1) This document describes fetal and placental development from the third month of pregnancy until birth. It details the growth and changes that occur in the fetus, fetal membranes, placenta, and umbilical cord during each month of the second and third trimesters. 2) The placenta develops from the chorion frondosum and decidua basalis. It functions to exchange gases, nutrients, electrolytes, and antibodies between the mother and fetus. 3) Near birth, the fetus is fully developed, weighing approximately 3,000-3,500g. Labor occurs around 38 weeks and has three stages, culminating in delivery of the placenta and membranes.

1. THIRD MONTH TO
BIRTH:
THE FETUS &
PLACENTA
Dr.Mohamed Boqore
MBBS, FAM.MED
Chapter 7
SCHOOL OF MEDICINE
1
GENERAL EMBRYOLOGY

2. • The period from the beginning of the ninth week
to birth is known as the fetal period. It is
characterized by maturation of tissues and organs
and rapid growth of the body.
• The length of the fetus is usually indicated as the
crown- rump length (CRL) (sitting height) or as
the crown-heel length (CHL), the measurement
from the vertex of the skull to the heel (standing
height).

3. All those measurements are CRL
• 9 – 12 wks = 5 – 8 cm
• 13 – 16 wks = 9 – 14 cm
• 15 – 19 wks = 15 – 19 cm

4. • In general, the length of pregnancy is consid-
ered 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.

5. MONTHLY CHANGES
• One of the most striking changes taking place
during fetal life is the relative slowdown in
growth of the head compared with the rest of
the body.

6. Relative size of the head
• 3 months – ½ of CRL
• 5 months – 1/3 of CHL
• Birth – ¼ of CHL
• Hence, over time, growth of the body
accelerates but that of the head slows down.

8. 3rd month
• During the third month, the face becomes
more human looking.
• The eyes, initially directed laterally, move to
the ventral aspect of the face, and the ears
come to lie close to their definitive position at
the side of the head.

9. • The limbs reach their relative length in
comparison with the rest of the body,
although the lower limbs are still a little
shorter and less well developed than the
upper extremities.

10. • Primary ossification centers are present in the
long bones and skull by the 12th week.
• Also by the 12th week, external genitalia
develop to such a degree that the sex of the
fetus can be determined by external
examination (ultrasound).

11. • During the sixth week, intestinal loops
cause a large swelling (herniation) in the
umbilical cord, but by the 12th week,
marks its completion and return of the
herniated content back into the abdominal
cavity.
– At the end of the third month, reflex activity
can be evoked in aborted fetuses, indicating
muscular activity.

14. 4th and 5th months
• During the fourth and fifth months, the fetus
lengthens rapidly, and at the end of the first
half of intrauterine life, its CRL is
approximately 15 cm, about half the total
length of the newborn.
• The weight of the fetus increases little during
this period and by the end of the fifth month
is still <500 g (expected 3,000 – 3,500 at birth)

15. • The fetus is covered with fine hair, called
lanugo hair; eye-brows and head hair are also
visible.
• Possibly, the most interesting moment is when
movement of the fetus is felt by a mother (4.5
months approximately)

16. 6th and 7th months
• During the sixth month, the skin of the fetus is
reddish and has a wrinkled appearance due to
non-existing underlying connective tissue.
• A fetus born early in the sixth month has great
difficulty surviving. Although several organ
systems are able to function, the respiratory
system and the central nervous system have not
differentiated sufficiently, and coordination
between the two systems is not yet well
established.

17. • By 6.5 to 7 months, the fetus has a CRL of
about 25 cm and weighs approximately 1,100
g.
• If born at this time, the infant has a 90%
chance of surviving.

19. 8th and 9th months
• During the last 2 months, the fetus obtains
well-rounded contours as the result of deposi-
tion of subcutaneous fat.
• By the end of intrauterine life, the skin is
covered by a whitish, fatty substance (vernix
caseosa) composed of secretory products
from sebaceous glands.

20. • At the end of the ninth month, the skull has
the largest circumference of all parts of the
body, an important fact with regard to its
passage through the birth canal.
• Fetus is rapidly increasing in size.

21. • 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.
• Genitalia re fully developed.
• Testes should descend into the scrotum.

