Your due date is supposed to be about 10 months from your last period start date. Or around that! They don't know when in the month of your last period you conceived so you could be on or off a month! If you go into natural labor around 36-37 weeks then you can estimate you're 9 months exactly when they said you were because 36 / 4 = 9 months! So you likely got pregnant directly after your last period. If you go to 40 weeks then you more than likely got pregnant a month after the start of your last period. When you go to your first prenatal doctors visit she will estimate your due date by what day your last period started on. Then after your first ultrasound they will take measurements to see the growth rate of the baby to compare its size to how far along it should be. I was estimated at 12 weeks when my doctor talked to me but ultrasound said I was closer to 12 weeks and 3 days. Instead of moving my due date she figured it wasn't enough to prove the due date was wrong. Now he's just considered a bigger baby. They say his growth is in the 55 percentile. So it's more likely I conceived three days before the day they think I conceived. Funny enough I do know when I got pregnant and it was three days before the day they picked. But none of this matters in the end! Some babies take longer to mature or they just want to stay in! I was told white male babies love to stay till the very end. Also first time moms normally have their babies stay till the very end or even past the due date. Yet also heavier moms (like me) normally go into labor early...so there are just FAR too many contributing factors in pregnancy to get an actual ACCURATE due date. It's an estimate. After 37 weeks with no problems you probably should be prepared any day to go into labor. That's what I've been told by my doctors.

probably around October 14th is when you conceived possibly

I am due June 11th and conceived on September 4th.

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I'm due July 14th and I got pregnant from what the dr says and my calendar Oct 7th

I'm not sure how it's calculated but I know they have websites that will figure it out for you