In nine months, a future human being develops from a ball of cells into a baby. Fetal development follows a well-documented chronology and is divided into trimesters.
First Trimester
The woman's menstrual period begins the first trimester. Sometime during the two weeks after her period, conception occurs. The sperm penetrates the egg and the two sets of DNA combine, which starts a whole cascade of changes. The fertilized egg travels down the fallopian tube and eventually implants itself into the uterus. Once implanted, the placenta and the amniotic sac, or the soft membrane that will contain the baby, begins to grow. According to the Mayo Clinic, by week six, the baby is developing facial features, a spinal cord and an immature nervous system is beginning. A tiny heart with a primitive circulatory system has started. This little being is still considered a zygote at this stage. Around week 11, the baby is called a fetus. The eyes are fused shut and widely spaced and the irises are developing. The baby is 2-1/2 inches long and weighs 1/2 ounce by week 12.
Second Trimester
The changes that the the fetus makes during this trimester are remarkable. The baby's kidneys are starting to function and amniotic fluid begins to fill the amniotic sac. The bones in the baby's head, arms and legs are beginning to form. By week 14, red blood cells are beginning to form in the baby's spleen. Around week 16, the baby's eyes are slowly moving forward to the front of the face and the ears are almost in the correct position. At around 18 weeks, the baby can hear. At 19 weeks, a female fetus will develop a uterus and vagina. By the end of this trimester, the baby's organs are completely formed, the fetus is able to swallow and move arms and legs and male testes are forming. A baby's movements will be felt by its mother around 20 weeks. At the end of this trimester, the lungs, liver and immune system are functioning.
Third Trimester
The main goal of the third trimester is to finish the organs and add weight to the fetus in order to assure survival at birth. Breathing practice can sometimes be seen on ultrasound and the baby's arm and leg movements will become strong. By week 28, the baby's eyes will open and eyelashes have formed. A baby's eyes can detect light by 33 weeks. At week 39, the male testes descend from the abdomen into the scrotum. By 40 weeks, the fetus has gained enough fat for adequate temperature control and acquired antibodies for protection against infections from the placenta. It is now time to be born.
