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Third Month to Birth: The Fetus and Placenta/1

Dr.Sumeya

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 Months

Growth in length is particularly striking during the third, fourth, and fifth months, while 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.
For the purposes of the following discussion, age is calculated from the time of fertilization and is expressed in weeks or calendar months.

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.
At the beginning of the third month, the head constitutes approximately half of the CRL
By the 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.

third 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 .

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.

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).
During the sixth week, intestinal loops cause a large swelling (herniation) in the umbilical cord, but by the 12th week, the loops have withdrawn into the abdominal cavity.
At the end of the third month, reflex activity can be evoked in aborted fetuses, indicating muscular activity.

3 Months

fourth and fifth 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.
The fetus is covered with fine hair, called lanugo hair; eyebrows and head hair are also visible.
During the fifth month, movements of the fetus can be felt by the mother .

second half of intrauterine life

During the second half of intrauterine life, weight increases considerably, particularly during the last 2.5 months, when 50% of the full term weight (approximately 3,200 g) is added.

During the sixth month, the skin of the fetus is reddish and has a wrinkled appearance because of the lack of 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,

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.

Parts of the Zygote

4 Weeks

Two Months

6 Weeks
7 Weeks
8 Weeks

7 weeks

8 weeks


During the last 2 months, the fetus obtains well-rounded contours as the result of deposition 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.

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.

At the time of birth, the weight of a normal fetus is 3,000 to 3,400g, 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.

3 Months

11 Weeks
12 Weeks

4 months

15 Weeks
16 Weeks
18 Weeks

6 Months

Seven Months

Eight Month

Time of Birth
The date of birth is most accurately indicated as 266 days, or 38 weeks, after fertilization.
The oocyte is usually fertilized within 12 hours of ovulation; however, sperm deposited in the reproductive tract up to 6 days prior to ovulation can survive to fertilize oocytes.
A pregnant woman usually will see her obstetrician when she has missed menstrual bleeds.
By that time, her recollection about coitus is usually vague, and it is readily understandable that the day of fertilization is difficult to determine.

The obstetrician calculates the date of birth as 280 days 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, they are categorized as premature; if born later, they are considered postmature.

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.

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.

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.

Smallest baby in the world born At just 22 weeks

She was 10 OUNCES when born and 9.5 inches. That's just longer than the length of your hand.
Her little feet

Third Month to Birth: The Fetus and Placenta/2

FETAL MEMBRANES AND PLACENTA

Changes in the trophoplast

The fetal component of placenta is derived from the trophoplast and extraembryonic mesoderm(chorionic plate);
the maternal component is derived from the uterine endometrium.

Human embryo at the beginning of the second month of development. At the embryonic pole, villi are
numerous and well formed; at the abembryonic pole, they are few in number and poorly developed.

Changes in trophoplast

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. These cells, released from the ends of anchoring villi invade the terminal ends of spiral arteries, where they replace maternal endothelial cells in the vessels’ walls, creating hybrid vessels containing both fetal and maternal cells.

To accomplish this process, cytotrophoblast cells undergo an epithelial-to-endothelial transition.

Invasion of the spiral arteries by cytotrophoblast cells transforms these vessels from small-diameter, high-resistance vessels to large diameter, lowresistance vessels that can provide increased quantities of maternal blood to intervillous spaces

During the following months, numerous small extensions grow out from existing stem villi and extend as free villi into the surrounding lacunar or intervillous spaces.
Initially, these newly formed free villi are primitive but by the beginning of the fourth month, cytotrophoblastic cells and some connective tissue cells disappear.
The syncytium and endothelial wall of the blood vessels are then the only layers that separate the maternal and fetal circulations

STRUCTURE OF STEM CHORIONIC VILLUS

chorion frondosum and Decidua basalis
In the early weeks of development villi cover the entire surface of the chorion .As pregnancy advances villi on the emberionic pole continue to grow and expand ,giving rise to the chorion frondosum (bushy chorion ).

Villi on the abemberyonic pole degenerate ,and by the third month , this side of the chorion ,now known as the chorion leave , is smooth.

The difference between the embryonic and abembryonic poles of the chorion is also reflected in the structure of the decidua, the functional layer of the endometrium, which is shed during parturition.
The decidua over the chorion frondosum, the decidua basalis, consists of a compact layer of large cells, decidual cells, with abundant amounts of lipids and glycogen.
This layer, the decidual plate, is tightly connected to the chorion. The decidual layer over the abembryonic pole is the decidua capsularis

With growth of the chorionic vesicle, this layer becomes stretched and degenerates. Subsequently, the chorion laeve comes into contact with the uterine wall (decidua parietalis) on the opposite side of the uterus, and the two fuse, obliterating the uterine lumen.
Hence, the only portion of the chorion participating in the exchange process is the chorion frondosum, which, together with the decidua basalis, makes up the placenta.
Similarly, fusion of the amnion and chorion to form the amniochorionic membrane obliterates the chorionic cavity. It is this membrane that ruptures during labor (breaking of the water).

Structure of placenta

By the beginning of the fourth month, the placenta has two components: (1) a fetal portion, formed by the chorion frondosum and (2) a maternal portion, formed by the decidua basalis.
On the fetal side, the placenta is bordered by the chorionic plate on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is most intimately incorporated into the placenta.
In the junctional zone, trophoblast and decidual cells intermingle. This zone, characterized by decidual and syncytial giant cells, is rich in amorphous extracellular material. By this time, most cytotrophoblast cells have degenerated

. Between the chorionic and decidual plates are the intervillous spaces, which are filled with maternal blood. They are derived from lacunae in the syncytiotrophoblast and are lined with syncytium of fetal origin. The villous trees grow into the intervillous blood lakes.
During the fourth and fi fth months, the decidua forms a number of decidual septa, which project into intervillous spaces but do not reach the chorionic plate.

