Your Dr. Will calculate your due date based on your LMP (Last Menstral Period) Plus 40 weeks from your LMP. It is just an estimated date with the average 28 day menstral cycle. If your cycle isn't the average 28 day cycle then it could be off by a couple weeks. Most women don't know when they conceived but almost all women know when they had their last period. You are only gestational pregnant for 38 weeks because from your LMP you are not pregnant for the first two weeks. An ultrasound performed within the first trimester are the most accurate with in 5 days. Later ultrasounds can be off by two weeks. Every baby grows at it's own pace and weight prediction is just a guess based on measurements and not all that accurate. The average full term baby in the US is 7.5lbs, but as we all know that isn't necessarily true. Some are lighter and some are heavier. Bottom line is your due date can be off by two weeks give or take from your LMP. I am currently pregnant with my 2nd child and had what I thought was a period in Dec. 08. Later discovered I was pregnant. I know I didn't conceive when my Dr thinks I did because my husband and I hadn't had sex that entire month of January until after I found out I was pregnant. My original due date was Oct. 4, they did an ultrasound in my 2nd trimester and moved it to Sept. 20th, based on my actual last LMP I should be due Aug. 31st. I know when we probably conceived because it was the only time hubby and I had sex without protection Dec 14th. Based on that date I will deliver around Sept 6th. So I think I will deliver sometime that week. I am already having Braxton Hicks contractions which can start a month before your due date. Go with your gut and don't worry about it too much, the baby will come out when he/she is ready.