Understanding Conception Dates & Paternity: Accuracy & Potential Scenarios

Question
How accurate are the "conception dates" doctors give pregnant women? A girl I slept with is pregnant and the Dr said something about the 30th of March being the date. I had sex with her one time late at night on the 29th so technically the 30th. I know that I could be the father. I also know that she had sex with a friend of mine on the 28th and was sexually invovled with her ex-boyfriend during that timfram as well. From what I have read on the net it seems that its hard to pinpoint the sexual experience that led to conception. I figure there are at least three men with their hat in the ring as potentially being the dad. A DNA test is forthcoming so we will find out then but Im just really nervous because it seems that she has decide that I am the father.

I am 28 yrs old and smoke cigarettes and marijuana regularly. I also had extensive chemotherapy as a child to treat leukemia. My father has always told me that when its time to start a family I will need a fertility test. These factors along with her lifestyle lead make me think that its probably not mine.

So basically the question I have is when the doctor determines the age of the fetus what is the margin of error?    Also, even if the estimated date of conception is exactly right, does that mean that the sexual encounter that I had with her is defineatly the one that caused conception?  

Answer
The accuracy of a conception date depends on the method used to calculate it.  A date determined from either the last menstrual period or from a late (> 10 weeks) ultrasounds can be off by several weeks.  A date determined from an early (<10 weeks) ultrasound is usually accurate to within a few days.  However, because sperm can survive in a woman for up to 4 days, this broadens further the range of possible dates.  So certainly any of the three of you could be the father of this young lady's baby, and it is good that a DNA test is being done to help sort things out.

I would not worry about your smoking or your history of chemotherapy, if you are concerned about an impact on the child.  If your sperm were healthy enough to fertilize the egg and create an embryo that did not spontaneously miscarry, then there shouldn't be any problems if the baby is yours.