Congrats on your grandbaby!
I just had my own baby a month ago
Since I had IUGR and my belly was very very very small throughout most of my pregnancy.. im not much help. I found out when I was 21 weeks though! and also it was SO hard for us to tell the sex because she was covering her privates (she was cupping them like how boys do when they get kick in the you know what! lol).. and she had her legs crossed! so i had to go to another place to get another ultrasound and they were able to tell..
its very exhilarating to find out the sex im excited for you! whatever the baby maybe, may he or she be safe and healthy