What major changes occur in a developing fetus during the second trimester of pregnancy?

The second trimester of pregnancy (the fourth, fifth, and sixth months) may be best described as the time when the fetus really begins to look like what we think of as a baby. During the first trimester, the embryo grows from just a small clump of cells to a recognizable fetus. Beginning with the fourth month, or about the fourteenth week of pregnancy, some major changes begin to take place! The fetal brain is beginning to fire nerve impulses, and the fetus may be gaining a sense of control over their facial muscles. The fetus may be observed moving on an ultrasound during the fourth month, though the mother may not feel it for several more weeks. The liver and pancreas begin producing secretions, and cartilage is beginning to harden into bone.

During the fifth month, the mother is more likely to feel "kicking" or quickening, as the fetus has increased control over their muscles and many of the bones and joints have hardened. The senses will also begin to develop around this time, and myelin is laid down as a protective coating of the nerves. Because the fetus' sense of hearing is developing during this time, some parents like to talk or play music and observe for fetal movement.

During the sixth month of gestation, the fetal heartbeat may now be heard with a stethoscope. A fine, peach-fuzz like fur covers all of the fetus' body and will help to keep them warm after birth. Meconium--the baby's first poo after being born--is developing inside the gastrointestinal tract. By this time, the fetus is far more recognizable as a baby, and even develops eyelashes and eyebrows. The nails also finish growing during this time. Towards the end of the sixth month, the fetus will begin to store fat in their soft tissues, and the lower airways of the lungs are beginning to develop.