Fetal Period:
The Ninth Week to Birth
YUSUF UTHMAN ADEMOLA
(B.Tech, M.Sc, PhD in view)
DEPARTMENT OF ANATOMY,
MULUNGUSHI UNIVERSITY
LIVING STONE CAMPUS
ZAMBIA
INTRODUCTION
• Development during the fetal period is concerned primarily with body growth and
differentiation of tissues, organs, and systems.
• Rudimentary organ systems have 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.
• Ultrasonographic measurements of the crown-rump length (CRL) can be used to
determine fetal size and probable age.
• The intrauterine period may be divided into days, weeks, or months, 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.
• Measurement of the CRL (crown- rump length) is the method of choice for
estimating fetal age until the end of the first trimester.
HIGHLIGHTS OF FETAL PERIOD
• There is not a formal staging system for the fetal period; however, it is
helpful to consider the main changes that occur in terms of periods of
4 to 5 weeks.
Nine to Twelve Weeks
• At the beginning of the ninth week, the head constitutes half of the CRL of the
fetus. Subsequently, growth in body length accelerates rapidly, and by the end of
12 weeks, the CRL has more than doubled.
• 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 in their mature fetal form
until 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
• 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.
Thirteen to Sixteen Weeks
• Growth is very rapid during the 13th to 16th weeks. 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 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 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 in them primordial
ovarian follicles that have oogonia. By 16 weeks, the eyes face anteriorly rather
than anterolaterally.
Seventeen to Twenty Weeks
• Growth slows down during weeks 17 to 20, but the fetus still increases its CRL by
approximately 50 mm. Fetal movements—quickening—are commonly felt by the
mother.
• The skin is now covered with a greasy material called vernix caseosa, which 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 called lanugo, which
helps to hold the vernix caseosa 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 newborn.
• This specialized adipose tissue produces heat by oxidizing fatty acids. By 18
weeks, the uterus is formed in female fetuses.
• By this time, many primordial ovarian follicles containing oogonia have formed.
• In male 20-week fetuses, 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 weeks 21 to 25 and the fetus is better
proportioned. At 21 weeks, rapid eye movements begin; blink-startle responses
have been reported at 22 to 23 weeks.
• 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.
• Although a 22- to 25-week fetus born prematurely may survive initially if given
intensive care support, the fetus may die because its respiratory system is still
immature.
• Infants born before 26 weeks of gestation have a high risk of
neurodevelopmental (functional) disability. Fingernails are also present by 24
weeks.
Twenty-Six to Twenty-Nine Weeks
• At 26 to 29 weeks, a fetus often survives if born 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 greatest neonatal mortality occur in low–birth weight infants (weighing 2500
g or less) and especially in very low–birth weight infants (weighing 1500 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.
Thirty to Thirty-Eight Weeks
• The pupillary light reflex of the eyes can be elicited by 30 weeks. Usually, by the
end of this period, the skin is pink and smooth, and the upper and lower limbs
have a chubby appearance. Fetuses 32 weeks and older usually survive if born.
Fetuses at 35 weeks have a firm grasp and exhibit a spontaneous orientation to
light.
• As term approaches (37–38 weeks), the nervous system is sufficiently mature to
carry out some integrative functions.
• Most fetuses during this “finishing period” are plump. By 36 weeks, the
circumferences of the head and the abdomen are approximately equal. Growth
slows as the time of birth approaches.
• Most fetuses weigh approximately 3400g at term. A fetus adds approximately 14
g of fat daily during the last weeks of gestation. The chest is prominent, and the
breasts protrude slightly in both sexes.
Expected Date of Delivery
• The expected date of delivery of a fetus is 266 days, or 38 weeks, after
fertilization (i.e., 280 days, or 40 weeks, after the LNMP).
• Approximately 12% of babies are born 1 to 2 weeks after the expected time of
birth.
FACTORS INFLUENCING FETAL
GROWTH
• The fetus requires substrates for growth and the production of energy. Gases and
nutrients pass freely to the fetus from the mother through the placental membrane.
• Glucose is a primary source of energy for fetal metabolism and growth; amino acids
are also required. Insulin, which is required for the metabolism of glucose, is
secreted by the fetal pancreas.
• Insulin, human growth hormone, and some small polypeptides (e.g., insulin-like
growth factor I) are believed to stimulate fetal growth.
