It's not surprising that calculating dates of conception and due dates in pregnancy can be as much an art as a science, since they are dependent on the whims of the developing fetus, but what is surprising is that unless you undergo artificial insemination or in vitro fertilization, calculating conception dates can be just as difficult. What's even more surprising is that gestational age, or the age of the fetus, isn't measured from conception at all.
Date of Conception
If you know when your last period began, it's relatively easy to calculate your approximate date of conception. Most women with a 28-day cycle ovulate about 14 days after the day their last period began. Because sperm can survive in the fallopian tubes for up to three days before fertilization, but the egg only survives for 24 hours after ovulation, women are most likely to conceive when they have intercourse two to three days before ovulation. As conception is only possible up to 24 hours after ovulation, if you have a regular period, you can usually narrow down date of conception to a three to four day window. If your cycle lasts longer than 28 days, take the number of days in your shortest cycle, subtract 18, and then whatever the result is, count ahead that many days from the beginning of your last period. That should indicate which days you were likely the most fertile.
Even when it's possible to calculate date of conception within a very narrow window, it still isn't used to measure gestational age. Instead, gestational age is measured from the first day of the last period. If the date of the beginning of the last period isn't know, it's possible to measure approximate gestational date with an ultrasound. Ultrasounds are most accurate at estimating gestational age when the fetus is between eight and 18 weeks old, but unusually small or large fetuses can skew the results.
Like gestation age, the due date is calculated from the date of the beginning of the last period rather than the date of conception. Physicians give the due date as 40 weeks from the beginning of the last menstrual cycle. The due date should only be seen as an approximation, however, as only five percent of babies are actually born on their due dates. Most are born in the two weeks before or after their due date.