This is a frequent question. Your conception would've been about 2 weeks after your last period. An ultrasound will easily pinpoint the date of conception.

Conception happened roughly 38 weeks before your given due date. Hope that this helps!

The conventions we use to 'date' the pregnancy are a little confusing. Before ultrasound was used to help with the dating process, the only thing the obstetrician had to go by was the patient's last menstrual period. We count 40 weeks from the first day of your last menstrual period to derive the due date. If the date of the last period is not certain, or if the ultrasound measurements don't match up with due date you start with, the ultrasound dating is used to determine the correct date. If the ultrasound is done in the first trimester, it will be quite accurate. If it is done later it is less accurate. The OB spends a lot of time trying to make sure the due date is accurate. Obviously you don't actually conceive until sometime after your period, usually 14 days . This means in a way we are adding 2 weeks, which is the confusing part. If what you are trying to determine is the exact date you conceived, you use the most accurate due date and then count back 38 weeks to estimate the date of conception. This is not super accurate, though. Sometimes patients in my practice are uncertain about who the father of their baby is and they need to know the exact date of conception. This is 'tricky business' , and I tell these patients that the only way to be certain about this is to have DNA paternity testing.

This is a great question. A term human pregnancy usually delivers 38 weeks after conception. So why do doctors say "40 weeks"? The reason is that most women do not know the exact date that they conceived but they usually remember the first day of their last menstrual period therefore two weeks are added on because the "average" menstrual cycle is 28 days and ovulation thus conception usually occurs around the 14th day of the cycle plus or minus a couple of days. But what if your cycle is not typically 28 days say it is 34 days which is not abnormal it is just not the average. Then you need to tel your OB that information because instead of conceiving in the middle of your cycle you would have gotten pregnant around day 20 and that would make your due date later. You see the second part of a woman's cycle is always 14 days long but the first part called the proliferative phase can vary in length. Whenever my patient's U/S dates are different from her "menstrual" dates I always ask her "are your cycles 28 days a part or are they shorter or longer than 28 days" because this simple question can clear up what due date we should be using. With regard to your question about can you become pregnant months after "the sexual relation" the answer is no. By the way with regard to implantation it should be remembered that we all got pregnant in our mother's tube (unless you are an IVF baby) and then about 6-7 days later moved into the uterus to set up shop. Good luck with your pregnancy.