Conception & Fetal Development

ByDr. Aida Abd El-

Razek

Learning Objectives Define the key terms. Summarize the process of

fertilization. Explain implantation and

nourishment of the embryo before development of the placenta.

Describe normal prenatal development from conception through birth.

Explain structure and function of the placenta, umbilical cord, and fetal membranes.

Identify the potential of teratogenic effects during period of embryonic and fetal development.

Describe prenatal circulation and the circulatory changes after birth.

Fertilization

It is the union of ovum and spermatozoon. It occurs at the ampulla of the fallopian tube.

The 23 chromosomes from the sperm mingle with 23 chromosomes from the ovum, restoring the diploid number to 46.

After intercourse, the spermatozoa ascend and reach the tubes within 30-40 minutes, but they are not able to fertilize the ovum immediately, and remain 2-6 hours during which they undergo certain enzymatic change called capacitation after which they are able to fertilize the ovum.

Capacitation of the sperm Is the process after which the sperm

becomes able to penetrate the zona pellucida, that surrounding the ovum and fertilize it.

The cervical and tubal secretions are mainly responsible for this capacitation.

Intrauterine development Is divided into three Stages

1. Pre-embryonic Stage

2. Embryonic Stage

3. Fetal Stage

1. Pre-embryonic Stage

Is the first 2 weeks after conception.

The period from fertilization through implantation.

Egg surrounded by sperm. A

sperm penetrates the egg and conception

occurs. It is called a zygote until it reaches

the uterus in 3-4 days.

Initiation of cell division The zygote divides into 2 cells,

then 4, then 8cells while in the fallopian tube.

Up to the 16 cell stage, the cells become smaller with each division, it is called a morula

Morula The outer cells of the morula

secrete fluid, creating a sac of cell (the blastocyst) that has an inner cell mass placed off-center within the sac

The inner cell mass of the blastocyst develops into the fetus (embryo).

The outer layer of blastocyst cells develops into placenta and fetal membranes. (trophoblast the feeding layer)

The morula enters the uterus about 3 to 4 days after conception, it lingers in the uterus another 2 to 4 days before beginning implantation.

Is in the secretary phase of the reproductive cycle, 1½ weeks before the woman would be gain her menstrual period.

Complete implantation is a gradually process that occurs between the 6 -10 days. Embryonic structures continue developing during implantation.

Immediately after implantation the endometrium called decidua.

Normal implantation occurs in the upper site of uterus. The upper uterus is the best area for implantation and placental development for three reasons:

The upper uterus is richly supplied with blood for optimal fetal gas exchange and nutrition.

The uterine lining is thick in the upper uterus preventing the placenta from attaching so deeply that it does not easily detach after birth.

Implantation in the upper uterus limits blood loss after birth because strong figure 8 fibers in this area.

Types of deciduaDecidua basalisDecidua capsularisDecidua parietalis or veraDecidual space

Function of the decidua It is the site of implantation. Protection of the uterine wall

against invasion by the chorionic villi

Nutrition of the embryo. Formation of the placenta

Second week Implantation is complete by the

end of the 2nd week. The most growth occurs in the

trophoblast, which eventually become the fetal part of the placenta.

The inner cells mass will develop into the embryonic disk

2. Embryonic Stage Extends from the beginning of

the 3rd week through the 8th weeks after conception.

Basic structures of all major body organs are completed during the embryonic period.

3rd week The embryonic disk develops

three layers (germ layers):

*Ectoderm

*Mesoderm

*Endoderm

*EctodermBrain and spinal cord.Peripheral nervous system.Pituitary gland.Sensory epithelium of the eye, ear, and

nose.Epidermis, hair, nails.Subcutaneous glands.Mammary glands

*Mesoderm Cartilage Bone. Connective & muscle tissue. Heart, blood vessels, blood cells. Lymphatic system, spleen, kidneys. Adrenal cortex, ovaries, testes. Reproductive system

*Endoderm Lining of gastrointestinal &

respiratory tracts. Tonsils, thyroid, parathyroid, liver,

pancreas. Lining of urinary bladder and

urethra Lining of ear canal

The central nervous system begins developing during the 3rd week.

A thickened flat neural plate appears, extending toward the end of the embryonic disk that will become the head

Early heart development begins beating at 21 to 22 days.

The neural plate develops but is still open

4th week The shape of the embryo changes. The embryo resembles C-shaped

cylinder by the end of 4th week. A tail is apparent during the embryonic

period because the brain and spinal cord develop more rapidly than other systems

The neural tube completes closure during 4th week.

Formation of the face and upper respiratory tract begins.

Beginning of the internal ear and the eye are apparent.

The upper extremities appear as buds.

Because the embryo is sharply flexed the heart is near the embryo mouth

The lower respiratory tract begins growth as a branch of the upper digestive tract, which is tubular at this time.

Gradually the esophagus and trachea separate completely.

5th week The head is very large because the

brain grows rapidly during 5th week. The heart is beating and developing

four chambers. Upper and lower limbs are paddle-

shaped

6th week The heart reaches its final four

chambered form. Upper and lower extremities

continue to become more defined. The eye continues to develop and the

beginning of the external ear is apparent.

7th weeks Extremities become longer and better defind. The face is now human-looking. The intestine grow faster than the abdominal

cavity. The eyelids begin to grow. The trunk elongates and straightens, although

a C-shaped spinal curve remains at birth.

8th weeks The external genitalia begin

to differentiate, but male and female characteristics are not distinct until after the 10th week. 3cm in length approximately.

