Normal lang yan, mama! At 26 weeks, mga 820 grams lang talaga ang average weight ng baby kaya you're right on track, ang ultrasound measurements ay always may slight difference din kasi depende sa accuracy ng machine at positioning ng baby. Pero since gusto mo ng peace of mind, better to bring this up sa OB mo during your next check-up para ma-confirm na lahat ay progressing well and walang concerns, okay?