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Stages of Fetal Development

Fetal growth and development is typically divided into three periods:

1. Pre-embryonic: first 2 weeks, beginning with fertilization

2. Embryonic (weeks 3 through 8).

3. Fetal ( from week 8 through birth).

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From ovulation to fertilizationOvum

From fertilization to implantationZygote

From implantation to 5–8 weeksEmbryo

From 5–8 weeks until termFetus

Developing embryo or fetus and placental structures throughout pregnancy

Conceptus

Stages of Fetal Development

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Fertilization: The Beginning of Pregnancy.

• Fertilization:

union of an ovum and a spermatozoon. This usually occurs in the outer third of a fallopian tube, the ampullar portion

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Implantation

• Once fertilization is complete, the zygote migrates over the next 3 to 4 days toward the body of the uterus

• The first cleavage occurs at about 24 hours

• By the time the zygote reaches the body of the uterus, it consists of 16 to 50 cells (morula).

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• The morula continues to multiply as it floats free in the uterine cavity for 3 or 4 more days.

• Blastocyst: is Large cells tend to collect at the periphery of the ball, leaving a fluid space surrounding an inner cell mass.

• The cells in the outer ring are known as trophoblast cells. They are the part of the structure that will later form the placenta and membranes.

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Extraembryonic Membranes

The embryo reaches the uterus on day 6. It penetrates the endometrial lining & initiates membrane formation.

1. Amnion: Encloses embryo

2. Chorion: Forms from the trophoblast, interacts with uterine tissue to form the placenta.

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Milestones of Fetal Growth and Development

• End of 4th G/W• End of 8th G/W• End of 12th G/W (First Trimester)• End of 16th G/W • End of 20th G/W • End of 24th G/W (Second Trimester)• End of 28th G/W • End of 32nd G/W• End of 36th G/W• End of 40th G/W (Third Trimester)

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Fetal Growth and Development

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• Length: 0.75 to 1 cm• Weight : 400 mg

• The spinal cord is formed and fused at the midpoint.

• Lateral wings that will form the body are folded forward to fuse at the midline.

• Head folds forward and becomes prominent, represent ing about one third of the entire structure.

End of 4th G/W

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• The back is bent so that the head almost touches the tip of the tail.

• The rudimentary heart appears as a prominent bulge on the anterior surface.

• Arms and legs are budlike structures.

• Rudimentary eyes, ears, and nose are discernible.

End of 4th G/W

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End of 8th G/W

• Length: 2.5 cm (1 in)• Weight : 20 g

• Organogenesis is complete.

• The heart, with a septum and valves, is beating rhythmically.

• Facial features are definitely discernible.

• Arms and legs have developed.

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• External genitalia are present , but sex is not distinguishable by simple observation.

• The primitive tail is regressing.

• Abdomen appears large because the fetal intestine is growing rapidly.

• Sonogram shows a gestational sac, diagnostic of pregnancy

End of 8th G/W

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End of 12th Gestational Week (First Trimester)

• Length: 7 to 8 cm• Weight : 45 g

• Nail beds are forming on fingers and toes

• Spontaneous movements are possible, although they are usually too faint to be felt by the mother.

• Some reflexes, such as the Babaniski reflex, are present.

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• Bone ossification centers are forming.

• Tooth buds are present.

• Sex is distinguishable by outward appearance.

• Kidney secretion has begun, although urine may not yet be evident in amniotic fluid.

• Heart beat is audible through Doppler technology.

End of 12th Gestational Week (First Trimester)

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End of 16th Gestational Week

• Length: 10 to 17 cm• Weight : 55 to 120 g

• Fetal heart sounds are audible with an ordinary stethoscope.

• Lanugo (the fine, downy hair on the back and arms of newborns, which apparently serves as a source of insulation for body heat) is well formed.

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• Liver and pancreas are functioning.

• Fetus actively swallows amniotic fluid, demonstrating an intact but uncoordinated swallowing reflex; urine is present in amniotic fluid.

•Sex can be determined by ultrasonography.

End of 16th Gestational Week

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End of 20th Gestational Week

• Length: 25 cm• Weight : 223 g

• Spontaneous fetal movements can be sensed by the mother.

