Hi, my first day of last period is March, 11 and I had sex on April, 2 and now on April, 26, I was pregnant. So I want to know how many weeks pregnant am I?

You had sex ONLY one day, April 2? Then you conceived anytime within a few days after that. It depends mainly on where your egg was in it's travels down your fallopian tube, and how fast the sperm swam to get to it. It can vary from 1 day to as much as 7, rarely. So usually about 2 or 3.

But it's too soon to know if you are pregnant, so I have a feeling that you also had sex between your last period and April 2. That means that you conceived approximately in the middle of what your typical number of days is for a cycle. There's no way to know the exact day, because you don't know exactly when you ovulate each month.

With your LMP being 3/11/15, you would have a due date of 12/16/15, making you 6 weeks and 5 days as of today.

Due Date Calculator (http://www.perinatology.com/calculators/Due-Date.htm)

With that said, if paternity is in question you will still need to have a DNA test after the baby is born.

If you only had sex one time, then you know the exact time you became pregnant. The number of weeks is an estimate based on last monthly cycle.

Assuming you only had sex one time, then it is not an issue. But conception estimates are not exact, if there are more than one date of sex

For the most part, gestational dating is done with regards to the LMP (Last Monthly Period). It is only an estimate based on a mathematical calculation. This calculation is not, in any way, a final decision. Due dates can be changed according to fetal growth on an ultrasound.

Typically gestation can vary two weeks one way or the other. I have had patients who knew they were pregnant as early as two weeks after conception.