The third trimester, from week 24 of the pregnancy until the birth of the new baby, is a period of intense growth. All the major organs are now developed, and the biggest risks during pregnancy are finally over. If the baby were to be born in the beginning of this period, he would thankfully even have a chance for survival.
Developing Personality
The fetus spends more and more time awake during the final trimester. At 20 weeks, the fetal heart rate reveals no period of alertness, but by the 28th week, the fetus is awake approximately 11 percent of the time. The time that the fetus spends awake eventually increases to about 16 percent towards the end of pregnancy. The fetus also begins to develop her own personality during this time. Dr. Groome showed in his study published in "Developmental Psychobiology" that higher fetal activity during the third trimester typically predicts a more active baby or even a toddler.
Responding to External Stimuli
From the sixth month on, the baby can hear and begins to respond to sounds. He might be frightened by loud, sudden sounds. After about 28 weeks of pregnancy, it has been reported that fetuses respond to nearby sound by blinking their eyes. Within six weeks, the unborn child is able to distinguish different tones and rhythms in speakers and show changes in heart rate when males, females, familiar and unfamiliar people are around or when they are played familiar melodies. Although the visual system matures more slowly that the auditory system, Dr. J.P. Lecanuet and Dr. B. Schaal showed in a study published in "The European Journal of Obstetrics, Gynecology, and Reproductive Biology" that fetuses can respond to light from the 26th week on. At the age of 23 weeks, a fetus can also start feeling pain.
Brain Growth
The last trimester is a period of intense growth for the baby's brain. The brain develops convolutions and groves, enlarging the surface area drastically, while minimizing actual growth in the size of the head. This helps keep the baby's head small enough to pass through the birth canal. In particular, the cerebral cortex, or the outer layer of the human brain, increases in size during this period.
If the Baby Were to Be Born
During the third trimester, a fetus born prematurely has a change to survive. The time when the baby is first able to survive outside her mother occurs between 22 and 26 weeks. This time, usually called the age of vitality, varies from baby to baby. Most children born between the seventh and eighth month, however, need assistance in breathing. Although the respiratory center in the brain is mature, the air sacs in their lungs are still too small and fragile to inflate and deflate on their own and can't exchange carbon dioxide and oxygen.
Changes in the Mother
The last trimester is a time of discomfort for most mothers. The growing baby is pressing against many of her internal organs, thus making it difficult for her to move and even breathe. Many women also report that they have sleeping difficulties, or they may suffer from varicose veins, swelling in the legs or leg cramps. Braxton-Hicks, or "practice" contractions, are common. During these contractions the uterus hardens for a brief period of time and then relaxes.
References
- "Child Development": Fetal Neurobehavioral Development; DiPietro, Hodgson, Costigan; vo. 67, 2553-2567, 1996.
- "Developmental Psychobiology": Spontaneous Motor Activity in the Prenatal Infant Before and After Birth: Stability in Individual Differences.; Groome, Swiber, Holland; vol. 35, 15-24, 1999.
- "11th European Confrence on Developmental Psychology Conference; Cardiac "Orienting" Response in Fetuses and Babies Following In Utero Melody-Learning; Granier-Deferre, Basserau, Ribeiro; 2003.
- "Infants and Children"; Laura E. Berk; 2008.
- "European Journal of Obstetrics and Gynecology and Reproductive Biology": Fetal Sensory Competencies.; Lecanuet, Schaal; vol. 68, 1-23, 1996.
