Stages of fetal development

By Karon West, R.N. on December 16, 2013
Pregnancy and human fetal development begin at the moment of conception. For the next nine months, you and your unborn child will experience fundamental changes.

FIRST TRIMESTER (one to 13 weeks)

This is a period of rapid growth from a single cell to a recognizable human fetus approximately three inches (7.5 cm) long, made up of billions of cells.

Four weeks: The embryo is just visible.
  • Development of the chorionic villi, from which the placenta is formed.
  • Production of the hCG hormone (human chorionic gonadotropin), which stops menstruation from occurring.

Five weeks:
The beginning of the embryo's nervous system becomes evident as a ridge of tissue that will develop into the brain and spinal cord.
  • Initial formation of the heart with faint beats. The nose and cheeks begin to show under the eyes.

Six weeks: The embryo grows to about 1/3- inch (less than one centimetre).
  • The spinal cord, brain, ears and eyes have started to develop.
  • The internal organs (lungs, digestive system, kidneys) begin to appear.
  • Tiny buds of arms and legs take shape.

Seven weeks: The major organs develop.
  • Distinct facial features appear with skin covering the eyes.
  • There are now clearly definable arms and legs with the emergence of developing fingers and toes.
  • The muscles, bones and blood vessels begin to develop. The heart of the tiny embryo beats rapidly at about 160 beats per minute.

Eight weeks: All the major organs are nearly formed.
  • The head is very large in proportion to the body as rapid brain growth occurs.
  • The embryo will now make slight movements and is about 3/5-inch (about two centimetres) long.
  • The amniotic sac fills about two-thirds of the uterus, sealing in the amniotic fluid which surrounds and protects the unborn infant during pregnancy.

Twelve weeks: The rapidly developing embryo is now called a fetus.

Nine to 13 weeks: By 12 weeks, the fetus is a recognizable human being.
  • Although gender was determined at conception, the sex organs begin to differentiate into male or female genital organs. The fetus starts to exercise its muscles.
  • By the end of this trimester, the fetus is a tiny but fully-formed human being which is three inches (7.5 cm) long and weighs one ounce.


SECOND TRIMESTER (13 to 26 weeks)

Sixteen weeks: The fetus is very active now. Fine, downy hair begins to grow over the fetus's delicate, semi-transparent skin.
  • Bone and circulatory growth continues rapidly. Fingernails and toenails are present at this stage.
  • The external genital organs are fully formed by this time and gender can be clearly distinguished.

Twenty weeks: Fetus swallows amniotic fluid. The kidneys produce fairly large quantities of urine, which is excreted into the amniotic fluid.
  • Scalp hair appears.

Twenty-four weeks:
At this stage, the fetus is termed viable (able to survive) and weighs about one to one-and-a-half pounds.
  • There is very little body fat.
  • The skin is wrinkled and covered with a white cheesy substance called vernix this protects and lubricates the delicate skin. It is still there at birth.
  • You may now feel the baby's vigorous movements.

THIRD TRIMESTER (27 to 40 weeks)

Twenty-eight weeks: Your unborn child will spend the next few weeks laying down fat under her skin.
  • The body undergoes rapid growth; size becomes more proportionate to her head.
  • Hiccuping is common; you experience this as a gently rhythmic sensation.
  • The lungs are beginning to function.

Thirty-Two weeks: A baby born at this stage has a good chance of survival with intensive neonatal care.
  • The baby weighs about four to four-and a-half pounds.

Thirty-six to 40 weeks: a great deal of fat continues to be laid down.
  • Your infants lungs have matured significantly in preparation for survival in the external environment.
  • The vernix is shed.
  • Your child at this time is fully developed with the characteristic features of the newborn infant.



By Karon West, R.N.| December 16, 2013

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