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Your doctor can use several methods to estimate how long
you have been pregnant. These methods can give an estimate of when you are
likely to deliver your baby (due date). The due date is only an estimate of
when you will deliver. Most women deliver within 14 days of their due
Methods for estimating the length
of your pregnancy and your due date include:
most common method of calculating your due date is by taking the first day of
your last menstrual period (LMP), adding 7 days, and then counting backward 3
months. For example, if your LMP started on March 20, you would add 7 days to
get March 27, and then subtract 3 months to get a due date of December
Another way to estimate your due date is to add 40 weeks to
the first day of your last menstrual period.
If you want help calculating your due date, use the Interactive Tool: What Is Your Due Date?
Around 12 weeks of pregnancy, the top of the
uterus (fundus) can be felt above the pelvic rim. At 20 weeks, the fundus will
be about as high as your umbilicus (belly button). After about 18 weeks, the
distance between the pubic bone and the fundus (in centimeters) is likely to be
about the same as the number of weeks since your last period.
Although fundal height is sometimes used to get a rough idea of how far
along a pregnancy is, it isn't an accurate way of predicting estimated
gestational age. There are a number of factors that
can make the fundal height seem higher or lower than expected, such as the
fetus's position or the presence of a
If you are not certain about
your last menstrual period or if the size of your uterus does not generally
correspond to the estimated length of your pregnancy, an ultrasound exam may be
ordered to find out your due date. Ultrasound testing is an accurate method of
finding out how long you have been pregnant, especially if it is done before 20
weeks of pregnancy.2 Some doctors do an ultrasound
routinely in early pregnancy.
During an ultrasound test, a small
instrument is moved back and forth over your abdomen. The instrument sends out
sound waves that bounce off the fetus. The sound waves are converted by a
computer into a picture of the fetus that is displayed on a TV screen.
Lund KJ, McManaman J (2008). Normal labor, delivery,
newborn care, and puerperium. In RS Gibbs et al., eds., Danforth's Obstetrics and Gynecology, 10th ed., pp. 23–42.
Philadelphia: Lippincott Williams and Wilkins.
American Academy of Pediatrics, American College of
Obstetricians and Gynecologists (2007). Antepartum care. In Guidelines for Perinatal Care, 6th ed., pp. 83–137. Elk Grove
Village, IL: American Academy of Pediatrics.
July 23, 2012
Sarah Marshall, MD - Family Medicine & Kirtly Jones, MD - Obstetrics and Gynecology
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