The most accurate means for determining paternity is using a DNA test from both men. If your cycles are irregular then it is impossible to determine the date of conception.


Conception occurs soon after ovulation (24 hour at most) . Ovulation usually happens about 14 days before the next period starts (there are always exceptions). The time period between ovulation and the next period is usually constant (about 14 days), no matter how long the cycles last... or if they are regular, or not. That’s why we can determine the day of ovulation (and possible conception) for women with regular periods. For example: if your last period started on the 6th of April and your cycles usually last 32 days, then your next period would come on or around the 7th of May. To calculate the ovulation day we count 14 days backward from the 7th of May... which means that you probably ovulated around the 23rd of April. Two days before and after the 23rd are considered highly possible days for conception, or "fertile" days. In women with irregular cycles, we can’t calculate the ovulation date because we don’t know when the next period will start. So, we don’t have a date from which we will count 14 days backward.


In your case, you can never be sure when you conceived. It was probably somewhere in May. Time distance between 10th and 13th of May is so short that gives, both actual or ex boyfriend, chances to be a father. I think that in your case only DNA-test can tell you who the real father is. The only precise fact that I can give you according to your last period date is that you are 16 weeks pregnant till now.