Fetal Development Timeline – Third Trimester

So many exciting things occur during the fetal development timeline during the first and second trimester, but now it’s the final stretch of development. Weeks 28-40 complete the development of the baby and prepare it to enter the world.

Weeks 28 to 31

The baby measures about 12 to 16 inches and weighs anywhere from 2 to 3 1/2 pounds. During an ultrasound you could possibly see the baby open and close it’s eyes. The testicles in male fetuses are starting to drop into the scrotum. The female genitalia is still developing but the clitoris is now prominent. Fat stores under the baby’s skin are starting to accumulate, which help regulate body temperature. The lungs are the last organ to mature.

Weeks 32 to 36

The baby is now really growing and is now about 16 to 19 inches long and weighs about 3 to 6 1/2 pounds. Male and female genitalia are finalizing their development. Movement is starting to slow as the baby runs out of room in the uterus. The brain is undergoing a growth spurt which will give the baby the motor skills it will need after birth. The baby may have moved into a head down position by now.

Weeks 37 to 40

The baby is completely developed and weighs between 6 and 9 pounds and measures 18 to 21 inches. The baby continues to practice breathing in the amniotic fluid in preparation for those first breaths of air after being born. He/she can also grip tightly and on an ultrasound, you may see the baby gripping the umbilical cord. The baby may have a full head of hair at this point, although some babies are born with very little. The vernix and lanugo have began to diminish, although it’s likely the baby will be born with some of both.