DEVELOPMENT OF FETUS
• Fetal period, 9th week (3rd month) to birth
• Maturation of tissues and organs and rapid growth of the
body
• Length of fetus, crown-crump length (CRL) or crown-heel
length (CHL), expressed in centimeters, correlated with the
age of fetus in weeks.
A. CROWN-CRUMP LENGTH (CRL)
• sitting height
• from vertex of skull to the heel
• up to 36 cm (for a full term baby) at 37-
38 week
B. CROWN- HEEL LENGTH (CHL)
• standing height
• from vertex of skull to heel
• up to 51 cm
Beginning of the third month: the head constitutes
approximately half of the CRL.
• Beginning of the fifth month: the size of the head is about
one third of the CHL, and at birth, it is approximately one
quarter of the CHL.
• Hence, over time, growth of the body accelerates but
that of the head slows down
• Growth in length is particularly striking during: 3rd, 4th, and
5th months.
• An increase in weight is most striking during the last 2
months of gestation.
• In general, the length of pregnancy is considered to be 280
days, or 40 weeks after the onset of the last normal
menstrual period (LNMP) or, more accurately, 266 days or
38 weeks after fertilization.
• Age is calculated from the time of fertilization and is
expressed in weeks or calendar months.
monthly changes
3
rd month to birth, the progress or the development of the
fetus is accounted month by month
• 3
rd month – primary ossification centers, external genitalia -
the testicles inside the scrotum is visible via the ultrasound
in the fetus.
o Fetus during 3rd month has the look of a human
being (with the face, the eyes etc) but the skin is
very thin that the blood vessels inside are
noticeable from outside.
o the face becomes more human looking. The limbs
reach their relative length. external genitalia
develop to such a degree that the sex of the fetus
can be determined by external examination
(ultrasound). At the sixth week, intestinal loops
cause a large swelling (herniation) in the umbilical
cord.
4
th, 5th month – lanugo hair, fetal movement felt by mother.
o If a mother goes to the doctor for a prenatal check-
up, and they’re not sure when the last menstrual
period was, then the month of pregnancy can be
determined by whether or not they can feel the fetal
movement.
o The fetus is covered with fine hair, called lanugo
hair; eyebrows and head hair are also visible.
• 6
th month-respiratory system and central nervous system
not yet well established, have differentiated fully, great
difficulty in surviving.
o The skin of the fetus is reddish and has a wrinkled
appearance (because of the lack of underlying
connective tissue).
7
th month – developmental events occur, 90% chance of
survival, the respiratory system, the sucking and the
swallowing mechanisms are already developed which is why
the survival is better than the 6th month.
• 8
th month- vermix caseosa, whitish fatty substance. When a
baby is delivered with whitish material covering the body, the
baby has crossed the 8 month mark.
9
th month- skull largest circumference of all parts of the
body. If the baby is born in breech position (feet first), the
circumference of the head should be known to confirm
whether or not the mother will be able to deliver the head.
• At 18th week, the fetus has subcutaneous tissue and the
attachment to placenta
• At the time of birth, the full term baby is 38 weeks after
fertilization of 266 days (38x7) or 40 weeks. This is computed
from the last normal menstrual period.
• Ovulation happens for 24hours but the sperm travels for 6
days. Not all eggs are fertilized right after ovulation within 24
hours.
• Window period is of about 6 days, and implantation is for 6-
7 days, so there’s an allowance of 10-14 days.
• During the last 2 months, the fetus obtains well-rounded
contours as the result of deposition of subcutaneous fat.
• At the time of birth, the weight of a normal fetus is 3,000 to 3,400 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
TIME OF BIRTH
Age of gestation:
• 266 days or 38 weeks after fertilization or 40
weeks from the first day of the LNMP. In women
with regular 28-day menstrual periods, the
method is fairly accurate, but when cycles are
irregular, substantial miscalculations may be
made.
• An additional complication occurs when the
woman has some bleeding about 14 days after
fertilization as a result of erosive activity by the
implanting blastocyst. Hence, the day of delivery
is not always easy to determine. Most fetuses are
born within 10 to 14 days of the calculated
delivery date.
• If they are born much earlier - Premature
• If born later – Postmature
• Ultrasound- A valuable tool for assisting in this
determination, which can provide an accurate (1 to 2
days) measurement of CRL during the 7th to 14th
weeks.
• Measurements commonly used in the 16th to 30th
weeks:
o Ultrasound (determines the age of gestation),
CRL, BPD (biparietal diameter), head,
abdominal circumference, femur length
• Average size of newborn: 2,500-4,000g: 51 cm length
• An accurate determination of fetal size and age is
important for managing pregnancy, especially if the
mother has a small pelvis or if the baby has a birth
defect
• Low Birth Weight (LBW): weight <2,500g regardless
of gestational age
Intrauterine Growth Restriction (IUGR): infants who do
not attain optimal intrauterine growth; pathologically
small, at risk for poor outcomes; gestational age taken
into account.
• Small for Gestational Age (SGA): birth weight below
10th percentile for gestational age; healthy but smaller
in size. no poor outcomes. Gestational period is
taken into account also.
• Intrauterine Growth Restriction (IUGR): infants who do
PLACENTA
Facilitates nutrient and gas exchange between maternal and
fetal compartments
• As the fetus begins the 9th week of development, its demand
for nutritional and other factors increase, causing the major
changes in placenta.
• The deposition of fetal membranes is also altered as
production of amniotic increases.
• Maternal blood is delivered to the placenta by spiral arteries
in the uterus.
• Erosion of these maternal vessels to release blood into
intervillous spaces is accomplished by endovascular
invasion by cytotrophoblast cells.
• Fetal component of placental is derived from trophoblast and
extraembryonic mesoderm, chorionic plate
• Maternal component is derived from uterine endometrium
• Fetal side covered by chorionic plate. Fetal side also has
blood vessels covered by the amnion connecting the fetus
and the placenta with the umbilical cord.
• Chorion frondosum, chorionic plate (lines the trophoblast)
• Decidua basalis, decidual plate
• Junctional zone, decidual septae, compartments or
cotyledons
III. TIME OF BIRTH
IV. PLACENTA
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