What are the characteristics of the first trimester in pregnancy?

The first trimester of pregnancy is defined as the period from conception to the end of 12 weeks gestation. This time frame encompasses crucial developmental milestones for the fetus during early pregnancy, including the formation of major organs and systems. Establishing the timeline as 1 to 12 weeks is essential for both prenatal care and for understanding potential risks and typical developmental changes that may occur during this stage.

Recognizing this period correctly also allows healthcare providers to offer appropriate screening tests, guidance on lifestyle modifications, and early assessments for conditions such as ectopic pregnancies or miscarriage, which are more common in the early weeks. Additionally, knowing the correct timeline helps in planning for necessary follow-up appointments and screenings that are standard as the pregnancy progresses. The other timeframes provided do not align with the established medical consensus on when the first trimester occurs.