Use this simple calculator to roughly estimate a time of conception. The average time from conception to birth is 38 weeks, so if you just enter a birth date and leave the settings at their default values, you will be shown the most likely time range for your conception. If you know you were born prematurely, or late, you can provide that information and an adjustment will be made.
Wikipedia article on pregnancy.
Wikipedia article on gestation.
If you know your birth weight, that could be a (very vague) clue as to your gestational age at birth: baby birth weights.
Article from BiomedCentral that contains a good chart showing the distribution of births by gestational age: article | chart
Recent change in distribution of births by gestational age: article from CDC