The three stages of the development of a fetus include the first trimester, second trimester, and the third trimester.
First trimester- the baby is the most vulnerable, up to 25% of pregnancies end in a miscarriage. In the first four weeks, the zygote is still the same size of an apple seed, the spinal cord, central nervous system, digestive system, heart and lungs develop. In a couple weeks, the zygote becomes an embryo with moving legs and arms, a beating heart, and developing organs and buds that eventually become teeth. Fingerprints and sweat glands have developed. The placenta develops in this time. At eight weeks, the embryo is now a fetus and is the size of a grape. The embryonic tail is gone, the hands can bend at the wrist and the feet no longer look webbed.
Second Trimester- The chance of miscarriage is gone. At fourteen weeks, red blood cells begin to form in the spleen. Sixteen weeks, fingernails, toenails, hair, eyelashes, and eyebrows have formed. Can feel the baby kick because it is bigger. Fifteen weeks, the skeleton begins to harden. Twenty weeks, you can hear the heartbeat with a stethoscope and the circulatory system is functioning. Twenty two weeks, hair called lanugo covers the body. Twenty three weeks, the reproductive parts have developed. By the end of the second trimester, the fetus is eleven to fourteen inches long, has sleep-wake cycles, senses have developed, can recognize voices and eyelids can open.
Third Trimester- Babies born at twenty eight weeks have a 90% chance of survival, but aren't full term until thirty seven weeks. At twenty nine weeks, the skeleton is fully developed. Thirty two, it responds to sound, the brain continues to develop, lanugo hair falls off. Thirty three, pupils are responsive to light. Thirty five, the Uterus protective skin called Vernix Caseosa begins to thicken in preparation for birth.
