To calculate due date after in vitro fertilization, 266 days or 38 weeks are added to the date of egg retrieval, according to the Advanced Fertility Center of Chicago. Because eggs are fertilized on this day and actual pregnancy is approximately 38 weeks, this is the most accurate method to calculating due date.
Due date is a calculation that gives the approximate day a woman may deliver. Calculations for women who have not had IVF add 40 weeks or 280 days because the date is calculated from the first day of their menstrual cycle rather than the date of conception. It is difficult to calculate the exact date of conception in women who have not had IVF. Because IVF requires fertilization of the egg outside of the womb, the date is known in those cases.
Besides manual calculations, an ultrasound can be done post-conception to calculate gestational age. A measurement of the fetus is done at this time. Some doctors use both methods and compare results for accuracy. The ultrasound method of determining gestational age can start in weeks five or six of pregnancy and continues to be accurate through week 18. After this, measurement is not as accurate. According to the American Pregnancy Association, only 5 percent of women actually give birth on the calculated due date. About 80 percent of women give birth anywhere from 37 to 42 weeks after the first day of their last menstrual period.