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.
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Major Developmental Events during the Third Trimester |
| 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, |
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calcium, and phosphorus |
| Week 30 |
Rate of weight gain increases to 0.5 pounds (227 grams) per week; |
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fetus practices breathing; hiccups are not uncommon |
| Week 31 |
Testes begin to descend into scrotum in boys; lungs continue to |
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mature |
| Week 32 |
Lanugo begins to fall off |
| Week 33 |
Pupils in the eyes constrict, dilate, and detect light; lungs are nearly |
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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 |
