Approximately 2-5% of pregnant women develop gestational diabetes; this number may increase to 7-9% of mothers who are more likely to have risk factors. The screening for this disease usually takes place between your 24th and 28th week of pregnancy. Doctors test for gestational diabetes during this time because the placenta is producing large amounts of hormones that may cause insulin resistance. If the results indicate elevated levels, further testing would be done to confirm a gestational diabetes diagnosis.