The screening for gestational diabetes should be conducted between the 24th and 28th weeks of pregnancy. This includes a 75g glucose test, where 75 grams of anhydrous glucose is dissolved in water and consumed within five minutes on an empty stomach. This test monitors fasting blood sugar, blood sugar one hour after the meal, and blood sugar two hours after the meal. A diagnosis of gestational diabetes is made if any of the following criteria are met or exceeded: fasting blood sugar is greater than or equal to 5.1 mmol/L, blood sugar one hour after the meal is greater than or equal to 10 mmol/L, and blood sugar two hours after the meal is greater than or equal to 8.5 mmol/L.