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Human Growth and Development

Prenatal Developmentfetal Stage

Major Developmental Events during the Third Trimester

During the third trimester of a pregnancy the fetus continues to grow, while the organ systems continue to develop to the point of being fully functional. Fetal movements become stronger and more frequent.

Time Period Major Developments
Week 28 Eyes begin to open and close; fetus has wake and sleep cycles
Week 29 Bones are fully developed, but still pliable; fetus begins to store iron, calcium, and phosphorus
Week 30 Rate of weight gain increases to 0.5 pounds (227 grams) per week; fetus practices breathing; hiccups are not uncommon
Week 31 Testes begin to descend into scrotum in boys; lungs continue to mature
Week 32 Lanugo begins to fall off
Week 33 Pupils in the eyes constrict, dilate, and detect light; lungs are nearly completely developed
Week 34 Vernix becomes thicker; lanugo has almost disappeared
Week 35 Fetus stores fat all over the body; weight gain continues
Week 36 Sucking muscles are developed
Week 37 Fat continues to accumulate
Week 38 Brain and nervous system are ready for birth
Week 39 Placenta continues to supply nutrients and antibodies to fight infection
Week 40 Fetus is fully developed and ready for birth


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