With the new early pregnancy tests, you can find out if you're pregnant within (IIRC) 24 hours. The problem with that is that many, many pregnancies normally end as miscarriages before a woman would even know that she was pregnant. She just thinks she was a few days late.
This happened to my close friend's son and daughter-in-law. Because they had been trying to get pregnant for awhile, they were using ept's constantly. She found out that she was pregnant (about 24 hours after the fact, IIRC), excitedly told everyone, and lost the baby a week later. Her doctor told her the information I gave above. So when they got pregnant again, they didn't tell anyone until the traditional 4-6 weeks later (after she'd missed a second period). They now have a beautiful baby boy, but the scare from the first miscarriage (which wouldn't even have been a scare if not for the info from the ept) tainted her entire pregnancy. Everyone was constantly terrified that she would miscarry again.
