How long is the first trimester of pregnancy?

The first trimester of pregnancy is commonly recognized as lasting 12 weeks. It begins on the first day of the last menstrual period (LMP) and continues until the end of the 12th week of gestation. This timeframe is significant as it encompasses crucial early developments in the fetus, including the formation of vital organs and systems.

Choosing 14 weeks or 16 weeks incorporates too much time into the first trimester, as these durations extend well into the second trimester, which starts after the 12th week. Similarly, while 10 weeks may seem close, it does not encompass the full, medically accepted guidelines for the first trimester's duration. Thus, identifying the first trimester as lasting 12 weeks is accurate and aligns with standardized medical practice.