponding to Figure 7-5C. B. Lateral view of an embryo at 28 days and corresponding to Figure 7-5E. C. Lateral view of embryo-fetus at 56 days, which marks the end of the embryonic period and the beginning of the fetal period. The liver is within the fine, white circle. (From Werth, 2002, with permission.)
CHAPTER 7
32 and 36 Gestational Weeks
At 32 weeks, the fetus has attained a crown-rump length of about 28 cm and a weight of approximately 1800 g. The skin surface is still red and wrinkled. In contrast, by 36 weeks, the fetal crown-rump length averages about 32 cm, and the weight is approximately 2500 g. Because of subcutaneous fat depo- sition, the body has become more rotund, and the previous wrinkled facial appearance has been lost.
40 Gestational Weeks
This is considered term from the onset of the last menstrual period. The fetus is now fully developed. The average crown-rump length is about 36 cm, and the weight is approximately 3400 g.