If you take 3 hour glucose tolerance test in 18 weeks are the results accurate or are they more accurate when you take it in 24-28 weeks?

It will be accurate if you do have a high glucose level. Did you have gestational diabetes before? Are you having any symptoms of gestational diabetes? Any family history? Did you fail the one hour test? These could be reasons that your doctor or midwife are doing the test a bit earlier than the norm.