Pregnancy consists of three stages called trimesters. Most healthy pregnancies progress at approximately the same rate, but many variables can affect fetal development during each trimester. For this reason, the American Pregnancy Association advises that you use standard milestones of development as a guide rather than as an exact timetable.
First Trimester
The first trimester of pregnancy covers weeks one through 12. As early as week three, the brain, heart, spinal cord and gastrointestinal tract begin to develop. A little later, arm and leg buds become visible, and eyes and ears begin to form. By the end of week five, the embryo has grown to approximately ¼ inch long. Lungs, nose, jaw and palate begin to form during week six, and web-like structures that will eventually become the fingers and toes develop. By week seven, the embryo still weighs less than an aspirin, but every vital organ has already begun to form.
By the end of week eight, the embryo has begun development of everything present in an adult human body. The eighth week marks the end of the embryonic period--the developing baby now officially becomes a fetus. By the end of the first trimester, the sex organs develop into male or female, but are not clearly visible on an ultrasound. The fetus has now grown to about three inches in length and weighs about one ounce. The head makes up almost half the entire fetus.
Second Trimester
As the fetus enters the second trimester, fine hair called lanugo forms on the head. The tiny fingers now have fingerprints that will distinguish the baby from all other humans. As the fetus begins to move around more, mom may feel flutters in her abdomen. By the end of week 16, the fetus has grown to six inches in length and weighs about four ounces.
During weeks 17 through 20, the eyelashes and eyebrows grow in, and tiny fingernails and toenails begin to form. A stethoscope can now detect a heartbeat. The fetus begins to look more like a newborn infant by week 23. The fetus now weighs about one pound and measures 10 to 11 inches in length.
Third Trimester
The beginning of the third trimester marks a significant milestone in your pregnancy. If delivered now, your baby has a 90 percent chance of surviving with no physical or neurological impairment, according to the Mayo Clinic. The fetus now has cycles of sleeping and being awake and has a startle reflex. Air sacs in the lungs begin to form, the brain starts to develop more rapidly and the nervous system can now control some functions. Breathing movements begin about this time, the eyelids open and bones become fully developed, although still soft and pliant.
Sometime during weeks 33 through 36, the baby descends into the head down position to prepare for birth. The skin sheds the fine lanugo hair and appears less red and wrinkled. At 38 weeks, the full term fetus has all organs developed and is ready for birth. Fetal movement may decrease because the fetus has very little room to move. Measuring 19 to 21 inches long, the fetus now weighs from 6 ¾ to 10 pounds.
