Prenatal Development


First 2 Weeks: Germinal Period

    After conception, the fertilized egg divides repeatedly into a mass of cells. The germinal period is the period of prenatal development that takes place in the first 2 weeks after conception. By 1 week after conception, the fertilized egg is composed of 100 to 150 cells. These cells have differentiated into 2 layers, an inner and outer layer. The blastocystis the inner layer of these cells that develop during this period and later develop into the embryo. The outer layer is the trophoblast which also develops during this period and later provides nutrition and support for the embryo. About 10 days after conception, the fertilized egg or now called a zygote, attaches to the inner wall of the uterus.

2 To 8 Weeks: Embryonic Period

    The embryonic period occurs 2 to 8 weeks after conception. During this period the rate of cell differentiation intensifies, with support systems and organs beginning to form. The zygote now forms another 2 layers of cells and is then called an embryo. The embryo’s new inner layer of cells, the endoderm, later develops into the digestive and respiratory systems. The outer layer is then further divided into 2: the ectoderm and the mesoderm. The ectoderm later becomes the nervous system and sensory receptors (ears, nose, and eyes, for example) and skin parts (hair and nails, for example). The mesoderm later becomes the circulatory system, bones, muscles, excretory system, and reproductive system. Every body part eventually develops from these 3 layers.

As the 3 layers form, the embryo’s life support systems begin to mature and develop rapidly. These life support systems include the placenta, the umbilical cord, and the amnion.
placenta – consists of tissues where blood vessels between the mother and baby intertwine.
umbilical cord – contains 2 arteries and a vein, connects the baby to the placenta. Nutrients from the mother’s blood and digestive wastes from the baby’s blood can now pass back and forth between mom and baby.
amnion – a pouch containing a clear fluid that envelopes and protect the developing embryo.
GROWTH: At the third week the neural tube that becomes the spinal cord begins to form. At 21 days, eyes begin to appear and at 24 days, cells for the heart begin to differentiate. During the fourth week, arm and leg buds emerge. When organs are being formed, they are vulnerable to environmental changes.

2 To 9 Months: Fetal Period

    Growth and development continue dramatically during the fetal period. Three months after conception, the fetus is about 3 inches long and weighs about 1 ounce. It has become very active, moving its arms and legs, opening and closing its mouth, and moving its head. The face, forehead, eyelids, nose, chin, arms, hands, and lower limbs are now distinguishable, and the genitals can be identified as male or female. By the end of the fourth month, the fetus is about 6 inches in length and weighs 4 to 7 ounces. By the end of the 5th month, the fetus then grows to 12 inches long and weighs close to a pound. Its toenails and fingernails have begun to form and is more active, showing a preference for a particular position in the womb. By the end of the 6th month, a thin layer of hair has formed and irregular breathing occurs. By the end of the 7th month, the fetus is now about 16 inches long and weighs about 3 pounds. During the 8th and 9th months, the fetus rapidly gains another 4 pounds. At birth, the average American baby weighs 7 pounds and is about 20 inches long.

Fetal Growth Chart by Trimester:

First Trimester (first 3 months)
Conception to 4 weeks

• Less than 1/10 inches long
• Spinal cord, nervous system, gastrointestinal
system, heart and lungs begin to develop
• Amniotic sac envelopes the body

8 weeks

• Less than 1 inch long
• Face is forming with rudimentary eyes, ears, mouth and teeth
• Arms and legs are moving
• Brain is forming
• Fetal heartbeat is detectable with ultrasound

12 weeks

• About 3 inches long, weighs 1 ounce
• Can move arms, fingers and toes
• Fingerprints are present
• Can smile. frown, suck and swallow
• Sex is distinguishable

Second Trimester (middle 3 months)
16 weeks

• About 5 1/2 inches long and weighs 4 ounces
• Strong heartbeat
• Thin, transparent skin formed
• Downy hair (lanugo) covers body
• Fingernails and toenails are forming
• Coordinated movements; is able to roll over in amniotic fluid

20 weeks

• 10 to 12 inches long and weighs 1/2 to 1 pound
• Heartbeat is audible with stethoscope
• Sucks thumb
• Hiccups
• Hair, eyelashes, and eyebrows have formed

24 weeks

• 11 to 14 inches long and weighs 1 to 1 1/2 pounds
• Skin is wrinkled are covered with protective coating
• Eyes are open
• Has strong grip


Third Trimester (last 3 1/2 months)
28 weeks

• 14 to 17 inches long and weighs 2 1/2 to 3 pounds
• Adding body fat
• Very active
• Rudimentary breathing present

32 weeks

• 16 1/2 to 18 inches long and weighs 4 to 5 pound
• Periods of sleep and wakefulness
• Responds to sounds
• May assume birth position
• Bones of head are soft and flexible
• Iron is being stored in liver

36 to 38 weeks

• 19 inches long and weighs 6 pounds
• Skin is less wrinkled
• Less active
• Gaining immunities from mother



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