Normal?

Loredana
I went to my doctors appt last week I was 31 weeks pregnant. Since 4 weeks ago baby is been measuring 1 week bigger. Wheb i went last week she told me that at around 36 weeks she is going to send me to do an ultrasound to see how much the baby weights. Is this normal? My sister in law never got that ultrasound. Is it because baby is measuring bigger?