Hello and thankyou for approaching DoctorSpring with your query.
I have gone through your question. Please can you give me more information about your situation.

You said you had unprotected sex on 9-10-11-13 of February. Following which you had your periods on 26th February.
You said you had protected sex on 12th February, did you mean 12th March?

If you had protected sex on 12th February and had your period on 26th february, you need not worry as this could not have resulted in pregnancy as you had your period after that.
Did you have protected or unprotected sex with your boyfriend after your periods on 26th February?

From what you have told me that you have your period every 26th of a month means you have a 30 day cycle. In that case your period in the month of March should be around the 28th of march plus or minus a few days as february had 28days. A slight delay can occur at times.

Please can you answer my questions and I will give you an informed opinion.
Please reply as a follow up.
Thankyou

Hello, thankyou for your clarification.
I have gone through your question and understand your concern.
It is unlikely that the person you had sex with on 12th of march would have resulted in your pregnancy as you used protection.
However condoms do have a failure rate, and it is difficult to say for sure who is the father based on this.
Even ultrasound will not help in establishing who the father is.
The only definite way to know would be by doing DNA paternity testing. DNA paternity tests are 100 % confirmatory.
Prenatal DNA paternity testing can be done as early as the late first trimester.

I hope this has helped. I will leave this consult open so that you can get back to me anytime.
Thankyou

Hello thankyou for the follow up.
These following clinics are well known in New York. You can give them a call and do the needful.
1) MedPro Connect
2) DNACLINIC

I hope this has helped.
Wishing you all the best.
Thankyou