These septa have a core of maternal tissue, but their surface is covered by a layer of syncytial cells, so that at all times, a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of the villi.
As a result of this septum formation, the placenta is divided into a number of compartments, or cotyledons Because the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained.

As a result of the continuous growth of the fetus and expansion of the uterus, the placenta also enlarges. Its increase in surface area roughly parallels that of the expanding uterus, and throughout pregnancy, it covers approximately 15% to 30% of the internal surface of the uterus.
The increase in thickness of the placenta results from arborization of existing villi and is not caused by further penetration into maternal tissues.

Third Month to Birth: The Fetus and Placenta/3

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 afterbirth.
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.


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. Rarely, however, does it insert into the chorionic membranes outside the placenta (velamentous insertion).

Maternal side

Fetal side

velamentous insertion

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.

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.

Collectively, the intervillous spaces of a mature placenta contain approximately 150 mL.

Functions Of The Placenta

Exchange of gases
Exchange of nutrition and electrolytes
Transmission of maternal antibodies
Excretion
Hormone production

Placental endocrine synthesis

The syncytiotrophoblast synthesizes protein &steroid hormones
The protein homones
1- human chorionic gonadotropin
2- h.c. somatomammotropin
3-h.c. thyrotropin
4-h.c. corticotropin
The steroid hormones
Progesterone & Estrogens

Amnion and umbilical cord

At the fifth week of development ,the following structures pass through the ring
(1)the connecting stalk, containing the allantois and the umbilical vessels, consisting of two arteries and one vein;
(2) the yolk stalk (vitelline duct), accompanied by the vitelline vessels;
(3) the canal connecting the intraembryonic and extraembryonic cavities

. During further development, the amniotic cavity enlarges rapidly at the expense of the chorionic cavity, and the amnion begins to envelop the connecting and yolk sac stalks, crowding them together and giving rise to the primitive umbilical cord

The abdominal cavity is temporarily too small for the rapidly developing intestinal loops, and some of them are pushed into the extraembryonic space in the umbilical cord. These extruding intestinal loops form a physiological umbilical hernia. At approximately the end of the third month, the loops are withdrawn into the body of the embryo, and the cavity in the cord is obliterated. When the allantois and the vitelline duct and its vessels are also obliterated, all that remains in the cord are the umbilical vessels surrounded by Wharton’s jelly. This tissue, which is rich in proteoglycans, functions as a protective layer for the blood vessels. The walls of the arteries are muscular and contain many elastic fi bers, which contribute to a rapid constriction and contraction of the umbilical vessels after the cord is tied off.

PLACENTAL CHANGES AT THE END OF PREGNANCY

At the end of pregnancy, a number of changes that occur in the placenta may indicate reduced exchange between the two circulations.
These changes include
1- an increase in fibrous tissue in the core of the villus
2- thickening of basement membranes in fetal capillaries, of the villi
3- deposition of fibrinoid on the surface of the villi in the juncional zone and in the chorionic plate.
Excessive fibrinoid formation frequently causes infarction of an intervillous lake or sometimes of an entire cotyledon. The cotyledone then assumes a whitish appearance.

Amniotic fluid

The amniotic cavity is filled with a clear ,watery fluid that is produced in part by amniotic cells but is derived primarily from maternal blood .
The amount of fluid increase from approximately 30 mL at 10 weeks of gestation to 450 mL at 20 weeks to 800 to 1000 mL at 37 weeks .During the early month of pregnancy ,the embryo is suspended by its umbilical cord in this fluid ,which serves as a protective cushion .
The fluid 1-absorbs jolts 2- prevents adherence of the embryo to the amnion 3- allows for fetal movements .

The volume of amniotic fluid is replaced every 3hours .

From the beginning of the fifth month ,and it is estimated that it drinks about 400mL a day ,about half of the total amount
.Fetal urine is added daily to the amniotic fluid in the fifth month, but this urine is mostly water , because the placenta is functioning as an exchange for metabolic wastes.
During childbirth, the amniochorionic membrane forms a hydrostatic wedge that helps to dilate the cervical canal.

Dizygotic Twins (Fraternal Twins)

* Two eggs are ovulated and each is fertilized by a sperm cell
No more genetically similar than any other sibling in the family (can be same/different sexes)
Maternal age, use of assisted reproductive technologies are factors

* Monozygotic Twin

One egg is fertilized by one sperm
Have identical genes and must be of the same sex
(Incidence: about 3 in every 1000 births)

They result from splitting of the zygote at various stages of development .The earliest separation at two cell stage.
The splitting of zygote usually occurs at the early blastocyst stage.The inner cell mass splits into two separate groups of cells within the sameblastocyst cavity.
The two embryos have a common placenta and a common chorionic cavity but separate amniotic cavities.
In rare cases, the separation occurs at the bilaminar germ disc stage, just before the appearance of the primitive streak.
This method of splitting results in formation of two partners with a single placenta and a common chorionic and amniotic sac.
Although the twins have a common placenta, blood supply is usually well balanced.

Although triplets are rare (about one per 7,600 pregnancies), birth of quadruplets, quintuplets, and so forth is rarer.
In recent years, multiple births have occurred more frequently in mothers given gonadotropins (fertility drugs) for ovulatory failure.

parturition (birth)

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

THE END



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