Aiyoh, 28 weeks and 39kg is memang quite light lah, better check with your doctor soon sebab weight gain during pregnancy is important untuk baby's development and your health juga. A few of my mum friends kena this situation, doktor dirang suggest increase calories, protein, and frequent small meals, plus maybe iron supplements if needed, so jangan worry, but do get checked properly ya, bukan sesuatu yg boleh tengok-tengok je.