The inaccuracy of ultrasound
First trimester: 7 days
14 - 20 weeks: 10 days
21 - 30 weeks: 14 days
31 - 42 weeks: 21 days
Calculating an accurate EDD
Recent research offers a more accurate method of approximating gestational length. In 1990 Mittendorf et Al. undertook a study to calculate the average length of uncomplicated human pregnancy. They found that for first time mothers (nulliparas) pregnancy lasted an average of 288 days (41
weeks 1 day). For multiparas, mothers who had previously given birth, the average gestational length was 283 days or 40 weeks 3 days. To easily calculate this EDD formula, a nullipara would take the LMP, subtract 3 months, then add 15 days. Multiparas start with LMP, subtract 3 months and add 10 days. The best way to determine an accurate due date, no matter which method you use, is to chart your cycles so that you know what day you ovulate. There are online programs available for this purpose (refer to links in resources section). Complete classes on tracking your cycle are also available through the Couple to Couple League.
ACOG and postdates
One of the most vital pieces of information to know when you are expecting is that ACOG itself (American College of Obstetricians and Gynecologists) does not recommend interfering with a normal pregnancy before 42 completed weeks. This is why knowing your true conception date and EDD is very important; if you come under pressure from a care provider to deliver at a certain point, you can be armed with ACOG's official recommendations as well as your own exact due date. This can help you and your baby avoid much unnecessary trauma throughout the labor and delivery. Remember, babies can't read calendars; they come on their own time and almost always without complication when left alone to be born when they are truly ready.
Sources:
Mittendorf, R. et al., "The length of uncomplicated human gestation," OB/GYN, Vol. 75, No., 6 June, 1990, pp. 907-932.
ACOG Practice Bulletin #55: Clinical Management of Post-term Pregnancy