Understanding Due Dates
Why does your midwife think you are 8 weeks pregnant when you are certain you are only 6? Your midwife does not count pregnancy as beginning from the date of conception.
Trying to determine the actual date of conception is nearly impossible unless you have been charting through a natural family planning method. Most women are not familiar enough with their cycle to know when they are ovulating, and most women do not keep track of which nights intercourse occurred.
But most women do know approximately when they had their last period. By knowing the first date of your last period, your doctor can guess that conception occurred about two weeks later, and so your baby will be born about 38 weeks after that. Instead of doing the math twice, your doctor just adds 40 weeks to the first date of your last period to get your due date.
This causes two problems. First, you can be 8 weeks pregnant (counting from the first day of the last period) even though your baby is only in the 6th week of gestation. That's because using the 40 weeks, you count two weeks before conception occurred. In other words, you were two weeks pregnant when you conceived your baby.
The second problem is that not every woman has a 28 day regular cycle. If your cycle is shorter or longer, your conception date would not be at the average 14 days later and so your due date will be off by a few days. If you were just coming off birth control medications, you may have missed a period or had unusually long periods as your body was adjusting to the new hormone levels. Again, this would change your due date.
Just remember that the due date is not a definite, only a best guess based on the average length of a pregnancy. Some baby's are born earlier, some later - that's how you get an average. Since most first time mom's give birth after their due date, you may want to consider your due date as the first possible due date.