At what gestational age is the first trimester defined?

The first trimester of pregnancy is defined as the period from conception to 13 weeks of gestation, which encompasses roughly the first three months of pregnancy. This timeframe is critical as it is during this period that the foundations for major organ systems and body structures are established. By the end of the first trimester, significant developmental milestones are typically achieved, such as the formation of essential organs and the development of the fetus from an embryo.

The definition of the first trimester at up to 13 weeks aligns with commonly used obstetric terminology and guidelines. This period is crucial for both maternal and fetal health, as it is also when early prenatal care and screenings typically begin. Accurate recognition of this timeframe is important for healthcare providers in monitoring the health of both the mother and the developing fetus, providing appropriate interventions, and preparing for the second trimester.