What does Morrison's rule indicate about fetal age?

Morrison's rule is a method used in forensic medicine to estimate the fetal age based on the length of the fetus, specifically measuring the crown-rump length (CRL). This measurement is considered an accurate predictor of fetal age during the first trimester. As the fetus grows, certain developmental milestones can be correlated with specific lengths, allowing for an estimation of how far along the pregnancy is. The accuracy of this rule is grounded in established fetal growth patterns, making it a valuable tool in cases where determining gestational age is essential, such as in legal or medical contexts.

In contrast, the other options do not align with the specific focus of Morrison's rule. The time until birth may be an implication of fetal age but is not the main focus of this rule. Similarly, while fetal weight can provide information about growth and development, it is not a direct measure as per Morrison's rule. Lastly, the health of the fetus can be influenced by a multitude of factors, and while length may provide some indirect information, it is not the primary purpose of Morrison's rule. Thus, the relationship of Morrison's rule to the length of the fetus is what makes it an important tool in estimating fetal age accurately.