I did the calculation and you would have been in the ovulation window to conceive. Normally the ovulation window is 5-7 days. Depends on your cycle, if you ovulate early or late. That's why they go based from your period not the conception day
Yes it adds up :) it's calculated by adding 40 weeks from the first day of your last menstrual cycle :) they should also be able to tell you how far along you are by your babies measurements in An ultrasound
Why are you worried, are you concerned that you will have the baby earlier than expected?
A doctor hearing that the first day of your last period was October 31 will do a manual calculation with a calendar or a little cardboard wheel and will tell you that it adds up to an August 7 due date. Unless he has ultrasound information to the contrary, he will continue to assume that is your due date, even if you might have ovulated earlier in your cycle than an "average" woman might ovulate.
Sperm can live in your body 5-6 days (some research suggests longer). This means a woman can have sex on November 5 and get pregnant from it on November 10 or even November 11. A standard conception date for the due date of August 7 would be November 14, which is not much later than November 10 or 11. (Is the baby a girl, by the way?)
If you were given your due date from an ultrasound, how late was it in your pregnancy? Ultrasounds are only accurate for determining estimated conception dates if they were done quite early in the pregnancy. After a while there is a margin for error that gets larger depending on how far along you are.
In short, the accuracy of your estimated due date depends on how the doctor got the due date he gave you, and on how early or late your ultrasound was (if you got one). But you might not be as far out of range as you seem to think you are. If the last sex you had was November 5, that is the sex that produced the baby even if you didn't get pregnant until November 11.
Definitely adds up. The last day off my menstrual was the 12th of November and I'm due the 19th August