Final answer:
HCG levels are crucial for monitoring pregnancy progression when a smaller gestational sac is observed on ultrasound.
Explanation:
The laboratory value that is useful when a smaller than expected gestational sac is seen with ultrasound is hCG (human chorionic gonadotropin).
HCG helps to maintain the corpus luteum and elevated levels of progesterone, which are crucial for the progression of pregnancy.
Monitoring hCG levels can provide valuable information about the viability and progression of the pregnancy, especially in cases where the gestational sac appears smaller than anticipated.
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