Check your blood sugars regularly, 1 hour after each major meal. 155 post dinner is definitely high and needs medications to control. Metformin is used in some places but no long term data on effects on baby. Insulin is safe and effective option.

You can wait for a few days and follow diet and exercise. But keep a strict check on home readings especially 2 hour post meals. Targets would be less than 130 for post meals. Since its detected in only 18 weeks of pregnancy, there are high chances of you requiring therapy. Meanwhile, do get an hba1c test done too. If its high, you shouldn't wait before starting therapy.