• Many factors—maternal, fetal, and environmental—may affect prenatal growth.
• In general, factors operating throughout pregnancy, such as cigarette smoking
and consumption of alcohol, tend to produce intrauterine growth restriction
(IUGR) and small infants, whereas factors operating during the last trimester (e.g.,
maternal malnutrition) usually produce underweight infants with normal length
and head size. Severe maternal malnutrition resulting from a poor-quality diet is
known to cause reduced fetal growth.
• Neonates (newborns) resulting from twin, triplet, and other multiple pregnancies
usually weigh considerably less than infants resulting from a single pregnancy.
• It is evident that the total requirements of two or more fetuses exceed the
nutritional supply available from the placenta during the third trimester.
• Repeated cases of IUGR in one family indicate that recessive genes may be the
cause of the abnormal growth.
• In recent years, structural and numeric chromosomal aberrations have also been
shown to be associated with cases of restricted fetal growth. IUGR is
pronounced in infants with trisomy 21 (Down syndrome)
PROCEDURES FOR ASSESSING
FETAL STATUS
• Ultrasonography
• Ultrasonography is the primary imaging modality in the evaluation of the fetus
because of its wide availability, low cost, and lack of known adverse effects.
Placental and fetal size, multiple births, abnormalities of placental shape, and
abnormal presentations can also be determined.
• Many developmental defects can also be detected prenatally by ultrasonography.
Diagnostic Amniocentesis
• Diagnostic amniocentesis is a common invasive prenatal diagnostic procedure
typically performed during the second trimester.
• For prenatal diagnosis, amniotic fluid is sampled by insertion of a hollow needle
through the mother’s anterior abdominal and uterine walls and into the amniotic
sac. A syringe is then attached to the needle and amniotic fluid is withdrawn.
• The procedure is relatively devoid of risk, especially when performed by an
experienced physician using ultrasonography as a guide for outlining the position
of the fetus and the placenta.
Chorionic Villus Sampling
• Biopsy of chorionic villi is performed to detect chromosomal abnormalities,
inborn errors of metabolism, and X-linked disorders.
• Chorionic villus sampling can be performed as early as 7 weeks after fertilization.
The rate of fetal loss is approximately 1%, slightly more than the risk associated
with amniocentesis.
• The major advantage of chorionic villus sampling over amniocentesis is that it
allows fetal chromosomal sampling to be performed several weeks earlier.
Cell Cultures
• Fetal sex and chromosomal aberrations can also be determined by
studying the sex chromosomes in cultured fetal cells obtained during
amniocentesis. These cultures are commonly performed when an
autosomal abnormality, such as occurs in Down syndrome, is
suspected. Inborn errors of metabolism and enzyme deficiencies in
fetuses can also be detected by studying cell cultures.
Percutaneous Umbilical Cord Blood
Sampling
• For chromosomal analysis, blood samples may be obtained from the
umbilical vessels by percutaneous umbilical cord blood sampling.
Ultrasonographic scanning is used to outline the location of the vessels.
Percutaneous umbilical cord blood sampling is often performed
approximately 20 weeks after the LNMP to obtain samples for
chromosomal analysis when ultrasonographic or other examinations
have shown characteristics of birth defects.
Magnetic Resonance Imaging
• When fetal treatment, such as surgery, is planned, computed
tomography and magnetic resonance imaging (MRI) may be used.
MRI has the advantage of not requiring ionizing radiation to produce
images. These studies can provide additional information about a
fetal abnormality detected ultrasonographically.
Fetal Monitoring
• Continuous fetal heart rate monitoring in high-risk pregnancies is
routine and provides information about the oxygenation of the fetus.
Fetal distress, as indicated by an abnormal heart rate or rhythm,
suggests that the fetus is in jeopardy.
Alpha Fetoprotein Assay
• Alpha fetoprotein, a glycoprotein that is synthesized in the fetal liver
and umbilical vesicle, escapes from the fetal circulation into the
amniotic fluid in fetuses with open neural tube defects, such as spina
bifida with myeloschisis. Alpha fetoprotein can also enter the
amniotic fluid from open ventral wall defects, as occurs with
gastroschisis and omphalocele. Alpha fetoprotein can also be
measured in maternal serum.