During the third trimester, the fetus continues to grow in preparation for birth. Weight triples and length doubles during this period as stores of protein, fat, iron and calcium increase substantially. Pregnant women should follow the same list of precautions as they did in the second trimester. It is likely that they will spend a greater amount of time visiting their healthcare provider during this trimester.

The third trimester unfolds as follows:

• Week 28: The eyes begin to open and close. The fetus sleeps for 20 to 30 minutes at a time. By this point, the average fetus is 15 inches (38.10 centimeters) long and weighs about 2 to 3 pounds (0.91 or 1.36 kilograms).
  
  
• Week 29: The fetus begins to store minerals such as iron, calcium and phosphorus. Bones are formed but soft at this point. The fetus may make more vigorous movements that occasionally may feel uncomfortable for the expectant mother.
  
  
• Week 30: The fetus practices breathing by moving the diaphragm rhythmically. When the fetus gets the hiccups, the expectant mother may feel small spasms of the uterus. The average fetus weighs about 3 pounds (1.36 kilograms) and will add a half-pound (0.23 kilograms) a week through the 37^th week.
  
  
• Week 31. Testicles in boys move through the groin toward the scrotum. The clitoris is prominent in girls. The lungs are better developed but are still not fully mature, and a fetus born at this point cannot breathe without the assistance of a ventilator.
  
  
• Week 32: The lanugo begins to be shed from the baby’s skin. The fetus’ movements may become constricted as space in the uterus begins to run out. However, a physician should be informed if the mother notices fewer than 10 movements in a two-hour period.
  
  
• Week 33: The pupils in the eyes are able to constrict, dilate and detect light. Lungs are more completely developed.
  
  
• Week 34: The vernix that coats and protects the skin begins to thicken. The lanugo has nearly disappeared.
  
  
• Week 35: The fetus continues to grow at a rapid pace. The punches and kicks that formerly were felt in the uterus have been replaced by stretches and wiggles.
  
  
• Week 36: The face begins to round out because of increased fat deposits. The fetus usually begins to descend in a head-down position with legs tucked into the chest and knees against the nose. This prepares the fetus for birth. The fetus also will begin to move lower under the pelvis, a process known as “lightening.” At this point, the average fetus is between 16 and 19 inches (40.64 to 48.26 centimeters) long and weighs about 6 to 6.5 pounds (2.95 kilograms).
  
  The obstetrician or other practitioner can usually determine the fetal position with a physical examination of the woman’s abdomen. Tests such as ultrasound may be used as confirmation. Sometimes the fetus is in an atypical position, such as a head-up breech presentation. Manual techniques such as external version may in some cases be recommended to try to move a fetus from the breech to head-down position. If the fetus is not in a head-down position near term, a Caesarean section may be needed for delivery.
  
  
• Week 37: Fat deposits continue to accumulate, giving the fetus a rounder look. A baby born after week 37 is considered to be full-term.
  
  
• Week 38: The fetus’ brain and nervous system continue to develop. The average fetus weighs about 7 pounds (3.18 kilograms).
  
  
• Week 39: The fetus now has enough fat under the skin to keep a constant body temperature.
  
  
• Week 40. This is the average gestation period in humans. However, only a small percentage of women deliver on this date. Many deliveries occur a week before or a week after the due date. (According to new analysis by the March of Dimes, the typical U.S. pregnancy is 39 weeks.)