From the time sperm and egg join to begin creating a baby to the time the baby is born, the unborn baby undergoes several changes in order to fully develop. The changes occur in stages, known as trimesters. Each trimester lasts for approximately 12 weeks. Most babies follow the same loose schedule of events in which different organs and body parts develop.
The First Trimester
The first trimester begins with the last period of the mother, according to the Mayo Clinic. Around two weeks after the mother's period, during the third week of the first trimester, a sperm penetrates an egg and they collectively become a zygote. By the fourth week, the zygote implants itself in the walls of the uterus and begins to grow. A circulatory system begins to develop during the fifth week, and the heart begins pumping blood in the sixth week. During the seventh week, the baby's head and facial features begin to develop and by the eighth week, the baby begins to move. The baby's toes, neck and genitals form between the ninth and 12th week. By the end of the first trimester, the baby is about 2 1/2 inches long and weighs about half of an ounce.
The Second Trimester
During the 13th week of pregnancy, the baby begins expelling waste through urine. A milestone in the pregnancy occurs when the sex is determinable during the 14th week of pregnancy. The baby develops functioning organs and body parts, such as bones, in the 15th week. Before the halfway point, the 20th week, the baby begins to form fat and gains the ability to hear in the 17th and 18th weeks. The baby develops fingerprints, footprints and hair between the 23rd and 24th weeks before ending the second trimester with an immune system and a length of around 8 inches in the 27th week, reports Medline Plus.
The Third Trimester
During the last trimester, the baby finishes developing its important organs and organ system. The baby becomes more aware of its surroundings after its eyes open in the 30th week and it begins detecting light in the 33rd week. Before the baby's due date in the 40th week, the baby rapidly gains weight in the 36th week and is full-term, or fully developed, in the 37th week. The baby may arrive any time within a week or two of its due date.
