WBC counts decline after how many hours post-birth?

• A. 6 hours
• B. 12 hours
• C. 24 hours
• D. 48 hours

Answer: (B)

White blood cell counts in newborns are high right after birth as part of the physiological stress response and adaptation to the external environment. This leukocytosis is not permanent; the count begins to fall quite rapidly during the first day. Around 12 hours after birth, you typically start to see a noticeable decline toward the normal range, with the trend continuing over the next day or so. That makes 12 hours the best match for the timing of the decline. The key idea is that the initial high WBC count is a transient newborn phenomenon, and the decline begins within the first half-day as the newborn physiologic system settles.