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. Here is your stage-by-stage guide to fetal development to help you navigate your pregnancy.

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.