•Antibody production is possible.

• Hair forms, extending to include eyebrows and hair on the head.

• Meconium is present in the upper intestine.

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• Brown fat, a special fat that will aid in temperature regulation at birth, begins to be formed behind the kidneys, sternum, and posterior neck.

• Vernix caseosa, which serves as a protective skin covering during intrauterine life, begins to form.

• Definite sleeping and activity patterns are distinguishable ( the fetus has developed biorhythms that will guide sleep/wake patterns throughout life).

End of 20th Gestational Week

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End of 24th Gestational Week (Second Trimester)

• Length: 28 to 36 cm• Weight : 550 g

• Passive antibody transfer from mother to fetus probably begins as early as the 20th week of gestation, certainly by the 24th week.

• Infants born before antibody transfer has taken place have no natural immunity and need more than the usual protection against infectious disease in the newborn period until the infant's own store of immunoglobulin can build up.

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• Meconuim is present as far as the rectum.

• Active production of lung surfactant begins.

• Eyebrows and eyelashes are well defined.

• Eyelids, previously fused since the 12th week, are now open.

• Pupils are capable of reacting to light.

• Hearing can be demonstrated by response to sudden sound.

End of 24th Gestational Week (Second Trimester)

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End of 28th Gestational Week

• Length: 35 to 38 cm• Weight : 1,200 g

• Lung alveoli begin to mature, and surfactant can be demonstrated in amniotic fluid.

• Testes begin to descend into the scrotal sac from the lower abdominal cavity.

• The blood vessels of the retina are thin and extremely susceptible to damage from high oxygen concentrations (an important consideration when caring for preterm infants who need oxygen).

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End of 32nd Gestational Week

• Length: 38 to 43 cm• Weight : 1,600 g

• Subcutaneous fat begins to be deposited (the former stringy, “little old man” appearance is lost).

• Active Moro reflex is present.

• Birth position (vertex or breech) may be assumed.

• Iron stores, which provide iron for the time during which the neonate ingests only milk after birth, are beginning to be developed.

• Fingernails grow to reach the end of finger tips.

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End of 36th Gestational Week

• Length: 42 to 48 cm• Weight : 1,800 to 2,700 g (5 to 6 lb)

• Body stores of glycogen, iron, carbohydrate, and calcium are deposited.

• Sole of the foot has only one or two crisscross creases, compared with the full crisscross pattern that will be evident at term.

• Amount of lanugo begins to diminish.

• Most babies turn into a vertex or head-downpresentation during this month.

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End of 40th Gestational Week (Third Trimester)

• Length: 48 to 52 cm (crown to rump, 35 to 37 cm)

• Weight : 3,000 g (7 to 7.5 lb)

• Fetus kicks actively, hard enough to cause the mother considerable discomfort.

• Fetal hemoglobin begins its conversion to adult hemoglobin. The conversion is so rapid that , at birth, about 20% of hemoglobin is adult in character.

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• Vernix caseosa is fully formed.

• Fingernails extend over the fingertips.

• Creases on the soles of the feet cover at least two thirds of the surface.

• In primiparas: the fetus often sinks into the birth canal during the last 2 weeks, giving the mother a feeling that the load she is carrying is less. This event , termed lightening

End of 40th Gestational Week (Third Trimester)

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Fetal Movement and Fetal Heart Rate

Fetal Movement

• Fetal movement that can be felt by the mother (quickening) begins at approximately 18 to 20 weeks of pregnancy and peaks at 28 to 38 weeks. A healthy fetus moves with a degree of consistency, or at least 10 times a day.

• In contrast, a fetus not receiving enough nutrients because of placental insufficiency has greatly decreased movements. Based on this, asking the mother to observe and record the number of movements the fetus makes daily offers a gross assessment of fetal well -being.

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Fetal Heart Rate:

• Fetal hearts beat at 120 to 160 beats per minute throughout pregnancy.

• Fetal heart sounds can be heard and counted as early as the 10th to 11th week of pregnancy by the use of an ultrasonic Doppler technique

Fetal Movement and Fetal Heart Rate