I've been wanted to start charting and learn NFP, and last night this came up with my mom. I knew she wasn't a fan of NFP and I found out why.
When she was trying to get pregnant with me, she was charting her temperatures. Her temps never changed, so she thought she wasn't ovulating. When she never got her period, she took a pregnancy test, even though her temps indicated she wasn't ovulating and wasn't pregnant. Turns out she was and 9 months later I was born.
I thought her temps are just wacky and she should have been looking at cervical mucus, but then she told me that my grandma did temps and mucus and everything. First she used it to get pregnant, then to avoid. She had 4 babies while using it to avoid, and knowing my grandma, I would think she would use NFP correctly. Interestingly, it took her 10 years with no method to get pregnant the first time, then she got pregnant 10 times (and carried 8 of them to term).
So, maybe this means I might not be able to use STM and my grandma wasn't good at reading cervical mucus. But are there some people that charting is very unreliable for? Also, my family has a history of thyroid issues, so I don't know if that would make a difference.
