Hi, if you get a conception date from a doctor (who doesn't know when your last period was) from an ultrasound before your 8th week or so, it should be pretty close. If you get a conception date from a doctor just looking at a calendar or a little cardboard wheel and no ultrasound is done, then it's all about how regular your cycles are. If your cycles are not regular or are not about every 28 days, counting forward on a calendar won't tell you much.
I'm having a ultrasound done and I know how many days my cycles have been for the past year. I've been using an app to help me keep track.
They r accurate depending on how many days your cycle is..
You can't know for sure exactly when you ovulated and sperm can live for up to 5 days. So even if you took the 'conception date' as 100% accurate (which it won't be, due to human error measuring the fetus), the actual window of sex would be at least at week.