Do all pregnant women rely on the doctor for a due date?

[mears]

I am asking this question because of a mix-up in a woman's dates the other day. She had been ginev a date by the doctor that was actually 2 weeks early so she was disappointed not to get induced.
Do women not chart their periods anymore? I still circle the date of my period on the calendar even though I can't have any more babies
If you know for sure the date of your last period, do you know how to calculate your due date? You add 7 days and 9 months. The obstetric wheel calendar that doctors and midwives use may give a date a day either side of that sometimes.
If you were sure of your date would you accept a change of date up to a week by scan?
I wonder is women have lost the art of knowing how to calculate their own pregnancies y relying totally on the doctor/midwife.
What are your experiences?

I have a book on pregnancy published by Queen Charlotte's Hospital that has a table that allows you to see your due date at a glance, provided you know the date of your LMP, so with DS1 and DS2 I used the table to check that date that the midwives had given me.

This time I actually felt the conception taking place - I o'd two days after the last time I had sex during fertile period - so I know I got pregnant four days after Day 14 - so I assume that my "real" due date is four days later than the midwife says - August 6 instead of August 2.

well I've worked out 20th October 2004 by the add 7 days and nine months method. Then 19th October by counting exactly 40 weeks from the day of LMP (leap year difference I guess) and I'll be intrigued to see what the doctor says as they've never agreed with me in the past! My cycle always used to be 28 days too, so I never could understand why they got a different date to me. Its now 27 days, so who knows what the doc will come up with??