First Trimester
A woman's pregnancy is typically dated in weeks, with the start date at her last menstrual period. The weeks up to the 12th week are typically known as the first trimester. The process begins with a sperm penetrating the egg, forming a zygote. The zygote travels through the fallopian tubes or in the uterus. At this point, it is called a morula. Once it burrows into the side of the uterus and attaches, beginning to build the placenta, it officially becomes an embryo. Embryonic growth is rapid and complex. The basic organs and nervous system evolves, including the brain, heart, gastrointestinal tract and spine. By week six, a vaginal ultrasound may reveal a fetal heartbeat. By week eight, the basics of human form are visible, including hands, feet and jaw. By week 13, sex organs are formed, the embryo is now considered a fetus, and the fetus weighs about an ounce and is about three inches long, according to the American Pregnancy Association.
Second Trimester
In the early weeks, the fetus begins basic sucking, a key reflex for all babies. Sweat glands develop and the liver and pancreas begin to function in a rudimentary fashion. By week 17 to 20, most women experience a feeling of movement from the fetus. The eyes begin to develop and the baby begins to put on weight as the second trimester development continues. By the 24th week, if the fetus were delivered, medical technology might be able to help the baby survive outside the womb, according to the American Pregnancy Association.
Third Trimester
The brain continues rapid development and the baby begins to put on weight. Most fetuses weigh about 4 lbs. by the 32nd week. The lungs continue to develop and the baby assumes a head-down position for normal delivery by week 33. By week 38, if you go into labor, your doctor will not stop labor from progressing. At this week, fetal development is considered complete. The lungs are capable of processing oxygen for the baby, all organs are developed and the fetus uses the placenta to receive antibodies to protect itself against infection.
