Calculating how long you have been pregnant
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 date.1
Methods for estimating the length of your pregnancy and your due date include:
- Determining the first day of your last menstrual period.
- Measuring the size of your uterus.
- Using ultrasound testing.
Last menstrual period
The 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 27.
Another way to estimate your due date is to add 40 weeks to the first day of your last menstrual period.
Size of uterus
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 uterine fibroid.
If you are uncertain 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 determine your due date. Ultrasound testing is an accurate method of determining 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.
Last Updated: November 28, 2008