9 Th Lecture 3rd Month to Birth

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3rd month to birth

Dr. eman khammas alsadi

Embryology lecturer

Missan universityCollage of medicine

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Third Month to Birth: The Fetus and PlacentaDEVELOPMENT OF THE FETUSThe period from the beginning of the ninth week to

birth is known as the fetal period.It is characterized by:

1-(maturation of tissues and organs )

2-( rapid linear growth of the body.)

length is particularly striking during the3rd 4th, and5th months,

weight is most striking during the last 2 months ofgestation.

the length of pregnancy is considered to be 280days, or 40 weeks after the onset of the last normalmenstrual period (LNMP)

or, more accurately, 266 days or 38 weeks afterfertilization.

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Monthly Change1-the relative slowdown in growth of the head compared with

the rest of the body.

1-third month, the head constitutes approximately 1\2 of the CRL2-fifth month, the size of the head is about 1\3 of the CHL,

3- at birth, it is approximately 1\4 of the length,

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During the third month:1- the head constitutes approximately half of the CRL

2-the face becomes more human-looking .

3-Primary ossification centers- presenta- in the long bones

b-and skull by the 12th week

4-external genitalia develop to such a degree that the sex of thefetus can be determined by external examination

(ultrasound).5-the intestinal loops have withdrawn into the abdominal cavity.

6-Started muscular activity

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During the fourth and fifth

months,

1- the fetus lengthens rapidly ,its CRL isapproximately 15 cm, about half the

total length of the newborn.

2-The weight by the end of the fifth

month is still less than 500 g.

3- The fetus is covered with fine hair,

called lanugo hair; eyebrows and head

hair are also visible.

4- During the fifth month, movements of

the fetus can be felt by the mother.

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During the second half of intrauterine life,

weight increases considerably, particularly

during the last 2.5 months, when 50% of the

full-term weight is addedDuring the sixth month,

1- the skin of the fetus is reddish and has a wrinkled

appearance because of the lack of underlying

connective tissue.

2- 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 yet.

During 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

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developmental functions age(weeks)

1. Taste buds appear 7

2. Swallowing 103. Respiratory movements 14-16

4. Sucking movements 24

5. Some sounds can be heard 24-266. Eyes sensitive to light 28

7. aRecognition of form and color occurs

post natally.

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During the last 2 months:

1-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.

At the time of birth:

1- the weight of a normal fetus is 3,000 to 3,400 g,

2-its CRL is about 36 cm,

3-CHL is about 50 cm.

4-Sexual characteristics are pronounced,

5-testes should be in the scrotum.

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Time of Birth

. 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.

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the age determination of an embryo or

small fetus must be determined. Bycombining data on

1- the onset of the LMP2- fetal length, weight, and other morphological

characteristics typical for a given month ofdevelopment, By using ultrasound:

during the 7th to 14th weeks. an accuratemeasurement depending on CRL

Measurements commonly used in the 16th to 30thweeks are

1-biparietal diameter (BPD),2-head and abdominal circumference,

3-and femur length.

An accurate determination of fetal size and age isimportant for managing pregnancy, especially ifthe mother has a small pelvis or if the baby hasa birth defect10


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fetal membranes and placenta

The placenta is the organ that facilitates nutrient and gas exhange between thematernal and fetal compartments.

At the beginning of the ninth week of development major changes in the placentaoccur involving:

1- an increase in surface area between maternal and fetal components to facilitateexchange.

2-The disposition of fetal membranes is also altered as production of amniotic fluidincreases.

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Cli i l C l


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Clinical Correlates:

Low Birth Weight. Most factors influencing length and weight are

1. genetically determined,

2. environmental factors also play an important role.

Intrauterine growth restriction (IUGR)1. is a term applied to infants who do not achieve their

genetically determined potential size.

2. These infants are pathologically small3. and at risk for poor outcomes.

small for gestational age (SGA)

that are below the 10th percentile for their gestational age.SGA babies are not pathologically small, but instead, they

are healthy and have achieved their expected amount ofgrowth based on their genetic potential.

The challenge is to differentiate the two conditions so thatSGA babies are not subjected to high-risk protocols.

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A i l i 10 b bi h IUGR d h f h


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Approximately one in 10 babies have IUGR and therefore have anincreased risk of

1-neurological deficiencies,

2- congenital malformations,

3-meconium aspiration,4-hypoglycemia,

5-hypocalcemia,

6-and respiratory distress syndrome (RDS).

7-There are also long-term effects on these infants.For example,

they have a greater chance of developing a metabolic disorder laterin life, such as

1. obesity,

2. hypertension,

3. hypercholesterolemia,

4. cardiovascular disease,

5. and type 2 diabetes

6. poor mental and physical health in general13


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Causative factors include1- chromosomal abnormalities (10%);

2-teratogens;

3-congenital infections (rubella, cytomegalovirus,toxoplasmosis, and syphilis)

4-poor maternal health (hypertension and renal and cardiacdisease);

5- the mother's nutritional status and socioeconomic level;

6- her use of cigarettes, alcohol, and other drugs;

7- placental insufficiency;8- and multiple births (e.g., twins, triplets).

The major growth-promoting factor during developmentbefore and after birth is insulin-like growth factor-I (IGF-I),

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Changes in the Trophoblast


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Changes in the TrophoblastThe fetal component of the placenta is derived from (the chorionic plate);

maternal component is derived from the uterine endometrium.

By the beginning of the second month, the trophoblast is characterized by

1- secondary and tertiary villi appear, which a radial appearance.

