This is a holdover from a time when much less was understood about conception.
No one understood that ovulation was required for conception or when ovulation occurred.
Determining the Date of Conception Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception.
However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age and, therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.
In addition, although a woman is most likely to become pregnant if she has sex on the day of ovulation conception may also occur from live sperm still in her reproductive tract on the day of ovulation if she had sex for up to five days before ovulation [26,27].
Determining the Estimated Due Date The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period.
Obviously, you aren’t pregnant until conception occurs. In many other places, the new knowledge has been incorporated into a dating system that starts with conception.
In that system, pregnancy starts on the day of ovulation (usually the day of conception) and lasts for 266 days (38 weeks). It only makes a difference in calculating how far along you are during a pregnancy.
Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7]. You can calculate how far along you are using our Pregnancy Calculator.Dating a pregnancy can cause confusion, because it can be done in either of two different ways.Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.To find your due date, use the drop down menus below to enter the date of the first day of your last period, and click 'calculate date' - the calculator will do the rest.