Key week- by- week changes that occur during embryonic and fetal development:

WEEK THREE OF GESTATION:

• brain, spinal cord, heart, gastrointestinal tract start to develop.

WEEKS FOUR TO SIX:

• arm and leg buds become visible.

• the brain develops into five areas; the eyes and ears begin to form.

• the heart continues to develop and now beats at a regular rhythm.

• nose, mouth and ears begin to form.

WEEKS SEVEN TO EIGHT:

• elbows and toes are visible.

• by the end of the second month, all major organs and body systems have begun to form; in the brain, nerve cells are branching out to connect with one another.

WEEKS NINE TO 12:

• eyelids close and won’t reopen until about the 28th week.

• the hands are more developed than the feet and the arms are longer than the legs.

ENTERING THE SECOND TRIMESTER:

WEEKS 13 TO 16:

• more muscle tissue and bones have developed.

• the arms and legs can flex.

• the mouth makes sucking motions.

WEEKS 17 TO 19:

• the fetus can hear sounds; the mother may feel fluttering in the lower abdomen.

WEEKS 20 TO 23:

• fetal heartbeat can be heard with a stethoscope.

• bone marrow begins to make blood cells.

• the lower airways of lungs develop but still do not produce surfactant, a substance that helps inflate the air spaces in the lungs.

ENTERING THE THIRD TRIMESTER:

WEEKS 24 TO 28:

• air sacs form in lungs; the lungs start to produce surfactant.

• rapid brain development occurs.

• the fetus kicks and stretches and can make gasping motions.

• eyes can open and close and sense changes in light.

WEEKS 29 TO 36:

• rapid increase in the amount of body fat.

• rhythmic breathing movements occur.

• bones are fully developed but still soft and pliable.

WEEKS 37 TO 40:

• fetus considered full term; the brain continues to develop until the time of delivery and in the first year of life after birth.