B

Twice-weekly surveillance (about every 3 days ≈ 72 h) is recommended for non-diabetic high-risk pregnancies; studies show that moving from once-weekly to twice-weekly NSTs markedly lowered still-births in such patients, supporting a 72-hour interval as optimal [Boehm 1986, PMID 3960430; Keegan 1987, PMID 3319323].