MIL advice needed -- LONG

Hey guys and gals:

OK, this may be more appropriate for the TTC board, but since boosh and I have forged relationships with many of you on this board, this is probably the best place to turn to for advice.

Early next year, boosh and I will begin TTC. We're going to the doc in September to talk about the preparation needed, prenatals, etc.

Both sides of parents are very excited about the thought of being grandparents and know that this will be happening. Here's the problem. My MIL has a BIG mouth. We have a great relationship and all, but when it comes to keeping things confidential, she can't do it. Two examples

Boosh is on a strict diet to lose weight in preparation for pregnancy. The only one we told about this was my MIL. We figured if we told too many people and Boosh didn't lose right away, people would make comments to my MIL about her not being dedicated. It's happened before. We didn't even tell my parents and asked my MIL not to tell anyone and named a few people we specifically did not want to know. Thursday, she admits to us that she slipped and told this person who we asked her not to, because the person was bragging about her daughter and she felt compelled to brag about Boosh. I was furious but since Boosh wasn't that mad, I let it slide. My MIL defended her decision by saying that at least she was honest in telling me the truth that she had slipped and that should make it OK.

Another example, before we got engaged, she knew I had the ring and knew when I was going to propose. She would call me 5 times a day, asking if she could tell people. I kept telling her no, but she kept bugging me so much that I relented and by the time we were engaged and told people, everybody knew and had to fake acting surprised.

So, now, we will be TTC and when Boosh gets pregnant, it will be a closely monitored three months because she has a thyroid condition and a few other things. Hence, we don't want to tell people until we're confident that Boosh is out of the woods and can carry the baby to term. This will probably be around the three month mark. We are planning on not telling both sets of parents until this point, when we'll be ready to tell everyone, because my MIL will definitely not be able to keep this confidential and if something should happen, god forbid, we don't want to have to go back and convey bad news. We know that she'll be hurt that she did not know sooner, but there really is no other way to make sure that no one finds out before we're ready to tell. Plus, it's our news, not hers.

Is this bad of us to wait this long to tell parents? What do you all think?