How far along are you? I was 18 weeks on Monday and had a scan. Baby measured at a little over 9oz. My MFM said that put her in the 91 percentile for weight at that gestation. Depending on what week you are, it might be accurate or inaccurate. If it seems inaccurate, you may want to consider a second opinion.
I would just view it as information at this point, it’s not “good” or “bad” it’s just where you baby is currently measuring. The last trimester will be a big growing time for your baby so the percentile could change and is only just an estimate until your baby arrives and they can take exact weight measurements. My 20 week ultrasound had my little guy measuring at 11oz which is all in range as is your 13 ounce measurement