Predicting the possible date of conception

According to the menstrual history of the women and coital history, the possible date of conception can be predicted to some extent.

Usually conception occurs around the time of ovulation.
With regular menstrual history the possibility of the ovulation can be predicted according to the previous cycles.
In case of regular 28 days menstrual cycles ovulation usually occurs between 12th to 16th day of the cycle, whereas in 30 days menstrual cycles ovulation can occur between 14th to 18th day of the cycle.
In case of regular lengthier cycles, usually ovulation is expected to occur during last 14 to 16 days of cycle.
So, if unprotected intercourse occurs around the period of ovulation, that can lead to pregnancy.
Sometimes spermatozoa can survive for around 4 to 5 days in female genital tract and ovum can be alive for 24 hours to 48 hours. This time period should also be considered while predicting the date of conception.
If menstrual cycles are irregular, it is difficult to predict the date of ovulation and conception.
In that case around three weeks prior to the date of first positive pregnancy test can be considered as possible conception period.
Ultrasound also helps in predicting the possible conception date.
Three weeks before appearance of gestational sac or four before appearance of fetal heart beat can be considered as possible conception period.
Uterus becomes abdominal organ usually at around 12 weeks. Ten weeks prior to the date of palpating the uterus per abdominally, can approximately give the date of conception.