Basically they may suspect your baby is not growing like he/she should. They are gonna try to gauge the baby's weight. IUGR stands for intrauterine growth restriction. I pray everything goes well for you and baby! They said my son had IUGR. They thought he was 5lb2 and they induced me at 37 weeks, turns out he was 5lb8. He's a healthy, thriving and intelligent 5yr old.
My doctor asked me to do this test (Intrauterine Growth Restriction) when i was towards the end of my pregnancy (around 8 months) because she felt my stomach was too compact and wanted to rule out any growth issues with the baby. I did it and it was all fine, my bump was quite compact throughout. My daughter is now 8 and she turned out alright at birth.
Your doctor may have asked for this to figure out if the baby is indeed growing as should be, and this sonography specifically tests for that. Good luck!