Q&A: I had sex with two people five days apart and want to know my conception date so I know who the father is.

First off, I have to tell you that you are not alone. Any time one of my patients is particularly curious about the date of conception, I know that she may have these same concerns.

Sometimes you can calculate a window of possible conception dates by looking at exactly how far along you are in the pregnancy and then counting back. When you are talking about a difference of only five days, both episodes may fall within the window, but it is possible that one man would be excluded. The most accurate estimate of when conception occurred can be made based on an early first trimester ultrasound, preferably that agrees with the dates calculated from a known last menstrual period in a woman with very regular cycles. Your doctor or midwife can help you identify the window of days during which conception could have occurred—generally a few days before through about one day after ovulation.

If the window for conception includes both dates, several other approaches could help you establish paternity. Since the possible fathers are only two men, sometimes blood typing of the men, you, and the baby, can exclude one of the men as the father. Beyond that, genetic paternity testing would be necessary.

From what you are saying, it doesn't sound like you are certain yet that you are pregnant. You probably already know this, but while anyone can get a sexually transmitted infection, women with multiple partners are particularly at risk. Condoms for all penetrative sex, knowing your partners well, and limiting the number of partners are all good strategies for preventing sexually transmitted infections. Condoms, however, are not the most effective method of birth control. You can get accurate information about birth control from Planned Parenthood.