Yes, if you have a long cycle they can differ. In my first pregnancy my last period was December 17th which made my due date 23rd September. My ultrasound (at 18 weeks) gave a date of October 4th. I had a 36 day cycle. 3 years later my cycle was 28 days and my ultrasound was spot on. An early ultrasound, around 6-7 weeks is extremely accurate. The later in pregnancy it is done the greater the variation as babies are all different sizes.
Doctors determine how far along you are by the first day of your last menstrual period. If you can remember the first day of your last menstrual period, then start to count from then to now. If you don't remember the first day of your last menstrual period, then you should get an ultrasound done and they will tell you. It's good to get ultrasounds done early in your pregnancy.
Menstrual cycles aren't a calculation, it's really just as simple as counting from the first day of your period up until the next period. You figure out the average over your last six cycles - anything up to a weeks variation from your average menstrual cycle is considered normal - but cycles can change, it's sometimes easier to use a menstrual tracking app on your phone to keep track for you.
It's not calculation, it's simply counting from day one of your last period up until your next period - depending on your average menstrual cycle length. You can also simply pay attention to your body, your body changes throughout your cycles so learning to recognise how it changes will tell you when your period is due even if your periods are irregular - learn to use fertility awareness method.
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