It seems that many midwives (and perhaps OBs, I just don't have firsthand experience going to an OB) insist on using the LMP for calculating the due date, but at the same time are quick to point out that the rhythm method doesn't work. Many are still taught that ovulation is always on day 14, and yet say a woman can't possibly know when she ovulates. Contradictory much? If the LMP is accurate and ovulation is always on day 14, then the rhythm method is accurate and women can know when they ovulate (well, they can know when they ovulate anyway). However, we know that the rhythm method is not accurate for all women, just for those lucky enough to have regular cycles, and even they may have at least one cycle that's thrown off by stress or illness or whatnot.
I know I've gotten into arguments with midwives and nurses about these topics. It's rather infuriating, to be honest. When I was pregnant with C, I told them that giving them the LMP wouldn't give an accurate date because I'd had a 35-day cycle that time, but that's what they wanted. Had I been thinking fast enough, I would've given them a date 2 weeks prior to Peak instead (and told them I was giving them that date) so I wouldn't have had to fight about the date for the rest of the pregnancy, especially as I got near my due date and they started bugging me about being induced since they thought I'd gone over my due date (I hadn't - C was born 1 day before her due date). Ultrasounds can also be a week off from what I've read, even earlier ultrasounds; in fact, the ultrasound tech with C told me as much when I told her the date she gave was 6 days earlier than the date from my chart. Unfortunately, it was like pulling teeth to get the midwives to listen to me about my dates, as one kept insisting that charting wasn't accurate. (Please note that I do understand that LMP is the best starting point if a woman isn't aware of when she ovulated, and ultrasounds can narrow it down further in that case, my rant is about thinking that those are more accurate than a woman's chart, when they aren't. A really early ultrasound can be quite accurate, but they aren't usually done before 11-12 weeks here.)
That leads to the other bit about teaching that ovulation is on day 14 but that the rhythm method isn't accurate (and falsely equating all charting and NFP with the rhythm method). Both of those statements can't be true. I really wish more medical professionals were familiar with modern methods of NFP, or at least knew that there are various signs that indicate a woman's fertile times and that these methods are accurate and reliable, even if they don't know much about the methods themselves. I have met one GP who was willing to look at my chart and take my word for it, though she didn't understand it, and I appreciated that. I know there are more out there, especially since there were a few doctors at the Billings training I attended last summer. Ah well, enough ranting for now.