This article details fetal development during the third trimester. It begins with week 28 and ends with week 40. A play-by-play of what you can expect.
Reprinted with permission from American Pregnancy Association
Weeks 28 thru 32 - Gestational Age (Fetal Age - Weeks 26 thru 30):
The fetus really fills out over these next few weeks, storing fat on the body, reaching about 15-17 inches long and weighing about 4-4½ lbs by the 32nd week. The lungs are not fully mature yet, but some rhythmic breathing movements are occurring. The bones are fully developed but are still soft and pliable. The fetus is storing its own calcium, iron and phosphorus. The eyelids open after being closed since the end of the first trimester. Weeks 33 thru 36 - Gestational Age (Fetal Age - Weeks 31 thru 34):
This is about the time that the fetus will descend into the head down position preparing for birth. The fetus is beginning to gain weight more rapidly. The lanugo hair will disappear from the skin and it is becoming less red and wrinkled. The fetus is now 16-19 inches and weighs anywhere from 5¾ lbs to 6¾ lbs.
Weeks 37 thru 40 - Gestational Age (Fetal Age - Weeks 35 thru 38):
At 38 weeks the fetus is considered full term and will be ready to make its appearance at any time. Mom may notice a decline in fetal movement as the fetus is now filling the uterus with little room to move. The fingernails have grown long and will need to be cut soon after birth. Small breast buds are present on both sexes. The mother is supplying the fetus with antibodies that will help protect against disease. All organs are developed, with the lungs maturing all the way until the day of delivery. The fetus is about 19 - 21 inches in length and weighs anywhere from 6¾ lbs to 10 lbs.