The reasons why my area do it are for any of these:
Glycosuria 1+ on more than 1 occassion.
Previous large infant (over 4.5kg or >95% for gestatonal age)
Previous gdm ogtt at 16-18 and 26-28 weeks,
First degree relative with diabetes
History of pcos
Obesity bmi > or booking weight > 100mg
Family origin with high prevalence of diabetes (south Asian, black caribe an and midle eastern)
Fasting blood glucose > 6.0mmol/l or random blood glucose > 7.0mmol/l
Postnatal for gdm only if postnatal fbg> 6 mmol/l
Other ( who knows what else!!?)