The first trimester is the initial stage of pregnancy, spanning from week 1 to the end of week 12 (roughly the first three months). This period begins on the first day of your last menstrual period, meaning that for the first two weeks, you are not yet technically pregnant as the body prepares for ovulation. This is a critical window of development where the fertilized egg rapidly divides and implants in the uterine wall, becoming an embryo and then a fetus. By the end of this trimester, all the baby's major organs and body systems have begun to form, and the heart starts beating around week five or six. For the pregnant person, the first trimester is often characterized by significant hormonal shifts, leading to common symptoms like extreme fatigue, breast tenderness, and "morning sickness" (nausea and vomiting). Because the foundations of life are being laid, it is a period of high vulnerability where the risk of miscarriage is statistically highest. Healthcare providers typically schedule the first "booking appointment" during this time to establish prenatal care, conduct initial blood tests, and confirm the due date via an early ultrasound scan.