This may sound awful but I could really use some help. Me and my partner have been together for a year now but in November we broke up. In that time I slept with an ex in the early hours of 23rd November but he did not finish. I came on my period the next day and got the emergency contraception pill on Monday to cover my back. From the start of December me and my partner decided to make things work again, we had regular sex and he was the only person I slept with since. I then found out I was pregnant on Boxing Day, and as of my 12 week scan it was worked out that as of yesterday (05th March 2014) I would have been 15 weeks and I am due on 27th August. I have used online calculators which show that I was not pregnant until around the 3rd to 8th of December but I would like a more opinion. Should I be worried? Thanks guys! Could really use the help xxx

Worried, yes, but slightly, since the sex was right before the period, that period could come. And online estimates, are not exact, and to make that even worst, the dating by the doctor is at best give or take one week.

So guess what, if you move that date, back one week, you are getting very close to the sex with the ex.

So is it possible, yes, you had sex, he does not need to finish to cause pregnancy.
Now it is more probable that it is the current boyfriend, but there is no way ( sorry) to know for sure at this point.

You will have to wait till the baby is born and do a DNA test just to be sure

Odds are greatly in your favour for several reasons. One, he didn't finish, which isn't a guarantee, but it cuts the numbers down considerably. Two, it was right before your period, which while not impossible, it would be unlikely for you to ovulate at that time. Three, you used an emergency contraceptive pill, which again is not a guarantee, but they do work quite well when taken appropriately. Four, an early scan is going to be more accurate than a later one for dating purposes. And five, just the more frequent sex with your partner, during a time when you would most likely be ovulating, as opposed to a single time with the ex, when it would be far less likely, increases the odds that your partner is the father.

If it were me, I would feel comfortable with the idea that my partner is the father.

Now, given all of that... which may help for your peace of mind, as Fr. Chuck said, the only way to know for certain would be to have a DNA test done.

As well most of doctors date to the beginning of your last menstrual period and not the date of conception, ovulation, or implantation. This makes things easier when you talk about developmental milestones during your pregnancy. Basically standardizing it all, so the date given would pretty much guarantee that the father wasn't your one night stand. Well that and the fact that your period came after he didn't finish in you. I would give having the one night stand as the father VERY low odds.