First trimester of pregancy occurs from 0 to 12 weeks. This includes the ovarian, embryonic, and fetal phases of development. During the ovarian phase, follicle growth occurs from FSH, which is then followed by ovulation induced by LH. The embryonic phase occurs from weeks 6-10, in which a number of activities develop; such as the neurotube, fetal cardaic activity, transition from flat and trimlaminar embryo to C-shape, and umbilical cord. The fetal phase occurs from weeks 11-12, in which mineralization of the fetal skull and femur occurs. Below are the following activites/developments during the first trimester in their approriate weeks.
Week 1 - follice growth
Week 2 - ovulation, followed by fertilization the next day
Week 3 - Implantation of blastocyst into decidualized endometrium
Week 4 - Implantation complete, and development of gestational sac occurs. Embryo begins to grow and elongate.
Weeks 4 to 5 - neuroplate develops, which froms the neurocrest and neurotube. The neurotube eventually forms the brain and spinal cord.
Week 5 - woman's normal menstrual flow is absent, and yolk sac develops within gestational sac (suspect of pregnancy)
Weeks 6 to 10 - critical phase of human development (beginining of embryonic phase), in which the primitive heart begins to beat.
Week 6 - neurotube develops brain that consists of three segments: proencephalon, mesencephalon, and rhombencephalon.
Weeks 7 to 8 - umbilical cord develops, and appearance of embryo changes from flat and trilaminar to C-shaped embryo with limb buds.
Week 8 - mineralization of fetal skeleton begins
Week 9 to 10 - choroid plexus and falx develops
Week 10 - functional fetal kidney tissue appears
Week 11 - fetal intestinal activity begins
Week 12 - fetal swallowing develops
Weeks 11 to 12 - mineralization of fetal skull and femur occurs