Okay I had sex on the 21st of August. It's was protected . And I came on my period September 2nd or 3rd ! Then I had sex again on the 18 of September! I took a pregnancy test and I'm pregnant but I got my first ultrasound December 10,2015 and the baby age was exactly 14 weeks which I was confused cause that that's a week ahead from when I had sex on the 18th ! Can someone help me I'm confused ! Why is it measuring bigger shouldn't it be smaller ?

Pregnancy is most often dated from your last period. This often adds a couple of weeks, give or take, to when you likely ovulated. The reasoning is because, unless you had medical assistance to become pregnant, such as IVF, your exact date of conception is unknown.

You may have a fairly good idea of when ovulation most likely occurred, but unless baby's measurements are off a fair amount, dating will still go from the last period.

This is also partly why, while you will have a due date, giving birth a bit before or after that date is perfectly normal... in fact you mostly likely won't give birth on your due date. Sometimes it can be better to say that you are due early-mid June instead of an exact date.

No the ultrasound is not "exactly" it is never exact. It is at best, at the very best plus or minus a week, and can be even more.

So one week is easy within the doctors 14 weeks.

No since you only had sex on that one day, it is obvious when the exact date is. (unless there was other sex dates not listed)

No no other dates of sexual intercours just the 18th in September around the time of ovulation.

Your period could have been implantation bleeding from the baby. It looks like a regular or light period

Can implantation be heavy ? And last 5 days ? But my periods are regular .

Implantation bleeding, and not everyone experiences it, is usually minimal and for a day or two.

Some women can have spotting or bleeding in early pregnancy however, which they may think is an off period as most often the amount and duration will be different from their norm.

On rare occasions, some women will have bleeding off and on throughout their pregnancy.

The dates you mentioned, however, all match up. Being considered 14 weeks on Dec. 10, would have put conception around the middle of September. Yes, fertilizational age would be closer to maybe 12 weeks going from likely conception, but most often pregnancy is dated from your last period, giving you the gestational age, unless baby's measurements show differently.

Did they give you a due date around June 10?

Yes I'm due June 09th