Pregnancy is a time of rapid growth and change for Mom and the baby. Pregnancy is divided into trimesters, and a full-term pregnancy is 40 weeks. It's normal to wonder about the growth of your fetus and his formation, and what bodily changes are happening when. From sperm and egg meeting to a full-grown baby, those 40 weeks involve incredible development.
First Trimester
After the sperm and egg form a zygote full of chromosomes, it travels to the uterus, where it's now known as a blastocyst. The blastocyst attaches itself to the uterine wall so it can obtain nourishment, and the placenta starts to form. Three weeks after conception, the embryonic period starts. This is when the spinal cord, brain, heart and various other organs start to form. Four weeks post-conception, the embryo's neural tube closes, and the heart begins to pump blood. The brain develops more, and the face begins to form. Genitals start forming in the first trimester, but they are not recognizable. At nine weeks post-conception, the embryo is now called a fetus. The genitalia are starting to form externally, fingernails start to develop, and the fetus starts to have a human profile.
Second Trimester
In the second trimester, the skin of the fetus is transparent, and fingerprints have formed on the fingers. Eyebrows and eyelashes start growing, as do fingernails and toenails. A fine, downy hair called lanugo starts to appear all over the head and body, and fat begins to form. The liver, pancreas and air sacs in the lungs are all forming and developing, and the brain and nervous system are becoming more developed. The fetus has a startle reflex, and movements can be felt by the mother.
Third Trimester
In the last trimester of pregnancy, the fetus starts storing fat, calcium, iron and phosphorus. The bones are fully developed but still soft. As the trimester goes on, the lanugo starts falling off the body and skin gets smoother. Eyelids have formed, and the eyes start to open and detect light. At 34 weeks post-conception, the baby starts to gain approximately 1/2 pound a week. Boys and girls develop small breast buds, and the lungs continue to develop until the baby is born.
