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How does my baby develop in the third trimester?

Your baby is growing bigger and bigger during the final trimester. Though they are almost at the birth length, your baby still has a way to go until he or she reaches their birth weight. Your baby's lungs and muscles are developing as their bones harden and head and brain grow. Here are some highlights:

  • Week 28 - Baby may be growing eyelashes and blinking. He/she has a good chance of being born healthy if you have a preterm delivery. 
  • Week 31 - Baby continues to add fat and is starting to get that newborn look. The central nervous system has fully developed, and he/she can now regulate its body temperature.
  • Week 32 - Baby is practicing breathing and will lose the soft hair known as lanugo that has been keeping their body warm.
  • Week 33 - Baby starts sensing when light changes. Their pupils dilate and contract with variances in light.
  • Week 35 - Baby has fully developed kidneys and a liver that can begin to filter waste. Baby has grown so big, you will experience less fetal somersaults, though he/she will still kick.
  • Week 36 - Baby now has a fully developed digestive system. Until now, he/she has been wrapped in a protective fluid called the vernix caseosa. He/she sheds these layers, swallows them, and combines it with other secretions to create their first bowel movement, called meconium.
  • Week 39 - Baby has distinct cycles of sleeping and waking. During waking hours, he/she will continue to kick, even though it is becoming quite cramped.

The third trimester is the period of greatest growth for your baby, who is starting to gain about one-half pound every week. Her organs are continuing to mature. She sucks her thumb, looks around her snug little world and has definite periods of activity and rest. By 36 weeks or so, she will start settling into her birth position, which is head-down for most births.

Dr. Deborah Raines, MSN
Nursing Specialist

During the third trimester the fetus is getting ready for birth and extrauterine life.

By the beginning of the third trimester all the major body organs are present but not yet functional for existence outside the uterus. For example the lungs are present and some rhythmic breathing movement are occurring, but the pulmonary vascular structure and surfactant production necessary for breathing air are not yet mature. Therefore during the third trimester the functional abilities of the organ systems are readied for life outside the uterus.

In the third trimester, the fetus’ appearance changes, taking on characteristics of a newborn. The fetus’ transparent, red, wrinkled skin smooths out and gains opacity as the fetus gains weight and develops a layer of underlying or subcutaneous fat. Also the lanugo or soft hair that has covered the fetus’ body begins to fall off. Finally, toe nails become visible, finger nails grow and small breast buds appear.

During the third trimester the fetus is growing. At the beginning of the third trimester the fetus is about 12-14 inches in length and weights a little over 2 pounds. By the end of the third trimester the fetus weight between 7-10 pounds and is approximately 19-21 inches in length. The fetus’ arms and legs become chubby and the mother may notice a change in fetal activity as the fetus is now filling the uterus. While fetal “flips” are no long possible, there will still be a lot of stretches, wiggles and rolls. A lack or decrease in fetal movement is a reason to contact the health care provider. During this time the fetus has developed fat stores as well as its own stores of calcium, iron, phosphorus, glucose and brown fat.

Finally, during the third trimester the fetus prepare for the journey through the birth canal. The majority of fetuses assume a head down position and settle into the pelvic bone awaiting the onset of labor and their journey to the outside world.

Lots of important things happen to your baby in the last few weeks of pregnancy. For example, your baby's brain and lungs are still developing. Being pregnant for at least 39 weeks gives your baby’s body the time it needs to grow and develop. Learn more at: marchofdimes.org/week-by-week

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.