📖 Definition
The first stage of pregnancy, from week 1 through week 13. The first trimester is the most critical period for fetal development — by the end of week 10, all major organs and body structures are present. Most miscarriages (about 80%) occur in the first trimester. Common symptoms include morning sickness, fatigue, breast tenderness, and frequent urination. First prenatal visits, genetic screening, and the NT scan occur during this period.