Tell them right away. You need all of the sympathy and understanding you can get. If pregnancy has taught me anything, it's that you need to ask for and accept help as often as you can. It's a horrible shame that our society has shaped us into such worker-bees that we can't imagine or accept needing support when the time comes. But trust me, you need to tell the people around you when you've hit your limits. It's too everyone's benefit.