22. TIME OF BIRTH
• The date of birth is most accurately indicated
as 266 days, or 38 weeks, after fertilization.
• Quite hard to know exactly.
• +/- 10 – 14 days could be used to determine
when a birth is expected.

23. • A pregnant woman usually will see her
obstetrician when she has missed two
successive menstrual bleeds.
• The obstetrician calculates the date of birth as
280 days or 40 weeks from the first day of the
LNMP.

24. • In women with regular 28-day menstrual
periods, the method is fairly accurate, but
when cycles are irregular, substantial
miscalculations may be made.

25. • 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.

26. • 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, they are
categorized as premature; if born later, they
are considered postmature.

27. • Occasionally, the age of an embryo or small
fetus must be determined.
• By combining data on the onset of the last
menstrual period with fetal length, weight,
and other morphological characteristics
typical for a given month of development, a
reasonable estimate of the age of the fetus
can be formulated.

28. • A valuable tool for assisting in this determination
is ultrasound, 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 are biparietal diameter (BPD), head
and abdominal circumference, and femur length.

29. • 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.

30. FETAL
MEMBRANES &
PLACENTA

31. CHANGES IN THE TROPHOBLAST
• By the beginning of the second month, the
trophoblast is characterized by a great
number of secondary and tertiary villi, which
give it radial appearance.

33. • Trophoblasts (from the Greek words trephein,
to feed, and blastos, germinator) are cells that
form the outer layer of a blastocyst.
• These cells provide nutrients to the embryo
and develop into a large part of the placenta.

34. • They are formed during the first stage of
pregnancy and are the first cells to
differentiate from the fertilized egg.
• This layer of trophoblasts is also collectively
referred to as the trophoblast, or, after
gastrulation, the trophectoderm, as it is then
contiguous with the ectoderm of the embryo.

35. • Trophoblasts play an important role in embryo
implantation and interaction with the
endometrium of the maternal uterus
following decidualization.
• The trophoblast is composed of two layers: an
inner cytotrophoblast and an outer
syncytiotrophoblast.

36. • The syncytiotrophoblast is non-proliferative
and thus relies on fusion of the underlying
cytotrophoblast cells to expand.
• The syncytiotrophoblast are the cells in direct
contact with the maternal blood that reaches
the placental surface, and thus facilitates the
exchange of nutrients, wastes, and gases
between the maternal and fetal systems.

37. • Cytotrophoblast in the tips of villi can
differentiate into another type of trophoblast
called the extravillous trophoblast.
• Extravillous trophoblasts grow out from the
placenta and penetrate into the decidualized
uterus.

38. • This process is essential not only for physically
attaching the placenta to the mother, but also
for altering the vasculature in the uterus to
allow it to provide an adequate blood supply
to the growing fetus as pregnancy progresses.

39. • Some of the trophoblast even replaces the
endothelial cells in the uterine spiral arteries
as they remodel these vessels into wide bore
conduits that are independent of maternal
vasoconstriction.
• This ensures the fetus receives a steady supply
of blood, and the placenta is not sensitive to
fluctuations in oxygen that could cause it
damage.

40. THE DECIDUA
• At the implantation location, the maternal
endometrium is changed by the decidual
reaction (epithelial transformation of the
fibroblasts of the uterine stroma, in that lipids
and glycogen accumulate) and is called
the decidua.
The decidua consists of various parts,
depending on its relationship with the embryo

41.  Decidua basalis, where the implantation
takes place and the basal plate is formed. This
can be subdivided into a zona compacta and a
zona spongiosa (where the detachment of the
placenta takes place following birth).
Decidua capsularis, lies like a capsule around
the chorion
Decidua parietalis, on the opposite uterus
wall

42. • At around the 4th month, the fetus is so large
that the decidua capsularis comes into contact
with the decidua parietalis.
• The merging of these two deciduae causes the
uterine cavity to disappear.

45. CHORION FRONDOSUM &
DECIDUA BASALIS
• Originally the entire chorionic surface is
covered with villi but those adjacent to the
capsularis degenerate to produce the smooth
(nonvillous) chorion laeve.
• The villi adjacent to the decidua basalis
persist, increase in size and produce
the chorion frondosum or fetal portion of the
placenta.

