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Fetal development

Alternative names

Zygote - Morula; Blastocyst; Embryo; Fetus


Once sperm is deposited in the vagina, it travels through the cervix and into
the Fallopian tubes. Conception usually takes place in the outer third of the
Fallopian tube.

A single sperm penetrates the mother's egg cell (ovum) and the developing child gets half of its genetic information (in the form of DNA) from the mother (this is contained in the egg), and half from the father (from the sperm). The resulting single cell is called a zygote.

The zygote spends the next few days traveling down the Fallopian tube and divides to form many attached cells. A ball of cells is produced, each cell including a copy of the genes that will guide the development of the baby. Once there are about 32 cells, the developing baby is called a morula.

With additional cell division, the morula becomes an outer shell of cells with
an attached inner group of cells. Now the developing baby is in the
"blastocyst" stage. The outer group of cells will become the membranes that nourish
and protect the inner group of cells, which will become the embryo (the next stage for the future baby).

The blastocyst reaches the uterus at roughly the fifth day, and implants into
the uterine wall on about day six. At this point in the mother's menstrual cycle,
the endometrium (lining of the uterus) has grown and is ready to support a
fetus. The blastocyst adheres tightly to the endometrium where it receives
nourishment via the mother's bloodstream.

During the time between implantation and the eighth week, the cells of what is now called the embryo not only multiply, but begin to take on specific functions. This
process is called differentiation, and is necessary to produce the varied cell types
that make up a human being (such as blood cells, kidney cells, nerve cells, etc.).

There is rapid growth, and the main external features begin to take form. It is during this critical period of differentiation (most of the first trimester) that the growing baby is most susceptible to damage from external sources including:

  • teratogens (substances that cause birth defects including alcohol and certain prescription and recreational drugs)
  • infection (such as rubella or cytomegalovirus)
  • radiation (ionizing radiation such as X-rays, radiation therapy, or accidental
    exposure to radiation)
  • nutritional deficiencies

Specific changes by week for the embryonic stage:

  • Week 3
    • beginning development of the brain, spinal cord, and heart
    • beginning development of the gastrointestinal tract
  • Weeks 4 to 5
    • formation of tissue that develops into the vertebra and some other bones
    • further development of the heart which now beats at a regular rhythm
    • movement of rudimentary blood through the main vessels
    • beginning of the structures of the eye and ears
    • the brain develops into five areas and some cranial nerves are visible
    • arm and leg buds are visible
  • Week 6
    • beginning of formation of the lungs
    • further development of the brain
    • arms and legs have lengthened with foot and hand areas distinguishable
    • hands and feet have digits, but may still be webbed
    • heart and circulation more fully developed
  • Week 7
    • straightening of trunk
    • nipples and hair follicles form
    • elbows and toes visible
    • bones more mature
    • all essential organs have at least begun to form
  • Week 8
    • rotation of intestines
    • facial features continue to develop
    • the eyelids are more developed
    • the external features of the ear begin to take their final shape

Note: The end of the eighth week marks the beginning of the "fetal period" and the end of the "embryonic period"

  • Weeks 9 to 12
    • the fetus reaches a length of 3.2 inches
    • the head comprises nearly half of the fetus' size
    • a neck is present
    • the face is well formed
    • eyelids close and will not reopen until about the 28th week
    • tooth buds appear for the baby teeth
    • limbs are long and thin
    • digits are well formed
    • the fetus can make a fist with its fingers
    • urogenital tract completes development
    • genitals appear well differentiated
    • red blood cells are produced in the liver
  • Weeks 13 to 16
    • the fetus reaches a length of about 6 inches
    • a fine hair develops on the head called lanugo
    • fetal skin is almost transparent
    • more muscle tissue and bones have developed, and the bones become harder
    • the fetus makes active movements
    • sucking motions are made with the mouth
    • amniotic fluid is swallowed
    • meconium is made in the intestinal tract
    • lungs further develop
    • sweat glands develop
    • the liver and pancreas produce their appropriate fluid secretions
  • Week 20
    • the fetus reaches a length of 8 inches
    • lanugo hair covers entire body
    • skin becomes less transparent as fat begins to deposit
    • eyebrows and lashes appear
    • nails appear on fingers and toes
    • the fetus is more active with increased muscle development
    • "quickening" usually occurs (the mother can feel the fetus moving)
    • fetal heartbeat can be heard with a stethoscope
  • Week 24
    • the fetus reaches a length of 11.2 inches
    • the fetus weighs about 1 lb. 10 oz.
    • hair on the head is longer
    • eyebrows and eyelashes are well formed
    • all the eye components are developed
    • the fetus has a hand and startle reflex
    • footprints and fingerprints forming
    • alveoli (air sacs) forming in lungs
  • Weeks 25 to 28
    • the fetus reaches a length of 15 inches
    • the fetus weighs about 2 lbs. 11 oz.
    • rapid brain development
    • nervous system developed enough to control some body functions
    • eyelids open and close
    • respiratory system, while immature, has developed to the point where gas exchange is possible
    • a baby born at this time may survive, but the possibilities for complications and death remain high
  • Weeks 29 to 32
    • the fetus reaches a length of about 15-17 inches
    • the fetus weights about 4 lbs. 6 oz.
    • rapid increase in the amount of body fat
    • increased central nervous system (CNS) control over body functions
    • rhythmic breathing movements occur
    • lungs are not fully mature
    • bones are fully developed, but still soft and pliable
    • fetus begins storing iron, calcium, and phosphorus
  • Week 36
    • the fetus reaches a length of about 16-19 inches
    • the fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.
    • lanugo begins to disappear
    • increase in body fat
    • fingernails reach the end of the fingertips
    • increased central nervous system (CNS) control over body functions
    • a baby born at 36 weeks has a high chance of survival, but may require some medical interventions
  • Weeks 38 to 40
    • considered full-term at 38 weeks
    • may be 19 to 21 inches in length
    • lanugo is gone except for on the upper arms and shoulders
    • fingernails extend beyond fingertips
    • small breast buds are present on both sexes
    • head hair is now coarse and thicker
    • mother supplies fetus with antibodies against disease
    • fetus fills entire uterus

Update Date: 5/31/2002

Adam Ratner, Adam Ratner, M.D., Children's Hospital of Philadelphia, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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Last updated: Tue, 06 Jan 2009 00:20:03 GMT