The transformation of an embryo to a fetus is gradual, but the name change is meaningful because it signifies that the primordia of all major systems have formed. Development during the fetal period is primarily concerned with rapid body growth and differentiation of tissues, organs, and systems. A notable change occurring during the fetal period is the relative slowdown in the growth of the head compared with the rest of the body. The rate of body growth during the fetal period is very rapid ( Table 6.1 ), and fetal weight gain is phenomenal during the terminal weeks. Periods of normal continuous growth alternate with prolonged intervals of absent growth.
|Age (Weeks)||Crown–Rump Length (mm) *||Foot Length (mm) *||Fetal Weight (g) †||Main External Characteristics|
|9||50||7||8||Eyelids closing or closed. Head large and more rounded. External genitalia are not distinguishable as male or female. Some of the small intestines are in the proximal part of umbilical cord. The ears are low set.|
|10||61||9||14||Intestines in abdomen. Early fingernail development.|
|12||87||14||45||Sex distinguishable externally. Well-defined neck.|
|14||120||20||110||Head erect. Eyes face anteriorly. Ears are close to their definitive position. Lower limbs well developed. Early toenail development.|
|16||140||27||200||External ears stand out from head.|
|18||160||33||320||Vernix caseosa covers skin. Quickening (first movements) felt by mother.|
|20||190||39||460||Head and body hair (lanugo) visible.|
|Viable Fetuses ‡|
|22||210||45||630||Skin wrinkled, translucent, and pink to red.|
|24||230||50||820||Fingernails present. Lean body.|
|26||250||55||1000||Eyelids partially open. Eyelashes present.|
|28||270||59||1300||Eyes wide open. Considerable scalp hair sometimes present. Skin slightly wrinkled.|
|30||280||63||1700||Toenails present. Body filling out. Testes descending.|
|32||300||68||2100||Fingernails reach fingertips. Skin smooth.|
|36||340||79||2900||Body usually plump. Lanugo (hairs) almost absent. Toenails reach toe tips. Flexed limbs; firm grasp.|
|38||360||83||3400||Prominent chest; breasts protrude. Testes in scrotum or palpable in inguinal canals. Fingernails extend beyond fingertips.|
‡ 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 rare for a baby to survive whose weight is less than 500 g or whose fertilization age is less than 22 weeks. Even fetuses born between 26 and 28 weeks have difficulty surviving, mainly because the respiratory system and the central nervous system are not completely differentiated.
Viability is defined as the ability of fetuses to survive in the extrauterine environment. Fetuses of less than 500 g at birth do not usually survive. In recent years, survival at gestational ages of 22 to 23 weeks has been increasingly reported, blurring the line at which the edge of viability is declared. If given expert postnatal care, many fetuses born at less than 1000 g may survive; such infants are referred to as extremely low-birth-weight infants. Many full-term, low-birth-weight infants result from intrauterine growth restriction (IUGR) . Consequently, if given expert postnatal care, some fetuses weighing less than 500 g may survive. Most fetuses weighing between 750 and 1500 g usually survive, but complications may occur.
Each year, approximately 500,000 preterm infants (<37 weeks) are born in the United States. Many of these infants suffer from severe medical complications or early mortality (death). The use of antenatal steroids and the postnatal administration of endotracheal surfactant have greatly lowered the rates of acute and long-term morbidity. Prematurity is one of the most common causes of morbidity and perinatal death .
Estimation of Fetal Age
Ultrasound measurements of the crown−rump length (CRL) of the fetus are taken to determine its size and probable age and to provide a prediction of the expected date of delivery . Fetal head measurements and femur length are also used to evaluate age. In clinical practice, gestational age is usually timed from the onset of the last normal menstrual period (LNMP) .
In embryology, gestational age based on the LNMP is superfluous because gestation (time of fertilization) does not begin until the oocyte is fertilized, which occurs around the middle of the menstrual cycle. This difference in the use of the term gestational age may be confusing; therefore, it is important that the person ordering the ultrasound examination and the ultrasonographer use the embryologic terminology (see Chapter 1 , Fig. 1.1 ).
