First Trimester
The first trimester is the stage in which an embryo officially becomes a fetus. It runs from week one to week 13, with the fetal stage beginning around week nine. At the end of this trimester, the fetus is about 3 inches long and it weighs about 1 oz., with its head making up about half of its total size. It will have moved from a microscopic being to a recognizable human that has developed most of its tissues and organs. It also has male or female genitals, although they won't show up on an ultrasound yet, and its spinal cord and digestive system are still developing. It has tooth buds and is able to make a fist. The Palo Alto Medical Foundation states the heartbeat may be detectable with the proper instruments as early as week 10. Although the fetus makes some movements, the mother will not be able to feel them yet.
Second Trimester
The second trimester of fetal development in the womb runs from week 14 through week 26. At this point, the fetus can suck and will swallow some amniotic fluid. Its pancreas and liver are beginning to function, and its lungs are developing air sacs. The Palo Alto Medical Foundation says its fingers and toes are now defined, and it has fingerprints. It will develop sleeping and waking patterns, and its mother will be able to feel its movements in the womb. It can even hiccup at this stage. It develops body fat, which makes its skin look less transparent, and its body is covered with fine hair. By the end of this trimester, American Pregnancy Association says the fetus is well developed enough to potentially survive outside of the womb with proper medical assistance. It will be 12 to 14 inches long and weigh up to 2 lbs.
Third Trimester
The third trimester of a fetus' development runs from week 27 through birth. The American Pregnancy Association says the fetus is begins to gain weight rapidly as its final development prepares it to survive outside of the womb without assistance. It begins to make some breathing motions as the lungs and other organs complete their development. It opens its eyes and loses the hair covering its skin. Its head and body are now more proportionate, and by the end of the trimester it will weigh up to 10 pounds. It will continue to receive antibodies in the womb from its mother until it is born, to help protest it from disease. At the end of this development stage, the Palo Alto Medical Foundation says the fetus will move into a head down position in preparation for birth.
