How long does the first trimester last?

The first trimester of pregnancy is defined as the period from the first day of the mother's last menstrual period (LMP) up until the end of the 12th week of gestation. This timeframe is crucial because it encompasses the early development stages of the embryo and the initial phase of the pregnancy, which include key developments such as organ formation and the establishment of critical bodily systems.

Using the first day of the last menstrual period provides a standardized starting point for calculating gestational age, which is important for both medical professionals and expectant mothers to track the pregnancy's progress. This period typically spans from week one to week twelve of gestation, aligning with the description provided in the correct choice.

Other options reflect different aspects of pregnancy but do not accurately describe the trimesters. For instance, stating that the first trimester lasts from conception to 8 weeks does not account for the total duration that includes weeks beyond the initial conception. Similarly, referring to the first ultrasound as a point of reference for determining the trimester is not standard, as ultrasounds can be conducted at various times and do not define the trimesters. Lastly, even though the first week to the 12th week encompasses the correct timeframe, it's essential to recognize that the counting starts from