I'm also confused by the letter itself. I agree (of course) that LW should put his wife and baby first, but he doesn't actually say why he and his wife thought it would be best to wait until they were home to tell the grandparents. How did that help?

I'm assuming it's because they would insist on visiting in hospital, even if he told them not too.

That the only way to fend off a hospital visit was not to tell them.