Aling part exactly ng left side ng balakang mo - yung joint area or lower back? Kasi 16 weeks, normal na kaunti-kunting aches dahil sa weight at hormones na nag-relax ng ligaments, pero kung consistent na siya or mas lumalaki, better to mention it sa OB mo para sure, a few of us dito nag-experience ng round ligament pain na parang ngalay, pero iba-iba talaga depende sa positioning ng baby at posture mo. Try adjusting how you sit and avoid staying in one position too long, pero don't worry, usually hindi yan serious, just uncomfortable lang.