Your due date is based on the FIRST day of your last period.

I'm going to assume you've had an ultrasound. Ultrasound dating is very accurate in the first few months.

I'm also going to assume you've had an internal exam. The size and shape of your uterus is another way your doctor can tell how far along you are.

A due date is an estimate. The baby can come /- 2 weeks either way.

ETA: You did have a scan - http://ehealthforum.com/health/topic118211 .html

yeh ive had a scan the first one i had was at what they said was 7 weeks im just confused as to whether im supposed to be adding on 2 extra weeks as this article said the gestational age is 2 weeks behind or something??

If your due date is July 16, then your due date is July 16.

This is how it works:

Women menstruate. This is the begining of the cycle. First day of your period is the first day of your cycle.

If you have a 'perfect' 28 day cycle, you ovulate right in the middle of the cycle - or 2 weeks after the first day of your cycle. By the time your period is late, you are considered to be 4 weeks pregnant even though you only ovulated 2 weeks ago.

You get the 'bonus' two weeks added on. Really this is just done for dating purposes. This is how pregnancies are dated, how due dates are estimated, etc.

You don't need to add on 2 weeks to anything because it has already been done for you.

so just because im blonde is this the same if you were on the pill because thats how i got pregnant by being on the pill.so i did conceive when i thought then?? around the middle of october

What did the doctor say your due date was?

the 16th july, so i made him to be conceived around the 16th of october just by counting back the weeks. thats why i was confused coz of the 2 week thing didnt know whether it was 28 weeks then add another 2 weeks for the actuall conception date. im sorry im rather dumb,

14 weeks ago you had sex and got pregnant. You are 16 weeks pregnant because you are given 2 'bonus' weeks.