By the end of the 8th week, all major organ systems are in place, and many are functioning in a simple way.

Development of the specialized structure is controlled by three factors:

The genetic information in the chromosomes received from the parents.

Interaction between adjacent tissue

And timing

Teratogenic effects Diagnosed procedures are

prescribed. Harmful drugs. Inadequate amount of folic acid

before or during pregnancy.

(Effects may cause prenatal damage)

3. Fetal Stage

Is begins 9 weeks after conception and ends with birth.

9 – 12 weeks Fetus approximately 6-8cm in length. At the beginning of this period, head

is large, about half the total length of the fetus.

The body begins growing faster than the head.

Extremities developed, fingers and toes differentiated.

External genitalia show signs of male or female sex.

The first fetal movement begin but are too slight for the mother to detect.

Blood formation occurs primarily in the liver during the 9th week but shifts to the spleen by the end of the 12 week.

The fetus begins producing urine during this period, excreting it into the amniotic fluid.

13 – 16 weeksFetus length 9-12 cm Weight 110 g. because the

fetus grows rapidlyQuickening.

Head and thorax can be identified by ultrasound.

The face looks human because the eye face forward.

17 –20 weeks Fetus length 19 cm. Weight 320 g. Heart beat can be heard with

fetoscope.

Meconium begins collecting in bowel.

This period very rapid growth.

Fetal movement feel like fluttering or butterflies.

Vernix caseosa, fatty, cheese like secretion of the fetal sebaceous glands, covers the skin to protect it from constant exposure to amniotic fluid.

Both vernix and lanugo hair diminish as the fetus reaches term.

Lanugo hair cover the fetal body to help the vernix adhere to the skin.

Eyebrows and head hair appear. Brown fat is heat producing fat

deposited on the back of the neck, behind the sternum, and around kidneys. Brown fat helps the neonate maintain temperature stability after birth.

21 – 24 weeks Fetus length 26 cm. Weight 630 g. Skin wrinkled and red, vernix present,

head and body covered with lanugo. The lungs begin to produce surfactant,

make it easier for the baby to breathe after birth.

25 – 28 weeks Fetus length 25-30 cm. Weight 1000 g. Eyes partially open, eyelashes

present. Respiratory system immature, but

fetus may survive if born.

29 – 32 weeksfetus length 32 cm.Weight 1700 g.Toenails present.Body filling out.

Testes descending. Iron, nitrogen, calcium stored. Vernix covers body. Chances of survival improving

33 – 36 weeks fetus length 32-35 cm. Weight 2000 – 2500 g. Skin thicker, less wrinkled as

subcutaneous fat accumulates. Excellent chance for survival.

37 – 40 weeks fetus length 36-50 cm. Weight 3400 g. Pulmonary system matures. Lanugo may be present over the

forehead, upper back and upper arms. Vernix may remain in major creases

such as groin and axillae.

Testes within scrotum. Female labia well developed, labia

majora cover labia minora. Full term ranges from the end of 38 to

40 weeks of fertilization age. Or 40 to 42 weeks of gestational age. Because conception occurs about 2

weeks after the first LMP.

Auxiliary Structures Three auxiliary structures

developed:

1. Placenta.

2. Umbilical cord.

3. And the fetal membranes

Placenta A full term placenta is a thick,

disk or circular cake-like shaped organ.

The placenta formation begins at outer layer of the blastocyst, called the trophoblastic cells.

Measuring 15-20 in diameter, about 2.5cm in thickness and 500 grams in weight.

During in early pregnancy, the placenta is larger than the fetus.

The fetus grows faster than the placenta, however, so that the placenta is about one-sixth the weight of the fetus at the end of a term of pregnancy.

Fetal Surface Is covered with amnion which

is thin smooth membrane. The umbilical cord is inserted

in the fetal surface of the placenta usually near its center.

Maternal Surface Is dull red in color and is

divided by grooves into 15 – 20 irregular lobes called maternal cotyledons.

The maternal surface is partly covered by a thin grey layer of deciduas called the basal plate

Function of the placenta

Metabolic function. Transfer of substance

between mother and fetus. Endocrine function.

Signs and SymptomsPregnancy – 3-8 weeks

A missing menstrual period Morning sickness and

nausea Frequent urination Uterus is size of a lemon Breasts enlarge Fatigue and dizziness Positive Chadwick’s,

Goodell’s & Hegar’s sign. Increased vaginal dischrage.

Woman’s Nausea usually decreases after 12

weeks. Uterus is size of an orange. Palpate above the symphysis

pubis. Vulva varcosities may appear.

Woman’s Relaxation of smooth muscles

of veins and bladder increases the chance of varicose veins and urinary tract infections.

Woman is more aware of fetal movements.

Woman’s Fetal movements may be felt at 16

weeks. Uterus has risen into the abdomen. Fundus midway between

symphysis pubis and umbilicus. Urinary frequency decreases. Blood volume increases. Uterine souffle heard.

Woman’s Skin pigmentation increases: areolae

darken, chloasma and linea nigra may be obvious.

Colostrum may be expressed. Braxton Hicks contractions palpable. Funds at level of umbilicus at about

20or 24 weeks.

Woman’s Heartburn common as uterus

presses on diaphragm and displaces stomach.

Braxton-hicks contraction more noticeable.

Lordosis increases, waddling gait develops due to incresed mobility of pelvic joints.

Woman’s

Shortness of breath caused by upward pressure on diaphragm.

Woman may have difficulty finding a comfortable position for sleep.

Umbilicus protrudes. Varicosities more pronounced. Ankle edema may be present.