The intrauterine period may be divided into days, weeks, or months ( Table 6.2 ), but confusion arises if it is not stated whether the age is calculated from the onset of the LNMP or from the estimated day of fertilization of the oocyte. Uncertainty about age arises when months are used, particularly when it is not stated whether calendar months (28 to 31 days) or lunar months (28 days) are meant. Unless otherwise stated, embryologic or fetal age in this book is calculated from the estimated time of fertilization .
|Reference Point||Days||Weeks||Calendar Months||Lunar Months|
|Last normal menstrual period||280||40||9.25||10|
* The common delivery date rule (Nägele’s rule) for estimating the expected date of delivery is to count back 3 months from the first day of the last normal menstrual period and add a year and 7 days.
Trimesters of Pregnancy
Clinically, the gestational period is divided into three trimesters, each lasting 3 months. By the end of the first trimester, one third of the length of the pregnancy, major systems have been developed (see Table 6.1 ). In the second trimester, the fetus grows sufficiently in size so that good anatomical detail can be visualized during ultrasonography . During this period, most major birth defects can be detected using high-resolution real-time ultrasonography . By the beginning of the third trimester, the fetus may survive if born prematurely. The fetus reaches a major developmental landmark at 35 weeks and weighs approximately 2500 g and usually survives if born prematurely.
Measurements and Characteristics of Fetuses
Various measurements and external characteristics are useful for estimating fetal age (see Table 6.1 ). CRL is the method of choice for estimating fetal age until the end of the first trimester because there is very little variability in fetal size during this period. In the second and third trimesters, several structures can be identified and measured ultrasonographically, but the most common measurements are biparietal diameter (diameter of the head between the two parietal eminences), head circumference , abdominal circumference, femur length, and foot length.
Weight is often a useful criterion for estimating age, but there may be a discrepancy between the age and weight, particularly when the mother has had metabolic disturbances such as diabetes mellitus during pregnancy. In these cases, the weight often exceeds values considered normal for the corresponding CRL. Fetal dimensions obtained from ultrasound measurements closely approximate CRL measurements obtained from spontaneously aborted fetuses. Determination of the size of a fetus, especially its head circumference, is helpful to the obstetrician for management of patients.
Highlights of Fetal Period
There is no formal staging system for the fetal period; however, it is helpful to describe the changes that occur in periods of 4 to 5 weeks.
Nine to Twelve Weeks
At the beginning of the fetal period (ninth week), the head constitutes approximately half of the CRL of the fetus ( Figs. 6.1 and 6.2 A ). Subsequently, growth in body length accelerates rapidly, so that by the end of 12 weeks, the CRL has almost doubled ( Fig. 6.2 B , and see Table 6.1 ). Although growth of the head slows down considerably by this time, the head is still disproportionately large compared with the rest of the body ( Fig. 6.3 ).
At 9 weeks, the face is broad, the eyes are widely separated, the ears are low set, and the eyelids are fused (see Fig. 6.2 B ). By the end of 12 weeks, primary ossification centers appear in the skeleton, especially in the cranium (skull) and long bones. Early in the ninth week, the legs are short, and the thighs are relatively small (see Fig. 6.2 ). By the end of 12 weeks, the upper limbs have almost reached their final relative lengths, but the lower limbs are still not so well developed, and they are slightly shorter than their final relative lengths.
The external genitalia of males and females appear similar until the end of the ninth week. Their mature form is not established until the 12th week. Intestinal coils are clearly visible in the proximal end of the umbilical cord until the middle of the 10th week (see Fig. 6.2 B ). By the 11th week, the intestines have returned to the abdomen (see Fig. 6.3 ).
At 9 weeks, the beginning of the fetal period, the liver is the major site of erythropoiesis (formation of red blood cells). By the end of 12 weeks, this activity has decreased in the liver and has begun in the spleen. Urine formation begins between the 9th and 12th weeks, and urine is discharged through the urethra into the amniotic fluid in the amnionic cavity. The fetus reabsorbs some amniotic fluid after swallowing it. Fetal waste products are transferred to the maternal circulation by passage across the placental membrane (see Chapter 7 , Fig. 7.7 ).
Thirteen to Sixteen Weeks
Growth is very rapid during this period ( Figs. 6.4 and 6.5 , and see Table 6.1 ). By 16 weeks, the head is relatively smaller than the head of a 12-week fetus, and the lower limbs have lengthened ( Fig. 6.6 A ). 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.