I've wondered this as well. I had a surprise pregnancy on FAM due to a sudden hormone change on a health supplement/juice. It made me ovulate 5 days early, caught 3 day old sperm (we used condoms for 8 days leading up to typical ovulation). I lost that baby and then we planned on waiting three cycles to try again. I continued the supplement that had changed my hormones (making for a much longer luteal phase), and continued to ovulate at the earlier time. The final wait cycle, I had all cervical position/fluid ovulation signs at the early time again. Then my fertile cm went away, my cervix lowered and closed. We pulled the goalie. Next morning, cervix went back up, opened, and cm returned...on the cycle day I had ovulated on for 5 years.
So, I either did the "impossible". Ovulated twice, 5 days apart, or my cervix and fluid showed all signs of fertility once while just gearing up, closed up, and then my body had reverted back to its old timing. It.was.weird.
Our handsome baby boy will be here in 6 weeks, lol. I would say it was a downside to not sleeping enough to temp, but I'm so excited about my 3rd son, it is hard to call it a downside.
My hubby wants me to get a paraguard now, though. Lol. I would still like to do FAM, just with more caution while making any diet or supplement changes, but hubby is definitely overwhelmed by the idea of a fourth kid. I think I'm probably feeling done, too.
Sorry, got OT. Just thought I would share my experience as far as cervical changes/ovulation.
Wife to Joe. Proud, tired mommy to Captain Destructive (6/09), Sir Screams-a-lot (8/10), and another little miracle due 3/13.
Trusting God brings life. Believing brings rest.