Baby bigger than glow says

Sonel • Mom of 2 boys aged 4 and 6 and expecting a miracle 3rd baby naturally after doctor said it was impossible. 4 failed iui’s and a corneal ectopic that severed my left tube
Hi ladies! Just had my 16 week scan and it shows baby is 189g (6 1/2 oz), but glow says baby should be 100g (3 1/2 oz) this week. How can it be so far wrong? My boy is  almost double what glow says. Anyone else experienced this? Why is my baby so big? Should I be worried about gestational diabetes?