Whether your parents and your significant other's have been anxiously awaiting your first pregnancy or are more relaxed about your timeline, chances are they will receive your big news with great excitement. There is no one right way to make the announcement, or a right time or place; how creatively you tell them and how long you wait are up to you and the other parent-to-be. Talk to your doctor about the risks of your particular pregnancy before you begin telling anyone, including the new grandparents -- knowing your situation can help you decide when to share the news.
Decide if you want to wait until the end of the first trimester -- or even longer -- to tell the grandparents. If your doctor says your pregnancy is relatively high-risk, or if you've struggled with infertility, you might not want to tell anyone for the first three months. On the other hand, you might want your parents' support in case something goes wrong. Determine what will make you most comfortable.
Consider if each set of grandparents will be willing and able to keep the news secret from the other set if they get the information first. If so, tell the different branches of the family in whatever order you prefer. If not, tell the more discreet grandparents first, or plan to tell them all at the same time.
Send distant grandparents a card or a gift that spills the beans when you're ready. For example, mail them matching "I'm the Grandma" and "I'm the Grandpa" shirts, and include a note saying "Do not wear until [your due date]," or send a bouquet of pink, white and blue flowers with a note saying "Your grandson or granddaughter can't wait to meet you on [your due date]." Mail the items several days before you plan to tell local grandparents.
Invite local grandparents to dinner around the time you expect distant grandparents to receive their items. Drop a series of hints during the dinner, and finish the meal with a final reveal. For example, you might come to dinner dressed in pink and your significant other in blue; set the table with cutlery designed for children instead of adults; and for dessert, bring out a cake that says "World's Best Grandparents" or something similar on it. See how long it takes them to figure it out.
Call your parents personally if you are estranged from them and you want them to know about your pregnancy, or send them a serious letter. Tell your parents how you feel about them being part of your child's life, and listen to what they have to say. If they seem distant, give them time to process the information before you contact them again.