No more than 10% of women deliver on their specified due date (usually estimated to be about 40 weeks of pregnancy). About 50% of women deliver within 1 week (before or after), and almost 90% deliver within 2 weeks of the due date.
Determining the length of pregnancy can be difficult because the precise date of conception often cannot be determined. Early in pregnancy, an ultrasound examination, which is safe and painless, can help determine the length of pregnancy. In mid to late pregnancy, ultrasound examinations are less reliable in determining the length of pregnancy.
Last full review/revision June 2013 by Julie S. Moldenhauer, MD