Fetal Growth Restriction

At 21w I had my anatomy scan, doctor said baby was measuring in the 8th% and I will need an MFM anatomy extended ultrasound in 2 weeks to check the growth of the baby. My first Child was fine born 2 weeks early at 6lbs 11oz, my 2nd child was in the 13-15th% born 3 weeks early at 5lbs 4oz. This is my 3rd pregnancy and I'm not sure why this is happening, I'm so confused doctor said everything is measuring fine but the weight. If anyone was experience with FGR that would be really helpful.