Question

Beta hCG is detected earliest in serum by which day of conception?

• A. 8 days
• B. 15 days
• C. 21 days
• D. 30 days

Hint:

Serum picks up hCG before urine does - think roughly a week after conception.

Answer 1

The Correct Option is A

Step 1: The hormone tested in pregnancy is human chorionic gonadotropin, made by the syncytiotrophoblast layer of the early conceptus. Its beta subunit is specific and is what assays measure.

Step 2: Trophoblastic tissue starts releasing hCG into the maternal bloodstream right after the blastocyst implants. Sensitive serum immunoassays pick it up at roughly 8 to 9 days post conception.

Step 3: Because the hormone must accumulate before it spills into urine, home urine kits turn positive only around 2 weeks after conception. So serum always wins on timing.

Step 4: Comparing the choices, 8 days matches the earliest serum threshold, while 15, 21 and 30 days are far beyond the first detectable point.

\[\boxed{\text{8 days}}\]