47. FULL TERM PLACENTA
• At full term, the placenta is discoid with a
diameter of 15 to 25 cm, is approximately 3
cm thick, and weighs about 500 to 600 g.
• At birth, it is torn from the uterine wall and,
approximately 30 minutes after birth of the
child, is expelled from the uterine cavity as the
after birth.

48. • When the placenta is viewed from the
maternal side, 15 to 20 slightly bulging areas,
the cotyledons, covered by a thin layer of
decidua basalis, are clearly recognizable.
• Grooves between the cotyledons are formed
by decidual septa.

49. • The fetal surface of the placenta is covered
entirely by the chorionic plate. A number
of large arteries and veins, the chorionic
vessels, converge toward the umbilical cord.
The chorion, in turn, is covered by the
amnion.
• Attachment of the umbilical cord is usually
eccentric and occasionally even marginal.

51. CIRCULATION OF THE PLACENTA
• Cotyledons receive their blood through 80 to
100 spiral arteries that pierce the decidual
plate and enter the intervillous spaces at more
or less regular intervals.
• Pressure in these arteries forces the blood
deep into the intervillous spaces and bathes
the numerous small villi of the villous tree in
oxygenated blood.

52. • As the pressure decreases, blood flows back
from the chorionic plate toward the decidua,
where it enters the endometrial veins.
• Hence, blood from the intervillous lakes drains
back into the maternal circulation through the
endometrial veins.

54. STRUCTURE OF PLACENTA
• The placenta consists of two components:
(1) a fetal portion, derived from the chorion frondosum
or villous chorion, and
(2) a maternal portion, derived from the decidua basalis.
• The space between the chorionic and decidual plates
is filled with intervillous lakes of maternal blood.

55. The fetal circulation is at all times separated
from the maternal circulation by :
(1) a syncytial membrane (a chorion derivative)
and
(2) endothelial cells from fetal capillaries.

56. • Intervillous lakes of the fully grown placenta
contain approximately 150 mL of maternal
blood, which is renewed three or four times
per minute.
• The villous area varies from 4 to 14 m2,
facilitating exchange between mother and
child.

57. FUNCTIONS OF THE PLACENTA
 Exchange of gases.
- oxygen and carbon dioxide
 Exchange of nutrients & electrolytes.
 Transmission of maternal antibodies; providing
the fetus with passive immunity
- IgG (maternal immunoglobulin G)


58. Hormone production, such as progesterone,
estradiol, and estrogen (in addition, it
produces hCG and somatomammotropin
‘placental lactogen’); and
 Detoxification of some drugs.

59. UMBLICAL CORD
• The umbilical cord, surrounded by the
amnion, contains
(1) two umbilical arteries,
(2) one umbilical vein, and
(3) Wharton’s jelly, which serves as a protective
cushion for the vessels.

60. AMNIOTIC FLUID
• The amnion is a large sac containing amniotic
fluid in which the fetus is suspended by its
umbilical cord.
• The fluid:
(1) absorbs jolts,
(2) allows for fetal movements, and
(3) prevents adherence of the embryo to
surrounding tissues.

61. • The fetus swallows amniotic fluid, which is
absorbed through its gut and cleared by the
placenta.
• The fetus adds urine to the amniotic fluid, but
this is mostly water. An excessive amount of
amniotic fluid (polyhydramnios) is associated
with anencephaly and esophageal atresia,
whereas an insufficient amount
(oligohydramnios) is related to renal agenesis.

62. FETAL MEMBRANES IN TWINS
• Fetal membranes in twins vary according to
their origin and time of formation.
• Two thirds of twins are dizygotic, or fraternal;
they have two amnions, two chorions, and
two placentas, which sometimes are fused.

64. • Monozygotic twins usually have two amnions,
one chorion, and one placenta.
• In cases of conjoined twins, in which the
fetuses are not entirely split from each other,
there is one amnion, one chorion, and one
placenta.

66. PARTURITION
• Signals initiating parturition (birth) are not clear,
but preparation for labor usually begins between
34 and 38 weeks.
• Labor itself consists of three stages:
(1) effacement and dilatation of the cervix,
(2) delivery of the fetus, and
(3) delivery of the placenta and fetal membranes.