I am a very regular person. And I know when I’m ovulating; I can feel it. If I’m paying attention, I know when my period should start to within about 12 hours, and if I’m really paying attention, I can get it down to about 4 hours. I would start hoping (for me, there’s no longer any way it could be worrying, though oh do I remember the agony and misery when it was), provided there was actually some way, at one week late, and get the test right then. Why prolong the suspense, y’know? But I would never do it before one week.
And I would tell my partner before I took the test, since I would want her as involved as she could possibly be, and that starts early. However, if I was in Abby’s situation, I’d probably do what she said; no one knowing but me would make it easier for me not to get hopeful or excited, and that might make it easier to stand another miscarriage. Under those circumstances, the partner would surely understand, and in any case be too happy to care how long you’d waited to say.
As for telling family (I know you didn’t ask, but just to be complete), I’d wait as long as possible, even to the extent of avoiding them once I started to show. I would do this for a very specific reason: my mother and sister just love to tell pregnancy and delivery horror stories, and I don’t enjoy hearing them even now. I know they’d send my anxiety level through the roof if I actually was pregnant. (I would tell my father, though, at three or four months. He knows better than to tell any horror stories at all, and he can keep a secret. Then I’d let him tell my mother and sister, and he could remind them of my long-stated rules: one horror story of any kind or description, and I walk out and don’t see them or speak to them again until after the baby is born.)