My wife has been really late with her cycle this year. Normally, she's pretty irregular and long anyway, but after a couple months, she decided a pregnancy test would be prudent. She ended up buying a pack of 3 and used one. Result: Negative.
A few weeks later, she was feeling a little nauseated, had some other possible-pregnancy symptoms, and still no cycle; so she used the second test. Result: Negative.
Now, she had put on quite a bit of weight over the past couple years. We figured it was probably her body telling her it was well past time to do something about that. So, we both started dieting (as I have a few pounds I could do without, as well). Her low-carb diet started with a two-week "boost" period, where everything was limited to an extreme, only to be brought back to normal more slowly later. The nausea was especially bad at this time, but she figured the diet was really stressing her body out. When the nausea persisted for the third or fourth week, I started pressuring her to call her doctor, as that's a long time to feel sick. (She's as stubborn as most men when it comes to seeing a doctor.)
So, Sunday, she decided maybe she'd try the last pregnancy test. It had been six or seven months by this point. Result: Positive.
So, at last, she made a doctor's appointment (although instead of her general practitioner, it was an obstetrician), and they did a quick ultrasound. The size of the baby surprised both her and her OB. Estimated age: 12 weeks. She's scheduled for a full ultrasound on Friday to get more definite measurements, but it seems that the first trimester has already come and gone before we even knew it.
Near as we can figure, we conceived almost immediately after she used her second pregnancy test, back in late February. At least now we know why she's been so consistently nauseated.
Addendum: A full ultrasound and measurements estimate the baby's age at 14 weeks, with a due date of mid-December.