How Do You Use Conception Calendars and Calculators?

A conception calculator is a simple method of estimating the gestational age of a pregnancy by placing the date of conception at the first day of the mother's last menstrual period, explains BabyMed. It is impossible to identify the date of conception for certain, but the conception calculator is the best method for a pregnancy older than 8 to 10 weeks, according to the American Pregnancy Association.

While the pregnancy calculator probably most accurately determines the gestational age of a pregnancy by the end of the first trimester, ultrasound readings may be more accurate earlier in the pregnancy. Because all pregnancies age differently, ultrasounds lose their accuracy after the 18th week or so, as the size of the fetus might change the estimated conception date, reports the American Pregnancy Association. The conception calculator's date of conception would not change, as it continues to use the first day of the last menstrual cycle.

If used to determine the parentage of the pregnancy, the conception calculator is only effective if the mother has had one sexual partner after her last menstrual cycle began. Mothers can find the most accurate conception date results by combining the conception calendar with ultrasound readings as the pregnancy progresses, advises the American Pregnancy Association.