When they tell you for example: you are 20 weeks pregnant, this means that the baby is actually 18weeks. That is just based on how the docs count it, but some people conceive before or after this time, but they go by what's average.
Your mestrual cycle does play a part.
A baby is concidered full term at 38 weeks but the longest the baby is concidered safe to be in your belly (for the most part) is 42 weeks. So they count to 40 weeks which is right in the middle. This would mean that the average pregnancy is 280days (lmp) and 280 days actually is closer to 9 months.
(if you count 4 weeks per month you leave out 2-3 days per month, by the end you will have counted 10 months but this is not so)
280days divided by 7 days of the weeks = 40 weeks
280 days divided by 9 months = 31.1 (in days)
i am 30 weeks
if I count by 4 weeks I would be 7 and a half months (some people do this and end up being 10 months by the end)
but if I count by actual calendar months I won't be 7 months until feb.23rd
what I did to help me was just write on my calendar to count by weeks. Find the first day of your last menstrual cycle and mark 1week and go down each week as 2, 3,4, ... That way you can keep track.