2-Stem (anchoring) villi extend from the mesoderm of the chorionic plate to the cytotrophoblast shell.3- The surface of the villi is formed by the syncytium, resting on a layer of cytotrophoblastic cells that in

turn cover a core of vascular mesoderm.

4-The capillary system developing in the core of the villous stems soon comes in contact with capillaries ofthe chorionic plate and connecting stalk, thus giving rise to the extraembryonic vascular system

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Maternal blood is delivered to the placenta by spiral arteries in the uterus.

Erosion of these maternal vessels to release blood into intervillous spaces isaccomplished by endovascular invasion by cytotrophoblast cells.

These cells, released from the ends of anchoring villi .invade the terminal ends ofspiral arteries, where they replace maternal endothelial cells in the vessels' walls,creating hybrid vessels containing both fetal and maternal cells. To accomplish thisprocess, cytotrophoblast cells undergo an epithelial-to-endothelial transition.Invasion of the spiral arteries by cytotrophoblast cells transforms these vessels fromsmall-diameter, high-resistance vessels to larger-diameter, low-resistance vessels thatcan provide increased quantities of maternal blood to intervillous spaces

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Clinical Correlates


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Clinical CorrelatesPreeclampsiais a condition characterized by

1. maternal hypertension,

2. proteinuria,3. and edema.

It may begin suddenly anytime from about 20 weeks' gestation to term andmay result in

fetal growth retardation,

fetal death,or death of the mother.

It is a trophoblastic disorder related to failed or incomplete differentiation ofcytotrophoblast cells, many of which do not undergo their normalepithelial-to-endothelial transformation.

As a result, invasion of maternal blood vessels by these cells is rudimentary.How these cellular abnormalities lead to hypertension and otherproblems is not clear.

1-placental mosaicism, in which trophoblast cells have genetic defects,

2-and maternal diseases that cause vascular problems, such as diabetes.

3- smoking women salso have a higher incidence of preeclampsia19


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Chorion Frondosum andDecidua Basalis

In the early weeks of development, villi coverthe entire surface of the chorion .

chorion frondosumAs pregnancy advances, villi of the

developing placenta on the embryonicpole continue to grow and expand, givingrise to the chorion frondosum (bushychorion).

the chorion laeveVilli of the developing placenta on the

abembryonic pole degenerate, and by thethird month, this side of the chorion, nowknown as the chorion laeve, is smooth.

The difference between the embryonic andabembryonic poles of the chorion.

1. in the structure of the decidua&

2. the functional layer of theendometrium which is shedduring birth20


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the decidua basalisThe decidua over the chorion frondosum, consists of a compact layer of large cells, decidual cells, with

abundant amounts of lipids and glycogen. This layer, is tightly connected to the chorion.

decidua capsularis

The decidual layer over the abembryonic pole With growth of the chorionic vesicle, this layer becomes

stretched and degenerates.

decidua parietalis

the chorion laeve comes into contact with the uterine wall (decidua parietalis) on the opposite side of theuterus, 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.

chorion frondosum+ decidua basalis= placenta

Similarly, fusion of the amnion and chorion to form the amniochorionic membrane obliterates thechorionic cavity(the membrane reptured during labure.

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Birth defect

congenital malformation,

and congenital anomaly

are synonymous terms used to describe structural, behavioral,functional, and metabolic disorders present at birth. .major structural anomalies occur in 2% to 3% of live born infants, anadditional 2% to 3% are recognized in children by age 5 years,

total 4% to 6% .

birth defects are the leading cause of infant mortality, accounting forapproximately 21% of infant deaths.


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Causes:-

1-unknown cause. 40% to 60%,

2- Genetic factors, 15%;

3-environmental factors 10%;

4-a combination of genetic and environmental influences(multifactorial inheritance) 20% to 25%;5-twinning causes 0.5% to 1%.


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Minor anomalies occur in approximately 15% of newborns.

These structural abnormalities,suchas

microtia (small ears), pigmented spots,

short palpebral fissures,in some cases, are associated with major defects.

For example, infants with

one minor anomaly have a 3% chance of having a major malformation; two minor anomalies have a 10% chance; three or more minor anomalies have a 20% chance.

Therefore, minor anomalies serve as clues for diagnosing more serious underlying

defects

T f Ab liti


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Types of AbnormalitiesA. MalformationsB. Disruptions

C. DeformationsD. A syndromeE. Association

1-Malformations:occur during formation of structures, for example,

during organogenesis.They may result in

1- complete or partial absence of a structure2-or in alterations of its normal configuration.caused by environmental and/or genetic factors.Occur during the third to eighth weeks of gestation


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Disruptions-2 result in morphological alterations of already formed

structures and are caused by destructive processes.

1- Vascular accidents leading to bowel Artesia

2-defects produced by amniotic bands are examplesof destructive factors that produce disruptions

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3-Deformations :

result from mechanical forces that mold a part of the

fetus over a prolonged period. Clubfeet,

4-A syndrome

is a group of anomalies occurring together that havea specific common cause. This term indicates that a

diagnosis has been made and that the risk of

recurrence is known.


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is the non random apperance of two or moreanomalies that occur together more frequently

than by chance alone. But the cause has not been

determind.like VACTERL associationA. Vertibral,

B. anal

C. cardiacD. tracheoesophageal

E. renal

F. limb anomalies.

5- association:

E i t l F t


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Environmental FactorsGerman measles affecting a mother during early

pregnancy caused abnormalities in the embryo,.

linked limb defects to the sedative thalidomide

Since that time, many agents have been identified

as teratogens (factors that cause birth defects)


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Thank you

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9 Th Lecture 3rd Month to Birth