Stage One
The first stage of a fetus begins toward the end of the first trimester of pregnancy. During the first 9 weeks of development, the growing baby is referred to as an embryo. In week 10, at the end of the first trimester, it officially becomes a fetus. According to Epigee Health & Fitness, the fetus has facial features at this stage, and its heart has already started beating. Its organs are now mostly formed and working. According to the Mayo Clinic, by the end of this stage it should be more than 2 inches long and weigh half an ounce.
Stage Two
The second stage of a fetus runs throughout the second trimester. According to Epigee, the risk of miscarriage goes down significantly at this point. The fetus develops its eyelids, nails and hair during this stage, and by the end of the trimester it will have fingerprints. According to the Mayo Clinic, it also starts to produce and eliminate urine. Its inner ear bones develop enough to detect sound, and it can now start to hear noises outside the womb. It will also start to suck its thumb and move around. During this stage, the mother will start to feel that movement. The brain goes into a rapid develop phase by the end of this stage. At this point, according to the Mayo Clinic, the fetus should be about 9 inches long and should weigh around 2 pounds.
Stage Three
The third stage of a fetus runs from the start of the third trimester through its birth. According to Epigee, it would have a 50 percent chance of survival if it were born at the beginning of this stage if it got proper medical assistance. This chance goes up significantly by the middle of the third trimester as the fetus is almost ready for birth. It will open its eyes and start blinking as it begins to detect light. It will practice breathing and may even swallow amniotic fluid, which can cause hiccups. Fat will build up to help it stay warm once it is born. The fetus is fully grown and formed by the end of the third trimester. According to the Mayo Clinic, it's normal for delivery to happen anywhere from 2 weeks before the predicted due date to 2 weeks afterward without any harm to the fetus.
