Ways to Estimate Date of Conception Why don't epidemiologists ask people when they had sex? Why do they estimate time of conception from the birth date? This seems to introduce unnecessary error. Wouldn't it be better to record the time you had sex in some sort of database (assuming it was the time it resulted in pregnancy)?
Note that fertilization of the egg by sperm can happen without sex.
 A: A number of things to consider:


*

*Sometimes we do. There are, on occasion, studies that ask you to keep what is, for lack of a better description, a diary of sexual activity.

*We don't just back calculate from birth date. There are a number of methods of estimating date of conception - the one I see far more frequently than your suggested method is using the last known menstrual period of the mother. But there are extremely precise methods we can use - one study had women submit a staggering number of urine samples to detect pregnancy hormones, for example, which give extremely precise estimates.

*Your suggested approach is neither free of error, nor necessarily better. There are two reasons for this - first, while sex is a necessary component of conception, it isn't sufficient. "Had sex" doesn't indicate conception occurred. If its a once in a blue moon event, then sure it might work, but humans have sex for many reasons besides reproduction. Which means:


A. The timing of your estimate is still imprecise, and will probably be based informally on birth date. "What was I doing 9 months ago...ah, after that party at the Hendersons..."
B. If a couple is frequently having sex, you're essentially guessing from a sample.
C. Many subjects won't recall, and there may be serious differences in the recall rates for people based on important pieces of information (the non-random error another poster mentions). For example, couples just having sex might not remember with any precision, yet couples trying to have a baby might be tracking these things quite carefully. Random error, like the kind introduced by back calculating from a birthdate is irksome. Non-random error is bad.
A: There is a wealth of gestational age and birth weight population data and the prediction is less error-prone that one may expect. Just like growth reference chart for children, there are growth reference chart for fetus as well.
In addition, when it comes to interview, it can be burdensome for respondents to disclose when they have sex, mostly due to memory fading or the pregnancy being socially marginalized (especially in situation like teenage or underage moms, out-of-wedlock conception, etc.) Some people have sex once a while, some have sex multiple times a day, each of them would be subjected to different memory biases. And especially the condition you said:

assuming it was the time it resulted in pregnancy, 

for those who have a lot of frequent sex, how can you even tell? It's not like the woman's nose will blink like Rudolph's if the sperm made it to her egg.
Keeping the estimation of conception date in-house can skip many awkward moments, and perhaps will result in a higher response rate and a much trust-worthy assessment.
