When is that baby due?

How accurate is ultrasound at setting due dates?
A study has shown that an ultrasound isn't any more accurate than a reliable menstrual history combined with a pelvic exam by an experienced obstetrician. Researchers confirmed this by looking at pregnancies with known conception dates and comparing due dates arrived at by ultrasound measurements with dates arrived at by menstrual history and pelvic exam (4).

The fact that the old-fashioned method for dating a pregnancy does just as well as ultrasound is a vital point. While a sonogram may be useful in cases where there is uncertainty about when conception occurred, first-trimester sonograms are currently used as the ultimate standard. Your due date will often be changed if it differs from the one derived from the sonogram no matter how the date was previously determined or how sure you are of when you conceived.

Even first-trimester sonograms have a range of plus or minus five days, or a ten-day window, around the calculated date (3). The range increases to plus or minus eight days in the second trimester and plus or minus ten days for third-trimester scans. For this reason, experts say the due date should not be altered based on results from an early scan unless the calculated date differs by two weeks or more from the date determined by physical signs and symptoms and menstrual history (3).

What does this mean to you?
The first lesson to be learned is have patience, unless there is a good reason not to wait for nature to take its course. When the fruit is ripe, it will fall from the tree. Inducing labor may be presented as a far more straightforward decision than it actually is. Think carefully about the risks and benefits of an induction recommended solely because you have not begun labor before some arbitrary cut-off date. Because it introduces risks, intervening in the natural process should only be undertaken to fix something that has gone awry.


  • Bastian LA et al. Diagnostic efficiency of home pregnancy test kits. A meta-analysis. Arch Fam Med 1998;7(5):465-9.
  • Mittendorf R et al. The length of uncomplicated human gestation. Obstet Gynecol 1990;75(6):929-32.
  • Otto C and Platt LD. Fetal growth and development. Obstet Gynecol Clin North Amer 1991;18(4):907-31.
  • Rossavik IK and Fishburne JI. 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. Obstet Gynecol 73(2):243-9.
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