Girls, hi! Today I had a CTG and ultrasound. The doctor said that the …

Girls, hi! Today I had a CTG and ultrasound. The doctor said that the CTG is all good. On the ultrasound they wrote - amniotic fluid index 31. (What does this mean?) They're putting me on low amniotic fluid. My due date is 34.4 weeks according to my cycle, but on the ultrasound it's 35.1. The baby is healthy and well-fed, we will keep monitoring. Who can explain what number 31 means? I look at tables online, I don't understand anything ??