Hello i need to know how to tell my exact last menstrual from last year? i have a 3 month old n can't predict when did i get pregnant with him

Start with birth: Most children will go almost exactly 40 weeks before being born. At that time they did a "ballard score" to see the level of maturity. Using those two numbers you can get an idea within 2-3 weeks of when conception likely happened. If you're trying to nail down an exact time to determine paternity, get a genetic paternity test done to know for sure.