What each trimester is doing
Pregnancy is conventionally divided into three trimesters of roughly thirteen weeks each. The first trimester is dominated by organogenesis, when the embryo's major organ systems form; this is also when the developing embryo is most vulnerable to certain exposures. The second trimester is largely a phase of rapid growth, when movement is often first felt and many anatomy assessments are done.
The third trimester is mainly about maturation and weight gain, as the lungs and other systems prepare for life outside the uterus and the fetus lays down fat stores.
Maternal and hormonal milestones
Hormones shift across the trimesters: hCG rises early and supports the corpus luteum, while progesterone and estrogen climb as the placenta takes over hormone production. Maternal blood volume expands substantially, and changes to the cardiovascular, respiratory, and musculoskeletal systems accumulate as the pregnancy progresses.
Timing of milestones varies between individuals, and dating is usually based on the last menstrual period or early ultrasound. This is general educational information, not medical advice; prenatal care and any concerns should be directed to a qualified clinician.