HCG can only determine whether there is a pregnancy or not, not the exact time of conception. HCG should be calculated according to the last menstrual period, not according to the time of coitus, because not all coitus can cause pregnancy. If the follicles are not monitored periodically to know when they are dominant and when they rupture, there is no way to project the time of conception. There is no way to calculate this time, whether by vaginal ultrasound after conception or by other calculations, such as ovulation calculations, which can be subject to some error. If there is any doubt about the time of conception, amniotic fluid can be drawn to determine this, which can be 99% accurate.